Compiler Construction I

Transcription

1 TECHNISCHE UNIVERSITÄT MÜNCHEN FAKULTÄT FÜR INFORMATIK Compiler Construction I Dr. Michael Petter, Dr. Axel Simon SoSe / 108

2 Organizing Master or Bachelor in the 6th Semester with 5 ECTS Prerequisites Informatik 1 & 2 Theoretische Informatik Technische Informatik Grundlegende Algorithmen Delve deeper with Virtual Machines Programmoptimization Programming Languages Praktikum Compilerbau Hauptseminars Materials: TTT-based lecture recordings the slides Related literature list online Tools for visualization of virtual machines Tools for generating components of Compilers 2 / 108

3 Organizing Zeiten: Lecture: Mo. 14:15-15:45 Tutorial: Tuesday morning and afternoon Exam Exam managed via TUM-online Successful tutorial exercises earns 0.3 bonus 3 / 108

4 Preliminary content Basics in regular expressions and automata Specification with regular expressions and implementation with automata Reduced context free grammars and pushdown automata Bottom-Up Syntaxanalysis Attribute systems Typechecking Codegeneration for stack machines Register assignment Basic Optimization 4 / 108

5 Topic: Semantic Analysis 5 / 108

6 Semantic Analysis Scanner and parser accept programs with correct syntax. not all programs that are syntacticallly correct make sense 6 / 108

7 Semantic Analysis Scanner and parser accept programs with correct syntax. not all programs that are syntacticallly correct make sense the compiler may be able to recognize some of these these programs are rejected and reported as erroneous the language definition defines what erroneous means 6 / 108

8 Semantic Analysis Scanner and parser accept programs with correct syntax. not all programs that are syntacticallly correct make sense the compiler may be able to recognize some of these these programs are rejected and reported as erroneous the language definition defines what erroneous means semantic analyses are necessary that, for instance: check that identifiers are known and where they are defined check the type-correct use of variables 6 / 108

9 Semantic Analysis Scanner and parser accept programs with correct syntax. not all programs that are syntacticallly correct make sense the compiler may be able to recognize some of these these programs are rejected and reported as erroneous the language definition defines what erroneous means semantic analyses are necessary that, for instance: check that identifiers are known and where they are defined check the type-correct use of variables semantic analyses are also useful to find possibilities to optimize the program warn about possibly incorrect programs 6 / 108

10 Semantic Analysis Scanner and parser accept programs with correct syntax. not all programs that are syntacticallly correct make sense the compiler may be able to recognize some of these these programs are rejected and reported as erroneous the language definition defines what erroneous means semantic analyses are necessary that, for instance: check that identifiers are known and where they are defined check the type-correct use of variables semantic analyses are also useful to find possibilities to optimize the program warn about possibly incorrect programs a semantic analysis annotates the syntax tree with attributes 6 / 108

11 Topic: Code Synthesis 7 / 108

12 Generating Code: Overview We inductively generate instructions from the AST: there is a rule stating how to generate code for each non-terminal of the grammar the code is merely another attribute in the syntax tree code generation makes use of the already computed attributes 8 / 108

13 Generating Code: Overview We inductively generate instructions from the AST: there is a rule stating how to generate code for each non-terminal of the grammar the code is merely another attribute in the syntax tree code generation makes use of the already computed attributes In order to specify the code generation, we require a semantics of the language we are compiling (here: C standard) the semantic of the machine instructions 8 / 108

14 Generating Code: Overview We inductively generate instructions from the AST: there is a rule stating how to generate code for each non-terminal of the grammar the code is merely another attribute in the syntax tree code generation makes use of the already computed attributes In order to specify the code generation, we require a semantics of the language we are compiling (here: C standard) the semantic of the machine instructions we commence by specifying machine instruction semantics 8 / 108

15 Code Synthesis Chapter 1: The Register C-Machine 9 / 108

16 The Register C-Machine (RCMa) We generate Code for the Register C-Machine. The Register C-Machine is a virtual machine (VM). there exists no processor that can execute its instructions... but we can build an interpreter for it we provide a visualization environment for the R-CMa the R-CMa has no double, float, char, short or long types the R-CMa has no instructions to communicate with the operating system the R-CMa has an unlimited supply of registers 10 / 108

17 The Register C-Machine (RCMa) We generate Code for the Register C-Machine. The Register C-Machine is a virtual machine (VM). there exists no processor that can execute its instructions... but we can build an interpreter for it we provide a visualization environment for the R-CMa the R-CMa has no double, float, char, short or long types the R-CMa has no instructions to communicate with the operating system the R-CMa has an unlimited supply of registers The R-CMa is more realistic than it may seem: the mentioned restrictions can easily be lifted the Java virtual machine (JVM) is similar to the R-CMa but has no registers an interpreter of R-CMA can run on any platform 10 / 108

18 Virtual Machines A virtual machines has the following ingredients: any virtual machine provides a set of instructions instructions are executed on virtual hardware the virtual hardware is a collection of data structures that is accessed and modified by the VM instructions... and also by other components of the run-time system, namely functions that go beyond the instruction semantics the interpreter is part of the run-time system 11 / 108

19 Components of a Virtual Machine Consider Java as an example: C 0 1 PC S 0 SP A virtual machine such as the JVM has the following structure: S: the data store a memory region in which cells can be stored in LIFO order stack. SP: ( = stack pointer) pointer to the last used cell in S beyond S, the memory containing the heap follows 12 / 108

20 Components of a Virtual Machine Consider Java as an example: C 0 1 PC S 0 SP A virtual machine such as the JVM has the following structure: S: the data store a memory region in which cells can be stored in LIFO order stack. SP: ( = stack pointer) pointer to the last used cell in S beyond S, the memory containing the heap follows C is the memory storing code each cell of C holds exactly one virtual instruction C can only be read PC ( = program counter) address of the instruction that is to be executed next PC contains 0 initially 12 / 108

21 Executing a Program the machine loads an instruction form C[PC] into an instruction register IR in order to execute it before evaluating the instruction, the PC is incremented by one while (true) { IR = C[PC]; PC++; execute (IR); } node: the PC must be incremented before the execution, since an instruction may modify the PC the loop is exited by evaluating a halt instruction that returns directly to the operating system 13 / 108

22 Code Synthesis Chapter 2: Evaluation of Expressions 14 / 108

23 Simple Expressions and Assignments Task: evaluate the expression (1 + 7) 3 that is, generate an instruction sequence that computes the value of the expression and stores it on top of the stack 15 / 108

24 Simple Expressions and Assignments Task: evaluate the expression (1 + 7) 3 that is, generate an instruction sequence that Idea: computes the value of the expression and stores it on top of the stack first compute the value of the sub-expressions store the intermediate result on top of the stack apply the operator 15 / 108

25 General Principle Evaluating an operation op(a 1,... a n ) the arguments a 1,... a n must be on top of the stack the execution of the operation op consumes its arguments any resulting values are stored on top of the stack iconst q q SP++; S[SP] = q; the instruction iconst q puts the int-constant q onto the stack 16 / 108

26 Binary Operators Operators with two arguments run as follows: 3 8 imul 24 SP--; S[SP] = S[SP] S[SP+1]; 17 / 108

27 Binary Operators Operators with two arguments run as follows: 3 8 imul 24 SP--; S[SP] = S[SP] S[SP+1]; imul expects two arguments on top of the stack, consumes them and puts the result on top of the stack 17 / 108

28 Binary Operators Operators with two arguments run as follows: 3 8 imul 24 SP--; S[SP] = S[SP] S[SP+1]; imul expects two arguments on top of the stack, consumes them and puts the result on top of the stack other arithmetic and logical operations iadd, isub, idiv, imod, etc. work analogously 17 / 108

29 Composition of Instructions Example: generate code for 1 + 7: iconst 1 iconst 7 iadd Execution of this instruction sequence: 7 iconst 1 1 iconst 7 1 iadd 8 18 / 108

30 Expressions with Variables Variables occupy a memory cell in S: z: y: x: 19 / 108

31 Expressions with Variables Variables occupy a memory cell in S: z: y: x: Associating addresses with variables can be done while creating the symbol table. The address is stored in any case at the node of the declaration of a variable. 19 / 108

32 Expressions with Variables Variables occupy a memory cell in S: z: y: x: Associating addresses with variables can be done while creating the symbol table. The address is stored in any case at the node of the declaration of a variable. For each use of a variable, the address has to be looked up by inspecting its declaration node. 19 / 108

33 Expressions with Variables Variables occupy a memory cell in S: z: y: x: Associating addresses with variables can be done while creating the symbol table. The address is stored in any case at the node of the declaration of a variable. For each use of a variable, the address has to be looked up by inspecting its declaration node. in the sequel, we use a mathematical map ρ, that contains mappings form a variable x to the (relative) address of x; the map ρ is called address environment (or simply environment). 19 / 108

34 Reading from a Variable The instruction iload k loads the value at address k, where k is relative to the top of the stack 13 iload k S[SP+1] = S[SP-k]; SP = SP+1; Example: Compute x + 2 where ρ = {x 1}: 20 / 108

35 Reading from a Variable The instruction iload k loads the value at address k, where k is relative to the top of the stack 13 iload k S[SP+1] = S[SP-k]; SP = SP+1; Example: Compute x + 2 where ρ = {x 1}: iload 1 iconst 2 iadd 20 / 108

36 Code Synthesis Chapter 3: Generating Code for the Register C-Machine 21 / 108

37 Motivation for the Register C-Machine A modern RISC processor features a fixed number of universal registers. 22 / 108

38 Motivation for the Register C-Machine A modern RISC processor features a fixed number of universal registers. arithmetic operations can only use these registers as arguments access to memory are done via instructions to load and store to and from registers unlike the stack, registers have to be explicitly saved before a function is called 22 / 108

39 Motivation for the Register C-Machine A modern RISC processor features a fixed number of universal registers. arithmetic operations can only use these registers as arguments access to memory are done via instructions to load and store to and from registers unlike the stack, registers have to be explicitly saved before a function is called A translation for a RISC processor must therefore: 22 / 108

40 Motivation for the Register C-Machine A modern RISC processor features a fixed number of universal registers. arithmetic operations can only use these registers as arguments access to memory are done via instructions to load and store to and from registers unlike the stack, registers have to be explicitly saved before a function is called A translation for a RISC processor must therefore: 1 store variables and function arguments in registers 2 save the content of registers onto the stack before calling a function 3 express any arbitrary computation using finitely many registers 22 / 108

41 Motivation for the Register C-Machine A modern RISC processor features a fixed number of universal registers. arithmetic operations can only use these registers as arguments access to memory are done via instructions to load and store to and from registers unlike the stack, registers have to be explicitly saved before a function is called A translation for a RISC processor must therefore: 1 store variables and function arguments in registers 2 save the content of registers onto the stack before calling a function 3 express any arbitrary computation using finitely many registers only consider the first two problems (and deal with the other two later) 22 / 108

42 Principle of the Register C-Machine The R-CMa is composed of a stack, heap and a code segment, just like the JVM; it additionally has register sets: local registers are R 1, R 2,... R i,... global register are R 0, R 1,... R j,... C 0 1 PC S 0 SP R loc R 1 R 6 R glob R 0 R 4 23 / 108

43 The Register Sets of the R-CMa The two register sets have the following purpose: 1 the local registers R i save temporary results store the contents of local variables of a function can efficiently be stored and restored from the stack 24 / 108

44 The Register Sets of the R-CMa The two register sets have the following purpose: 1 the local registers R i save temporary results store the contents of local variables of a function can efficiently be stored and restored from the stack 2 the global registers R i save the parameters of a function store the result of a function 24 / 108

45 The Register Sets of the R-CMa The two register sets have the following purpose: 1 the local registers R i save temporary results store the contents of local variables of a function can efficiently be stored and restored from the stack 2 the global registers R i save the parameters of a function store the result of a function Note: for now, we only use registers to store temporary computations 24 / 108

46 The Register Sets of the R-CMa The two register sets have the following purpose: 1 the local registers R i save temporary results store the contents of local variables of a function can efficiently be stored and restored from the stack 2 the global registers R i save the parameters of a function store the result of a function Note: for now, we only use registers to store temporary computations Idea for the translation: use a register counter i: registers R j with j < i are in use registers R j with j i are available 24 / 108

47 Translation of Simple Expressions Using variables stored in registers; loading constants: instruction semantics intuition loadc R i c R i = c load constant move R i R j R i = R j copy R j to R i 25 / 108

48 Translation of Simple Expressions Using variables stored in registers; loading constants: instruction semantics intuition loadc R i c R i = c load constant move R i R j R i = R j copy R j to R i We define the following translation schema (with ρ x = a): code i R c ρ = loadc R i c code i R x ρ = move R i R a code i R x = e ρ = code i R e ρ move R a R i 25 / 108

49 Translation of Simple Expressions Using variables stored in registers; loading constants: instruction semantics intuition loadc R i c R i = c load constant move R i R j R i = R j copy R j to R i We define the following translation schema (with ρ x = a): code i R c ρ = loadc R i c code i R x ρ = move R i R a code i R x = e ρ = code i R e ρ move R a R i Note: all instructions use the Intel convention (in contrast to the AT&T convention): op dst src 1... src n. 25 / 108

50 Translation of Expressions Let op = {add, sub, div, mul, mod, le, gr, eq, leq, geq, and, or}. The R-CMa provides an instruction for each operator op. op R i R j R k where R i is the target register, R j the first and R k the second argument. Correspondingly, we generate code as follows: code i R e 1 op e 2 ρ = code i R e 1 ρ coder i+1 e 2 ρ op R i R i R i+1 26 / 108

51 Translation of Expressions Let op = {add, sub, div, mul, mod, le, gr, eq, leq, geq, and, or}. The R-CMa provides an instruction for each operator op. op R i R j R k where R i is the target register, R j the first and R k the second argument. Correspondingly, we generate code as follows: code i R e 1 op e 2 ρ = code i R e 1 ρ coder i+1 e 2 ρ op R i R i R i+1 Example: Translate 3*4 with i = 4: code 4 R 3 *4 ρ = code 4 R 3 ρ code 5 R 4 ρ mul R 4 R 4 R 5 26 / 108

52 Translation of Expressions Let op = {add, sub, div, mul, mod, le, gr, eq, leq, geq, and, or}. The R-CMa provides an instruction for each operator op. op R i R j R k where R i is the target register, R j the first and R k the second argument. Correspondingly, we generate code as follows: code i R e 1 op e 2 ρ = code i R e 1 ρ coder i+1 e 2 ρ op R i R i R i+1 Example: Translate 3*4 with i = 4: code 4 R 3 *4 ρ = loadc R 4 3 loadc R 5 4 mul R 4 R 4 R 5 26 / 108

53 Managing Temporary Registers Observe that temporary registers are re-used: translate 3*4+3*4 with t = 4: where we obtain code 4 R 3 *4+3*4 ρ = code 4 R 3 *4 ρ code 5 R 3 *4 ρ add R 4 R 4 R 5 code i R 3 *4 ρ = loadc R i 3 loadc R i+1 4 mul R i R i R i+1 code 4 R 3 *4+3*4 ρ = 27 / 108

54 Managing Temporary Registers Observe that temporary registers are re-used: translate 3*4+3*4 with t = 4: where we obtain code 4 R 3 *4+3*4 ρ = code 4 R 3 *4 ρ code 5 R 3 *4 ρ add R 4 R 4 R 5 code i R 3 *4 ρ = loadc R i 3 loadc R i+1 4 mul R i R i R i+1 code 4 R 3 *4+3*4 ρ = loadc R 4 3 loadc R 5 4 mul R 4 R 4 R 5 loadc R 5 3 loadc R 6 4 mul R 5 R 5 R 6 add R 4 R 4 R 5 27 / 108

55 Semantics of Operators The operators have the following semantics: add R i R j R k sub R i R j R k div R i R j R k mul R i R j R k mod R i R j R k R i = R j + R k R i = R j R k R i = R j /R k R i = R j R k R i = sgn(r k )k wobei R j = n R k + k n 0, 0 k < R k le R i R j R k R i = if R j < R k then 1 else 0 gr R i R j R k R i = if R j > R k then 1 else 0 eq R i R j R k R i = if R j = R k then 1 else 0 leq R i R j R k R i = if R j R k then 1 else 0 geq R i R j R k R i = if R j R k then 1 else 0 and R i R j R k R i = R j & R k // bit-wise and or R i R j R k R i = R j R k // bit-wise or 28 / 108

56 Semantics of Operators The operators have the following semantics: add R i R j R k sub R i R j R k div R i R j R k mul R i R j R k mod R i R j R k R i = R j + R k R i = R j R k R i = R j /R k R i = R j R k R i = sgn(r k )k wobei R j = n R k + k n 0, 0 k < R k le R i R j R k R i = if R j < R k then 1 else 0 gr R i R j R k R i = if R j > R k then 1 else 0 eq R i R j R k R i = if R j = R k then 1 else 0 leq R i R j R k R i = if R j R k then 1 else 0 geq R i R j R k R i = if R j R k then 1 else 0 and R i R j R k R i = R j & R k // bit-wise and or R i R j R k R i = R j R k // bit-wise or Note: all registers and memory cells contain operands in Z 28 / 108

57 Translation of Unary Operators Unary operators op = {neg, not} take only two registers: code i R op e ρ = codei R e ρ op R i R i 29 / 108

58 Translation of Unary Operators Unary operators op = {neg, not} take only two registers: code i R op e ρ = codei R e ρ op R i R i Note: We use the same register. 29 / 108

59 Translation of Unary Operators Unary operators op = {neg, not} take only two registers: code i R op e ρ = codei R e ρ op R i R i Note: We use the same register. Example: Translate -4 into R 5 : code 5 R -4 ρ = code5 R 4 ρ neg R 5 R 5 29 / 108

60 Translation of Unary Operators Unary operators op = {neg, not} take only two registers: code i R op e ρ = codei R e ρ op R i R i Note: We use the same register. Example: Translate -4 into R 5 : code 5 R -4 ρ = loadc R 5 4 neg R 5 R 5 29 / 108

61 Translation of Unary Operators Unary operators op = {neg, not} take only two registers: code i R op e ρ = codei R e ρ op R i R i Note: We use the same register. Example: Translate -4 into R 5 : code 5 R -4 ρ = loadc R 5 4 neg R 5 R 5 The operators have the following semantics: not R i R j R i if R j = 0 then 1 else 0 neg R i R j R i R j 29 / 108

62 Applying Translation Schema for Expressions Suppose the following function void f(void) { is given: int x,y,z; x = y+z*3; } Let ρ = {x 1, y 2, z 3} be the address environment. Let R 4 be the first free register, that is, i = 4. code 4 x=y+z*3 ρ = code 4 R y+z *3 ρ move R 1 R 4 30 / 108

63 Applying Translation Schema for Expressions Suppose the following function void f(void) { is given: int x,y,z; x = y+z*3; } Let ρ = {x 1, y 2, z 3} be the address environment. Let R 4 be the first free register, that is, i = 4. code 4 x=y+z*3 ρ = code 4 R y+z *3 ρ move R 1 R 4 code 4 R y+z*3 ρ = move R 4 R 2 code 5 R z *3 ρ add R 4 R 4 R 5 30 / 108

64 Applying Translation Schema for Expressions Suppose the following function void f(void) { is given: int x,y,z; x = y+z*3; } Let ρ = {x 1, y 2, z 3} be the address environment. Let R 4 be the first free register, that is, i = 4. code 4 x=y+z*3 ρ = code 4 R y+z *3 ρ move R 1 R 4 code 4 R y+z*3 ρ = move R 4 R 2 code 5 R z *3 ρ add R 4 R 4 R 5 code 5 R z*3 ρ = move R 5 R 3 code 6 R 3 ρ mul R 5 R 5 R 6 30 / 108

65 Applying Translation Schema for Expressions Suppose the following function void f(void) { is given: int x,y,z; x = y+z*3; } Let ρ = {x 1, y 2, z 3} be the address environment. Let R 4 be the first free register, that is, i = 4. code 4 x=y+z*3 ρ = code 4 R y+z *3 ρ move R 1 R 4 code 4 R y+z*3 ρ = move R 4 R 2 code 5 R z *3 ρ add R 4 R 4 R 5 code 5 R z*3 ρ = move R 5 R 3 code 6 R 3 ρ mul R 5 R 5 R 6 code 6 R 3 ρ = loadc R / 108

66 Applying Translation Schema for Expressions Suppose the following function void f(void) { is given: int x,y,z; x = y+z*3; } Let ρ = {x 1, y 2, z 3} be the address environment. Let R 4 be the first free register, that is, i = 4. code 4 x=y+z*3 ρ = code 4 R y+z *3 ρ move R 1 R 4 code 4 R y+z*3 ρ = move R 4 R 2 code 5 R z *3 ρ add R 4 R 4 R 5 code 5 R z*3 ρ = move R 5 R 3 code 6 R 3 ρ mul R 5 R 5 R 6 code 6 R 3 ρ = loadc R 6 3 the assignment x=y+z*3 is translated as move R 4 R 2 ; move R 5 R 3 ; loadc R 6 3; mul R 5 R 5 R 6 ; add R 4 R 4 R 5 ; move R 1 R 4 30 / 108

67 Code Synthesis Chapter 4: Statements and Control Structures 31 / 108

68 About Statements and Expressions General idea for translation: code i s ρ : generate code for statement s code i R e ρ : generate code for expression e into R i Throughout: i, i + 1,... are free (unused) registers 32 / 108

69 About Statements and Expressions General idea for translation: code i s ρ : generate code for statement s code i R e ρ : generate code for expression e into R i Throughout: i, i + 1,... are free (unused) registers For an expression x = e with ρ x = a we defined: code i R x = e ρ = code i R e ρ However, x = e is also a statement: move R a R i 32 / 108

70 About Statements and Expressions General idea for translation: code i s ρ : generate code for statement s code i R e ρ : generate code for expression e into R i Throughout: i, i + 1,... are free (unused) registers For an expression x = e with ρ x = a we defined: code i R x = e ρ = code i R e ρ However, x = e is also a statement: Define: move R a R i code i e 1 = e 2 ρ = code i R e 1 = e 2 ρ The temporary register R i is ignored here. More general: code i e ρ = code i R e ρ 32 / 108

71 About Statements and Expressions General idea for translation: code i s ρ : generate code for statement s code i R e ρ : generate code for expression e into R i Throughout: i, i + 1,... are free (unused) registers For an expression x = e with ρ x = a we defined: code i R x = e ρ = code i R e ρ However, x = e is also a statement: Define: move R a R i code i e 1 = e 2 ρ = code i R e 1 = e 2 ρ The temporary register R i is ignored here. More general: code i e ρ = code i R e ρ Observation: the assignment to e 1 is a side effect of the evaluating the expression e 1 = e / 108

72 Translation of Statement Sequences The code for a sequence of statements is the concatenation of the instructions for each statement in that sequence: code i (s ss) ρ = code i s ρ code i ss ρ code i ε ρ = // empty sequence of instructions Note here: s is a statement, ss is a sequence of statements 33 / 108

73 Jumps In order to diverge from the linear sequence of execution, we need jumps: jump A A PC PC PC = A; 34 / 108

74 Conditional Jumps A conditional jump branches depending on the value in R i :!0 Ri jumpz Ri A!0 Ri PC PC 0 Ri PC jumpz Ri A if (R i == 0) PC = A; 0 Ri A PC 35 / 108

75 Management of Control Flow In order to translate statements with control flow, we need to emit jump instructions. during the translation of an if (c) construct, it is not yet clear where to jump to in case that c is false 36 / 108

76 Management of Control Flow In order to translate statements with control flow, we need to emit jump instructions. during the translation of an if (c) construct, it is not yet clear where to jump to in case that c is false instruction sequences may be arranged in a different order minimize the number of unconditional jumps minimize in a way so that fewer jumps are executed inside loops replace far jumps through near jumps (if applicable) 36 / 108

77 Management of Control Flow In order to translate statements with control flow, we need to emit jump instructions. during the translation of an if (c) construct, it is not yet clear where to jump to in case that c is false instruction sequences may be arranged in a different order minimize the number of unconditional jumps minimize in a way so that fewer jumps are executed inside loops replace far jumps through near jumps (if applicable) organize instruction sequence into blocks without jumps 36 / 108

78 Management of Control Flow In order to translate statements with control flow, we need to emit jump instructions. during the translation of an if (c) construct, it is not yet clear where to jump to in case that c is false instruction sequences may be arranged in a different order minimize the number of unconditional jumps minimize in a way so that fewer jumps are executed inside loops replace far jumps through near jumps (if applicable) organize instruction sequence into blocks without jumps To this end, we define: Definition A basic block consists of a sequence of statements ss that does not contain a jump a set of outgoing edges to other basic blocks where each edge may be labelled with a condition 36 / 108

79 Basic Blocks and the Register C-Machine The R-CMa features only a single conditional jump, namely jumpz. code ss c Outgoing edges must have the following form: 37 / 108

80 Basic Blocks and the Register C-Machine The R-CMa features only a single conditional jump, namely jumpz. code ss c Outgoing edges must have the following form: 1 a single edge (unconditional jump), translated with jump 37 / 108

81 Basic Blocks and the Register C-Machine The R-CMa features only a single conditional jump, namely jumpz. code ss c Outgoing edges must have the following form: 1 a single edge (unconditional jump), translated with jump 2 two edges, one with c = 0 as condition and one without condition, translated with jumpz and jump, respectively 37 / 108

82 Basic Blocks and the Register C-Machine The R-CMa features only a single conditional jump, namely jumpz. code ss c Outgoing edges must have the following form: 1 a single edge (unconditional jump), translated with jump 2 two edges, one with c = 0 as condition and one without condition, translated with jumpz and jump, respectively 3 a set of edges and one default edge, used for switch statement, translated with jumpi and jump (to be discussed later) 37 / 108

83 Formalizing the Translation Involving Control Flow For simplicity of defining translations of instructions involving control flow, we use symbolic jump targets. This translation can be used in practice, but a second run through the emitted instructions is necessary to resolve the symbolic addresses to actual addresses. 38 / 108

84 Formalizing the Translation Involving Control Flow For simplicity of defining translations of instructions involving control flow, we use symbolic jump targets. This translation can be used in practice, but a second run through the emitted instructions is necessary to resolve the symbolic addresses to actual addresses. Alternatively, we can emit relative jumps without a second pass: relative jumps have targets that are offsets to the current PC sometime relative jumps only possible for small offsets ( near jumps) if all jumps are relative: the code becomes position independent (PIC), that is, it can be moved to a different address the generated code can be loaded without relocating absolute jumps 38 / 108

85 Formalizing the Translation Involving Control Flow For simplicity of defining translations of instructions involving control flow, we use symbolic jump targets. This translation can be used in practice, but a second run through the emitted instructions is necessary to resolve the symbolic addresses to actual addresses. Alternatively, we can emit relative jumps without a second pass: relative jumps have targets that are offsets to the current PC sometime relative jumps only possible for small offsets ( near jumps) if all jumps are relative: the code becomes position independent (PIC), that is, it can be moved to a different address the generated code can be loaded without relocating absolute jumps generating a graph of basic blocks is useful for program optimization where the statements inside basic blocks are simplified 38 / 108

86 Simple Conditional We first consider s if ( c ) ss....and present a translation without basic blocks. Idea: emit the code of c and ss in sequence insert a jump instruction in-between, so that correct control flow is ensured code i s ρ = code i R c ρ jumpz R i A code i ss ρ A :... code R jumpz code for ss for c 39 / 108

87 General Conditional code c code tt code ee Translation of if ( c ) tt else ee. code i if(c) tt else ee ρ = A : B : code i R c ρ jumpz R i A code i tt ρ jump B code i ee ρ code R for c jumpz code for tt jump code for ee 40 / 108

88 Example for if-statement Let ρ = {x 4, y 7} and let s be the statement if (x>y) { /* (i) */ x = x - y; /* (ii) */ } else { y = y - x; /* (iii) */ } Then code i s ρ yields: 41 / 108

89 Example for if-statement Let ρ = {x 4, y 7} and let s be the statement if (x>y) { /* (i) */ x = x - y; /* (ii) */ } else { y = y - x; /* (iii) */ } Then code i s ρ yields: (i) (ii) (iii) move R i R 4 move R i+1 R 7 move R i R 4 move R i+1 R 7 A : move R i R 7 move R i+1 R 4 gr R i R i R i+1 jumpz R i A sub R i R i R i+1 move R 4 R i jump B B : sub R i R i R i+1 move R 7 R i 41 / 108

90 Iterating Statements We only consider the loop s while (e) s. For this statement we define: code i while(e) s ρ = A : code i R e ρ jumpz R i B code i s ρ jump A B : code R for e jumpz code for s jump 42 / 108

91 Example: Translation of Loops Let ρ = {a 7, b 8, c 9} and let s be the statement: while (a>0) { /* (i) */ c = c + 1; /* (ii) */ a = a - b; /* (iii) */ } Then code i s ρ evaluates to: 43 / 108

92 Example: Translation of Loops Let ρ = {a 7, b 8, c 9} and let s be the statement: while (a>0) { /* (i) */ c = c + 1; /* (ii) */ a = a - b; /* (iii) */ } Then code i s ρ evaluates to: (i) (ii) (iii) A : move R i R 7 loadc R i+1 0 gr R i R i R i+1 jumpz R i B move R i R 9 loadc R i+1 1 add R i R i R i+1 move R 9 R i B : move R i R 7 move R i+1 R 8 sub R i R i R i+1 move R 7 R i jump A 43 / 108

93 for-loops The for-loop s for (e 1 ; e 2 ; e 3 ) s is equivalent to the statement sequence e 1 ; while (e 2 ) {s e 3 ; } as long as s does not contain a continue statement. Thus, we translate: code i for(e 1 ; e 2 ; e 3 ) s ρ = code i R e 1 ρ A : B : code i R e 2 ρ jumpz R i B code i s ρ code i R e 3 ρ jump A 44 / 108

94 The switch-statement Idea: Suppose choosing from multiple options in constant time if possible use a jump table that, at the ith position, holds a jump to the ith alternative in order to realize this idea, we need an indirect jump instruction 45 / 108

95 The switch-statement Idea: Suppose choosing from multiple options in constant time if possible use a jump table that, at the ith position, holds a jump to the ith alternative in order to realize this idea, we need an indirect jump instruction q Ri B PC jumpi Ri A PC = A + R i ; q Ri A+q PC 45 / 108

96 Consecutive Alternatives Let switch s be given with k consecutive case alternatives: switch (e) { case c 0 : s 0 ; break;. case c k 1 : s k 1 ; break; default: s; break; } that is, c i + 1 = c i+1 for i = [0, k 1]. 46 / 108

97 Consecutive Alternatives Let switch s be given with k consecutive case alternatives: switch (e) { case c 0 : s 0 ; break;. case c k 1 : s k 1 ; break; default: s; break; } that is, c i + 1 = c i+1 for i = [0, k 1]. Define code i s ρ as follows: code i s ρ = code i R e ρ A 0 :. A k 1 : check i c 0 c k 1 B code i s 0 ρ jump D. code i s k 1 ρ jump D B : jump A 0.. jump A k 1 C : 46 / 108

98 Consecutive Alternatives Let switch s be given with k consecutive case alternatives: switch (e) { case c 0 : s 0 ; break;. case c k 1 : s k 1 ; break; default: s; break; } that is, c i + 1 = c i+1 for i = [0, k 1]. Define code i s ρ as follows: code i s ρ = code i R e ρ A 0 :. A k 1 : check i c 0 c k 1 B code i s 0 ρ jump D. code i s k 1 ρ B : jump A 0 jump D check i l u B checks if l R i < u holds and jumps accordingly.. C :. jump A k 1 46 / 108

99 Translation of the check i Macro The macro check i l u B checks if l R i < u. Let k = u l. if l R i < u it jumps to B + R i l if R i < l or R i u it jumps to C B : jump A 0. C :. jump A k 1 47 / 108

100 Translation of the check i Macro The macro check i l u B checks if l R i < u. Let k = u l. if l R i < u it jumps to B + R i l if R i < l or R i u it jumps to C we define: check i l u B = loadc R i+1 l geq R i+2 R i R i+1 jumpz R i+2 E sub R i R i R i+1 loadc R i+1 k geq R i+2 R i R i+1 jumpz R i+2 D E : loadc R i k D : jumpi R i B B : jump A 0.. jump A k 1 C : 47 / 108

101 Translation of the check i Macro The macro check i l u B checks if l R i < u. Let k = u l. if l R i < u it jumps to B + R i l if R i < l or R i u it jumps to C we define: check i l u B = loadc R i+1 l geq R i+2 R i R i+1 jumpz R i+2 E sub R i R i R i+1 loadc R i+1 k geq R i+2 R i R i+1 jumpz R i+2 D E : loadc R i k D : jumpi R i B B : jump A 0.. jump A k 1 C : Note: a jump jumpi R i B with R i = k winds up at C. 47 / 108

102 Improvements for Jump Tables This translation is only suitable for certain switch-statement. In case the table starts with 0 instead of u we don t need to subtract it from e before we use it as index if the value of e is guaranteed to be in the interval [l, u], we can omit check can we implement the switch-statement using an L-attributed system without symbolic labels? 48 / 108

103 Improvements for Jump Tables This translation is only suitable for certain switch-statement. In case the table starts with 0 instead of u we don t need to subtract it from e before we use it as index if the value of e is guaranteed to be in the interval [l, u], we can omit check can we implement the switch-statement using an L-attributed system without symbolic labels? difficult since B is unknown when check i is translated use symbolic labels or basic blocks 48 / 108

104 General translation of switch-statements In general, the values of the various cases may be far apart: generate an if-ladder, that is, a sequence of if-statements 49 / 108

105 General translation of switch-statements In general, the values of the various cases may be far apart: generate an if-ladder, that is, a sequence of if-statements for n cases, an if-cascade (tree of conditionals) can be generated O(log n) tests 49 / 108

106 General translation of switch-statements In general, the values of the various cases may be far apart: generate an if-ladder, that is, a sequence of if-statements for n cases, an if-cascade (tree of conditionals) can be generated O(log n) tests if the sequence of numbers has small gaps ( 3), a jump table may be smaller and faster 49 / 108

107 General translation of switch-statements In general, the values of the various cases may be far apart: generate an if-ladder, that is, a sequence of if-statements for n cases, an if-cascade (tree of conditionals) can be generated O(log n) tests if the sequence of numbers has small gaps ( 3), a jump table may be smaller and faster one could generate several jump tables, one for each sets of consecutive cases 49 / 108

108 General translation of switch-statements In general, the values of the various cases may be far apart: generate an if-ladder, that is, a sequence of if-statements for n cases, an if-cascade (tree of conditionals) can be generated O(log n) tests if the sequence of numbers has small gaps ( 3), a jump table may be smaller and faster one could generate several jump tables, one for each sets of consecutive cases an if cascade can be re-arranged by using information from profiling, so that paths executed more frequently require fewer tests 49 / 108

109 Translation into Basic Blocks Problem: How do we connect the different basic blocks? Idea: translation of a function: create an empty block and store a pointer to it in the node of the function declaration 50 / 108

110 Translation into Basic Blocks Problem: How do we connect the different basic blocks? Idea: translation of a function: create an empty block and store a pointer to it in the node of the function declaration pass this block down to the translation of statements 50 / 108

111 Translation into Basic Blocks Problem: How do we connect the different basic blocks? Idea: translation of a function: create an empty block and store a pointer to it in the node of the function declaration pass this block down to the translation of statements each new statement is appended to this basic block 50 / 108

112 Translation into Basic Blocks Problem: How do we connect the different basic blocks? Idea: translation of a function: create an empty block and store a pointer to it in the node of the function declaration pass this block down to the translation of statements each new statement is appended to this basic block a two-way if-statement creates three new blocks: 1 one for the then-branch, connected with the current block by a jumpz-edge 2 one for the else-branch, connected with the current block by a jump-edge 3 one for the following statements, connect to the then- and else-branch by a jump edge 50 / 108

113 Translation into Basic Blocks Problem: How do we connect the different basic blocks? Idea: translation of a function: create an empty block and store a pointer to it in the node of the function declaration pass this block down to the translation of statements each new statement is appended to this basic block a two-way if-statement creates three new blocks: 1 one for the then-branch, connected with the current block by a jumpz-edge 2 one for the else-branch, connected with the current block by a jump-edge 3 one for the following statements, connect to the then- and else-branch by a jump edge similar for other constructs 50 / 108

114 Translation into Basic Blocks Problem: How do we connect the different basic blocks? Idea: translation of a function: create an empty block and store a pointer to it in the node of the function declaration pass this block down to the translation of statements each new statement is appended to this basic block a two-way if-statement creates three new blocks: 1 one for the then-branch, connected with the current block by a jumpz-edge 2 one for the else-branch, connected with the current block by a jump-edge 3 one for the following statements, connect to the then- and else-branch by a jump edge similar for other constructs For better navigation in later stages, it can be necessary to also add backward edges. 50 / 108

115 Code Synthesis Chapter 5: Functions 51 / 108

116 Ingredients of a Function The definition of a function consists of a name with which it can be called; a specification of its formal parameters; possibly a result type; a sequence of statements. In C we have: Observe: code i R f ρ = loadc _f with _f starting address of f function names must have an address assigned to them since the size of functions is unknown before they are translated, the addresses of forward-declared functions must be inserted later 52 / 108

117 Memory Management in Functions int fac(int x) { if (x<=0) return 1; else return x*fac(x-1); } int main(void) { int n; n = fac(2) + fac(1); printf("%d", n); } At run-time several instance may be active, that is, the function has been called but has not yet returned. The recursion tree in the example: main fac fac fac fac fac printf 53 / 108

118 Memory Management in Function Variables The formal parameters and the local variables of the various (instances) of a function must be kept separate Idea for implementing functions: 54 / 108

119 Memory Management in Function Variables The formal parameters and the local variables of the various (instances) of a function must be kept separate Idea for implementing functions: set up a region of memory each time it is called 54 / 108

120 Memory Management in Function Variables The formal parameters and the local variables of the various (instances) of a function must be kept separate Idea for implementing functions: set up a region of memory each time it is called in sequential programs this memory region can be allocate on the stack 54 / 108

121 Memory Management in Function Variables The formal parameters and the local variables of the various (instances) of a function must be kept separate Idea for implementing functions: set up a region of memory each time it is called in sequential programs this memory region can be allocate on the stack thus, each instance of a function has its own region on the stack 54 / 108

122 Memory Management in Function Variables The formal parameters and the local variables of the various (instances) of a function must be kept separate Idea for implementing functions: set up a region of memory each time it is called in sequential programs this memory region can be allocate on the stack thus, each instance of a function has its own region on the stack these regions are called stack frames) 54 / 108

123 Organization of a Stack Frame stack representation: grows upwards SP points to the last used stack cell SP FP PCold FPold EPold local memory callee organizational cells local memory caller 55 / 108

124 Organization of a Stack Frame stack representation: grows upwards SP points to the last used stack cell SP FP PCold FPold EPold local memory callee organizational cells local memory caller FP = frame pointer: points to the last organizational cell use to recover the previously active stack frame 55 / 108

125 Organization of a Stack Frame stack representation: grows upwards SP points to the last used stack cell SP FP PCold FPold EPold local memory callee organizational cells local memory caller FP = frame pointer: points to the last organizational cell use to recover the previously active stack frame EP has to do with the heap, will come to that later 55 / 108

126 Split of Obligations Definition Let f be the current function that calls a function g. f is dubbed caller g is dubbed callee The code for managing function calls has to be split between caller and callee. This split cannot be done arbitrarily since some information is only known in that caller or only in the callee. Observation: The space requirement for parameters is only know by the caller: Example: printf 56 / 108

127 Principle of Function Call and Return actions taken on entering g: 1. compute the start address of g 2. compute actual parameters 3. backup of caller-save registers 4. backup of FP, EP 5. set the new FP 6. back up of PC and jump to the beginning of g 7. setup new EP 8. allocate space for local variables actions taken on leaving g: 1. compute the result 2. restore FP, EP, SP 3. return to the call site in f, that is, restore PC 4. restore the caller-save registers 5. clean up stack } } saveloc mark are in f call } } } enter are in g alloc return are in g } } } restoreloc are in f pop k 57 / 108

128 Managing Registers during Function Calls The two register sets (global and local) are used as follows: automatic variables live in local registers R i intermediate results also live in local registers R i parameters global registers R i (with i 0) global variables: 58 / 108

129 Managing Registers during Function Calls The two register sets (global and local) are used as follows: automatic variables live in local registers R i intermediate results also live in local registers R i parameters global registers R i (with i 0) global variables: let s suppose there are none convention: 58 / 108

130 Managing Registers during Function Calls The two register sets (global and local) are used as follows: automatic variables live in local registers R i intermediate results also live in local registers R i parameters global registers R i (with i 0) global variables: let s suppose there are none convention: the i th argument of a function is passed in register R i 58 / 108

131 Managing Registers during Function Calls The two register sets (global and local) are used as follows: automatic variables live in local registers R i intermediate results also live in local registers R i parameters global registers R i (with i 0) global variables: let s suppose there are none convention: the i th argument of a function is passed in register R i the result of a function is stored in R 0 58 / 108

132 Managing Registers during Function Calls The two register sets (global and local) are used as follows: automatic variables live in local registers R i intermediate results also live in local registers R i parameters global registers R i (with i 0) global variables: let s suppose there are none convention: the i th argument of a function is passed in register R i the result of a function is stored in R 0 local registers are saved before calling a function 58 / 108

133 Managing Registers during Function Calls The two register sets (global and local) are used as follows: automatic variables live in local registers R i intermediate results also live in local registers R i parameters global registers R i (with i 0) global variables: let s suppose there are none convention: the i th argument of a function is passed in register R i the result of a function is stored in R 0 local registers are saved before calling a function Definition Let f be a function that calls g. A register R i is called caller-saved if f backs up R i and g may overwrite it callee-saved if f R i does not back up g must restore it before it returns 58 / 108

134 Translation of Function Calls A function call g(e 1,... e n ) is translated as follows: code i R g(e 1,... e n ) ρ = code i R g ρ code i+1 R e 1 ρ. code i+n R e n ρ move R 1 R i+1. move R n R i+n saveloc R 1 R i 1 mark call R i restoreloc R 1 R i 1 move R i R 0 59 / 108

135 Translation of Function Calls A function call g(e 1,... e n ) is translated as follows: code i R g(e 1,... e n ) ρ = code i R g ρ code i+1 R e 1 ρ. code i+n R e n ρ move R 1 R i+1. move R n R i+n saveloc R 1 R i 1 mark call R i restoreloc R 1 R i 1 move R i R 0 New instructions: saveloc R i R j pushes the registers R i, R i+1... R j onto the stack mark backs up the organizational cells call R i calls the function at the address in R i restoreloc R i R j pops R j, R j 1,... R i off the stack 59 / 108

136 Translation of Function Calls A function call g(e 1,... e n ) is translated as follows: code i R g(e 1,... e n ) ρ = code i R g ρ code i+1 R e 1 ρ. code i+n R e n ρ move R 1 R i+1. move R n R i+n saveloc R 1 R i 1 mark call R i restoreloc R 1 R i 1? = code i R e 1 ρ move R 1 R i. code i R e n ρ move R n R i code i R g ρ saveloc R 1 R i 1 mark call R i restoreloc R 1 R i 1 move R i R 0 move R i R 0 New instructions: saveloc R i R j pushes the registers R i, R i+1... R j onto the stack mark backs up the organizational cells call R i calls the function at the address in R i restoreloc R i R j pops R j, R j 1,... R i off the stack 59 / 108

137 Rescuing EP and FP The instruction mark allocates stack space for the return value and the organizational cells and backs up FP and EP. FP EP e FP EP e e mark S[SP+1] = EP; S[SP+2] = FP; SP = SP + 2; 60 / 108

138 Calling a Function The instruction call rescues the value of PC+1 onto the stack and sets FP and PC. FP q p q Ri call Ri Ri PC p PC q SP = SP+1; S[SP] = PC; FP = SP; PC = Ri; 61 / 108

139 Result of a Function The global register set is also used to communicate the result value of a function: code i return e ρ = code i R e ρ move R 0 R i return 62 / 108

140 Result of a Function The global register set is also used to communicate the result value of a function: alternative without result value: code i return e ρ = code i R e ρ move R 0 R i return code i return ρ = return 62 / 108

141 Result of a Function The global register set is also used to communicate the result value of a function: alternative without result value: code i return e ρ = code i R e ρ move R 0 R i return code i return ρ = return global registers are otherwise not used inside a function body: advantage: at any point in the body another function can be called without backing up global registers disadvantage: on entering a function, all global registers must be saved 62 / 108

142 Return from a Function The instruction return relinquishes control of the current stack frame, that is, it restores PC, EP and FP. PC FP EP p e return PC FP EP p e PC = S[FP]; EP = S[FP-2]; SP = FP-3; FP = S[SP+2]; 63 / 108

143 Translation of Functions The translation of a function is thus defined as follows: code 1 t r f(args){decls ss} ρ = enter q move R l+1 R 1.. move R l+n R n code l+n+1 ss ρ Assumptions: return 64 / 108

144 Translation of Functions The translation of a function is thus defined as follows: code 1 t r f(args){decls ss} ρ = enter q move R l+1 R 1.. move R l+n R n code l+n+1 ss ρ Assumptions: the function has n parameters return 64 / 108

145 Translation of Functions The translation of a function is thus defined as follows: code 1 t r f(args){decls ss} ρ = enter q move R l+1 R 1.. move R l+n R n code l+n+1 ss ρ Assumptions: the function has n parameters return the local variables are stored in registers R 1,... R l 64 / 108