Compilation 2013 Translation to IR

Transcription

1 Compilation 2013 Erik Ernst Aarhus University

2 Intermediate Representation Translation source/target uses IR as a bridge Simplification: n+m combinations, not n m Java SML Pascal C C++ SPARC MIPS Pentium Alpha 2

3 Intermediate Representation Translation source/target uses IR as a bridge Simplification: n+m combinations, not n m Java SML Pascal C C++ IR SPARC MIPS Pentium Alpha 3

4 Intermediate Representation Translation source/target uses IR as a bridge Simplification: n+m combinations, not n m Even more important: Divide and conquer Trade-off, consider: near-source near-target Java SML Pascal C C++ IR SPARC MIPS Pentium Alpha 4

5 Role of IR in Translation Compiler frontend: Translate source to IR, enforce wellformedness Compiler backend: Translate IR to assembly, often optimizing IR should be near both, still independent of source language and of machine details Source, Target: Incongruent complexity IR: clear, simple! IR 5

6 Tiger Compiler IR A simple tree expression language: signature TREE = sig type label = Temp.label datatype stm = MOVE of exp * exp EXP of exp JUMP of exp * label list CJUMP of relop * exp * exp * label * label SEQ of stm * stm LABEL of label and exp = CONST of int NAME of label TEMP of Temp.temp BINOP of binop * exp * exp MEM of exp CALL of exp * exp list ESEQ of stm * exp and binop = PLUS MINUS MUL DIV AND OR LSHIFT RSHIFT ARSHIFT XOR and relop = EQ NE LT GT LE GE ULT ULE UGT UGE... end 6

7 Surprising Bits IR allows calling, but not defining routines! Why is stm =.. EXP of exp.. useful? Why is relop =.. ULT.. useful? 7

8 Translation into IR Core part of compilation! Tempting starting point: Each Absyn.exp translates into a Tree.exp Not optimal no need to manage unit values, maybe not even boolean values.. Important idea: Encode tiny-domain values into the program counter 8

9 Expression Kinds Partition tree expressions into 3 kinds: Statements (Nx): no result, arbitrary effects Expressions (Ex): delivers general-form result Choices (Cx): represents tiny-domain value by behavior NB: customizable, will receive the labels later Some flexibility: Can transform these forms datatype exp = Ex of Tree.exp Nx of Tree.stm Cx of Temp.label * Temp.label -> Tree.stm 9

10 Tree Example & Issue Consider a>b c<d, translated into this: Cx (fn (t,f) => SEQ ( CJUMP (GT, translate(a), translate(b), t, z), SEQ ( LABEL z, CJUMP ( LT, translate(c), translate(d), t, f)))) Label z is fresh: Completely relation-oriented Whole expression returned by translate, type problem? 10

11 Unwrapping Translate.exp Almost all combinations plausible: May have Ex, need Cx, or vice versa Introduce unwrapping functions: val unex: exp -> Tree.exp val unnx: exp -> Tree.stm val uncx: exp -> (Temp.label * Temp.label -> Tree.stm) Similar to pattern matching, but tries to work even on values that do not match 11

12 Example Unwrapper Destruct as (Ex _) from given Translate.exp fun unex (Ex e) = e unex (Cx genstm) = let val r = Temp.newtemp () val t = Temp.newLabel "unex_t" val f = Temp.newLabel "unex_f" in T.ESEQ ( seq [ T.MOVE (T.TEMP r, T.CONST 1), genstm (t, f), T.LABEL f, T.MOVE (T.TEMP r, T.CONST 0), T.LABEL t], T.TEMP r) end unex (Nx s) = T.ESEQ (s, T.CONST 0) 12

13 Translating a Simple Variable For local InFrame(k) variable: MEM (BINOP (PLUS, TEMP fp, CONST k)) NB: No reference to size, works only with all-scalar setup In Translate: val simplevar: access * level -> exp 13

14 Translating a Simple Variable For local InFrame(k) variable: MEM (BINOP (PLUS, TEMP fp, CONST k)) Note important principle: MEM used for both read and write: Depends on context Works because of special discipline.. 14

15 Translating a Simple Variable Consider non-local InFrame(k) variable: Must navigate via static link until correct level found NB: Use unit ref trick as usual to enable level comparison 15

16 Tiger L-Value Management Basic concepts: L-Value: May be LHS & on RHS of assignment R-Value: Only RHS of assignment Consider: Cannot assign to 7 L-Value: Should be the address Issue: How about registers? Trick: Use MEM for both value & address! Left child of MOVE: write; otherwise read 16

17 Using a Translated Simple Variable Dual use of MEM (..): Has no meaning of its own, but one in MOVE (MEM (..),..)and another meaning elsewhere 17

18 Information Management Intended disciplin in module structure: Frame, Temp machine dependent Translate uses Frame and Temp Semant never uses Frame nor Temp directly! Semant Recall structure: Translate Frame Temp 18

19 Record and Array Assignments Arrays: Reference semantics let in end type intarray = array of int var a := intarray [12] of 0 var b := intarray [12] of 7 a := b What is now a[5]? Is there a memory leak? Record assignments: Reference semantics, too 19

20 Variable Sizes General issue: C struct, Pascal array,... Arbitrary size possible for variables Assignment:.. depends.. (typical: bit-copy) Tiger variables: All scalar (word-sized) Many simplifications follow: Array element, record field lookup; local variable allocation/access;... Semantics affected! cf. array assignment 20

21 Array Element Lookup General approach to read a[i]: MEM (address(a) + (i-lower_bound)*elm_size) Tiger, taking index out of bounds into account let in end type arrtype = array of int var arr := arrtype [10] of 7 arr[3] 21

22 Conditionals Very good example for Ex/unEx etc. Cases needed: Consider unex in Cx fun ifthenelse2ir (test, thenexp, elseexp) = let val test' = uncx test val labelthen = Temp.newLabel "if_then" val labelelse = Temp.newLabel "if_else" val labeljoin = Temp.newLabel "if_join" in case (test', thenexp, elseexp) of (_, Cx _, Cx _) => Cx (fn (t, f) => seq [ test' (labelthen, labelelse), T.LABEL labelthen, uncx thenexp (t, f), T.LABEL labelelse, uncx elseexp (t, f)]) (_, Ex _, Cx _) => Cx (fn (t, f) => seq [ test' (labelthen, labelelse), T.LABEL labelthen, uncx thenexp (t, f), T.LABEL labelelse, uncx elseexp (t, f)])... end 22

23 Array and Record Creation Lifetime: Not limited by execution time for creating function Must use heap allocation (GC welcome!); use runtime.o Note var arr := arrtype [12] of x sets every element to x Surprising? Useful? 23

24 Using C Functions in runtime.c Ex, creating array with arguments a, b: CALL (NAME (Temp.namedlabel ("initarray")), [a,b]) May need underscore in names etc.: externalcall: string * Tree.exp list -> Tree.exp int *initarray(int size, int init) { int i; int *a = (int *)malloc((size+1)*sizeof(int)); a[0] = size; for (i = 1; i < size; i++) a[i] = init; return (a+1); } 24

25 While Obvious pseudo-code template will work transexp, transdec gets extra break argument: A label to jump to when breaking For-loop similar, except for maxint corner case # Pseudo-code template: test: if not(condition) goto done body goto test done: 25

26 Calling Functions Typical invocation: CALL (NAME mylabel, [ static_link, arg1, arg2,.., argk]) let in end function f(n: int): int = 42 f(10) 26

27 Declarations Why make changes here?! Variable declaration initialization expression requires running code transdec now also returns exp list, containing var init code, executed before let-body Function declarations do not contribute 27

28 Declarations Serious matter: Completing functions with prologue, body, epilogue (procentryexit1): 1. Pseudo-instructions starting function (segment, debug info,..) 2. Label: function name 3. Allocate frame: SP := SP - framesize 4. Copy escaping arguments into frame, others into registers 5. Store calleesave registers 6. BODY 7. RV :=.. (result so far) 8. Restore calleesave registers 9. Deallocate frame: SP := SP + framesize 10. return instruction 11. Pseudo-instructions to finish function (e.g., size) 28

29 Fragments Used to gather translated code for functions datatype frag = PROC of {body: Tree.stm, frame: frame} STRING of Temp.label * string Gathered using side-effects, use getresult to fetch them all before generating asm code 29

30 Summary Tough IR requirements: simple, independent, between target/source (n+m vs. n m cost) For Tiger: Simple tree language Kinds of expressions: Ex, Nx, Cx, with un.. Simple variable access contains all static link stuff L-Values: scalar, using MEM-both-r&w trick Conditionals: Important to preserve Cx/Ex Loops, break Function calls Declarations: init code; procentryexit1 30