& Simulator. Three-Address Instructions. Three-Address Instructions. Three-Address Instructions. Structure of an Assembly Program.

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1 & Simulator Copyright 2011, Pedro C. Diniz, all rights reserved. Students enrolled in the Compilers class at the University of Southern California (USC) have explicit permission to make copies of these materials for their personal use. Mapping of High-level Constructs Register-based (Temporaries) IR for Expression Evaluation Infinite Number of Virtual Registers Still Independent of Target Architecture Parameter Passing Discipline either on Stack or via Registers Addresses and Instructions Symbolic Names are addresses of the corresponding source-level variable. Various constants, such as numeric and offsets (known at compile time) Generic Instruction Format: label: x = y op z or if exp goto L Statements can have Symbolic Labels Compiler inserts Temporary Variables (any variable with t prefix) Type and Conversions dealt in other Phases of the Code Generation 2 Assignments: x = y op z (binary operator) x = op y (unary) x = y (copy) Operations: x = &y; x = *y and *x = y; for assignments via pointer variables. Control Transfer and Function Calls: goto L (unconditional); if (a relop b) goto L (conditional) where relop is a relational operator consistent with the type of the variables a and b; y = call p, n for a function or procedure call instruction to the name or variable p p might be a variable holding a set of possible symbolic names (a function pointer) the value n specifies that before this call there should exist n putparam instructions to load the values of the arguments. the param x instruction specifies a specific value in reverse order (i.e, the param instruction closest to the call is the first argument value. Later we will talk about parameter passing disciplines (Run-Time Env.) 3 4 Simulator c3esim Structure of an Assembly Program Model: Addressable as a Unit not Bytes Unlimited Symbolic Variables Simple Instructions with Basic Elements Base Types: Character, Integer, Double : Operands are Either Symbols (Variables) or (Typed) Constants Control Flow Instructions Unconditional/Conditional Jump or Goto Call/Return using Address and Argument Stacks Arithmetic and Logic Instructions Addition, Subtraction, Multiplication, Division Relational Operators - results integer: Zero is False, non-zero is True local variables for main local variables for procedure add Data section header! Comments! Text (Code) section header! 5 6 1

2 Structure of an Assembly Program Structure of an Assembly Program local variables for main local variables for procedure add Data Declaration and Allocation Section variable name basic type (word, byte,ascii,skip) initial value(s) ascii directive reserves and initializes space skip directive just reserves space local variables for main local variables for procedure add Data Declaration and Allocation Section variable name basic type (word, byte,ascii,skip) initial value(s) ascii directive reserves and initializes space skip directive just reserves space Text (Code) Section entry labels for procedures code starts executing at main 7 8 main: x = call iread, 0 y = call iread, 0 if x < 8 goto L2 goto L3 L if y goto L4 L t = 0 L t = 1 end: Conditional Control-Flow Predicate Evaluation Constants or Variables Operands No Side Effects True non-zero False zero main: x = call iread, 0 y = call iread, 0 if x < 8 goto L2 goto L3 L if y goto L4 L t = 0 L t = 1 end: Conditional Control-Flow Predicate Evaluation Constants or Variables Operands No Side Effects True non-zero False zero Example: if y goto L4 is equivalent to if y!= 0 goto L4 How does this code behave? 9 10 main: x = call iread, 0 y = call iread, 0 if x < 8 goto L2 goto L3 L if y goto L4 L t = 0 L t = 1 end: Conditional Control-Flow Predicate Evaluation Constants or Variables Operands No Side Effects True non-zero False zero Example: if y goto L4 is equivalent to if y!= 0 goto L4 How does this code behave? Reads x and y values Outputs 1 iff (x >= 8 and y!= 0) Outputs 0 otherwise a = 3 local variables for main c = local variables for procedure add x = y = t = putparam a

3 local variables for main local variables for procedure add a = 3 c = x = y = t = local variables for main local variables for procedure add a = 3 c = x = y = t = value(a) pc+1 local variables for main local variables for procedure add a = 3 c = x = y = t = 5 3 pc+1 local variables for main local variables for procedure add a = 3 c = x = y = t = 5 pc+1 3 address(c) local variables for main local variables for procedure add a = 3 c = x = y = t = 5 pc+1 3 0x0010 local variables for main local variables for procedure add a = 3 c = x = y = 5 t = pc+1 3 0x

4 local variables for main local variables for procedure add a = 3 c = x = 3 y = 5 t = pc+1 0x0010 local variables for main local variables for procedure add a = 3 c = x = 3 y = 5 t = 8 pc+1 0x local variables for main local variables for procedure add a = 3 c = x = 3 y = 5 t = 8 pc+1 0x0010 local variables for main local variables for procedure add a = 3 c = 8 x = 3 y = 5 t = Predefined I/O Functions Predefined I/O Functions local variables for main y:.byte z:.byte Six Basic I/O Functions cread: character read cwrite: character write iread: integer read iwrite: integer write fread: float/double read fwrite: float/double write local variables for main y:.byte z:.byte Six Basic I/O Functions cread: character read cwrite: character write iread: integer read iwrite: integer write fread: float/double read fwrite: float/double write Strings: Mapped to a sequence of chars with null terminated character. No predefined I/O functions

5 Predefined I/O Functions local variables for main y:.byte z:.byte Six Basic I/O Functions cread: character read cwrite: character write iread: integer read iwrite: integer write fread: float/double read fwrite: float/double write Strings: Mapped to a sequence of chars with null terminated character. No predefined I/O functions x = 0 y = z = local variables for main c:.byte main: p = &str p = p + i c = *p c = str[i] Use Operation Instructions c = local variables for main c:.byte Use Operation Instructions local variables for main c:.byte Use Operation Instructions main: p = &str p = p + i c = *p c = str[i] main: p = &str p = p + i c = *p c = str[i] c = c = c = c = c = local variables for main c:.byte Use Operation Instructions Use Operation Instructions main: p = &str p = p + i c = *p c = str[i] c = c = c = c =

6 Use Operation Instructions Use Operation Instructions local variables for main n:.skip 4 c:.byte x/y: n = f: g: local variables for main n:.skip 4 c:.byte x/y: n = f: g: x/y: n = f: g: h: main: p = &n p = p + 2 c = *p c = n->g with offset of g = 2 h: main: p = &n p = p + 2 c = *p c = n->g with offset of g = 2 h: UNIX Simulator Implementation Use Operation Instructions local variables for main y:.byte z:.byte >./c3esm -debug -infile sum.c3e -outfile sum.txt local variables for main n:.skip 4 c:.byte main: p = &n p = p + 2 c = *p c = n->g with offset of g = 2 x/y: f: g: h: n = x/y: f: g: h: n = x/y: f: g: h: p = 5 n = Uses lex and yacc UNIX tools Limited error checking (work in progress) A lot of run-time checks - it is slow (who cares!?)