Compilerconstructie najaar 2012 http://www.liacs.nl/home/rvvliet/coco/ Rudy van Vliet kamer 124 Snellius, tel. 071-527 5777 rvvliet(at)liacs.nl college 6, dinsdag 23 oktober 2012 Intermediate Code Generation 1
6. Intermediate Code Generation • Front end: generates intermediate representation • Back end: generates target code Intermediate Static intermediate Code ✲ ✲ ✲ ✲ ✲ Code Parser Checker Generator code Generator ✲ ✛ front end back end 2
Intermediate Representation • Facilitates efficient compiler suites: m + n instead of m ∗ n • Different types, e.g., – syntax trees – three-address code: x = y op z • High-level vs. low-level • C for C++ High Level Low Level Source ✲ Target ✲ . . . ✲ ✲ Intermediate Intermediate Program Code Representation Representation 3
6.2 Three-Address Code • Linearized representation of syntax tree / syntax DAG • Sequence of instructions: x = y op z Example: a + a ∗ ( b − c ) + ( b − c ) ∗ d Syntax DAG Three-address code t1 = b - c + ✟ ❍❍❍❍ ✟ ✟ t2 = a * t1 ✟ ✟ ❍ + ∗ t3 = a * t2 ❅ � ❅ ❅ ❅ � ∗ d � t4 = t1 * d � � ❅ � ❅ � a − t5 = t3 + t4 � ❅ � ❅ c b 4
Addresses At most three addresses per instruction • Name: source program name / symbol-table entry • Constant • Compiler-generated temporary: distinct names 5
Three-Address Instructions 1 Assignment instructions x = y op z 2 Assignment instructions x = op y 3 Copy instructions x = y 4 Unconditional jumps goto L 5 Conditional jumps if x goto L / ifFalse x goto L 6 Conditional jumps if x relop y goto L / ifFalse . . . 7 Procedure calls and returns param x 1 param x 2 . . . param x n call p, n return y 8 Indexed copy instructions x = y [ i ] / x [ i ] = y 9 Address and pointer assignments x = & y , x = ∗ y , ∗ x = y Symbolic lable L represents index of instruction 6
Three-Address Instructions (Example) do i = i+1; while (a[i] < v); Syntax tree. . . Two examples of possible translations: Symbolic labels Position numbers L: t1 = i+1 100: t1 = i+1 i = t1 101: i = t1 t2 = i * 8 102: t2 = i * 8 t3 = a [ t2 ] 103: t3 = a [ t2 ] if t3 < v goto L 104: if t3 < v goto 100 7
Implementation of Three-Address Instructions Quadruples: records op , vararg 1 , vararg 2 , result Example: a = b * - c + b * - c Syntax tree. . . vararg 1 vararg 2 op result Three-address code 0 minus c t 1 t1 = minus c 1 ∗ b t 1 t 2 t2 = b * t1 2 minus c t 3 t3 = minus c 3 ∗ b t 3 t 4 t4 = b * t3 4 + t 2 t 4 t 5 t5 = t2 + t4 5 = t 5 a . . . a = t5 8
Implementation of Three-Address Instructions vararg 1 vararg 2 Three-address code op result 0 minus c t 1 t1 = minus c 1 ∗ b t 1 t 2 t2 = b * t1 2 minus c t 3 t3 = minus c 3 ∗ b t 3 t 4 t4 = b * t3 4 + t 2 t 4 t 5 t5 = t2 + t4 5 = t 5 a . . . a = t5 Exceptions 1. minus , = 2. param 3. jumps Field result mainly for temporaries. . . 9
Implementation of Three-Address Instructions Triples: records op , vararg 1 , vararg 2 Example: a = b * - c + b * - c Syntax tree. . . vararg 1 vararg 2 op Three-address code 0 minus c t1 = minus c 1 ∗ (0) b t2 = b * t1 2 minus c t3 = minus c 3 ∗ (2) b t4 = b * t3 4 + (1) (3) t5 = t2 + t4 5 = a t 5 . . . a = t5 10
Implementation of Three-Address Instructions vararg 1 vararg 2 op Three-address code 0 minus c t1 = minus c 1 ∗ (0) b t2 = b * t1 2 minus c t3 = minus c 3 ∗ (2) b t4 = b * t3 4 + (1) (3) t5 = t2 + t4 5 = a t 5 . . . a = t5 Equivalent to DAG Special case: x [ i ] = y or x = y [ i ] Pro: temporaries are implicit Con: difficult to rearrange code 11
Implementation of Three-Address Instructions Indirect triples: pointers to triples Example: a = b * - c + b * - c Syntax tree. . . vararg 1 vararg 2 op instruction Three-address code 0 minus 35 (0) c t1 = minus c 1 ∗ (0) 36 (1) b t2 = b * t1 37 (2) 2 minus c t3 = minus c 38 (3) 3 ∗ (2) b t4 = b * t3 39 (4) 4 + (1) (3) t5 = t2 + t4 40 (5) 5 = (4) a . . . a = t5 . . . 12
6.3.3 Declarations • Three-address code is simplistic It assumes that names of variables can be easily resolved by the back end in global or local variables • We need symbol tables to record global and local declarations in procedures, blocks, and structs to resolve names • Symbol table contains type and relative adress of names Example: → T id ; D | D ǫ record ′ { ′ D ′ } ′ T → B C | int float B → | [ num ] C C → ǫ | 13
Structure of Types (Example) record ′ { ′ D ′ } ′ T → B C | int float B → | → | [ num ] C C ǫ int[2][3] T ✏ PPPPPPPPPPP ✏ ✏ ✏ ✏ ✏ ✏ ✏ ✏ ✏ ✏ ✏ P B C ✟ ❍❍❍❍❍❍ ✟ ✟ ✟ ✟ ✟ ❍ [ 2 ] int C ✟ ❍❍❍❍❍❍ ✟ ✟ ✟ ✟ ✟ ❍ [ 3 ] C ǫ 14
Storage Layout at Compile Time • Storage comes in blocks of contiguous bytes • Width of type is number of bytes needed → { t = B. type ; w = B. width ; } T B { T. type = C. type ; T. width = C. width ; } C → int { B. type = integer ; B. width = 4; } B float B. type = float ; B. width = 8; } B → { C. type = t ; C. width = w ; } C → ǫ { [ num ] C 1 C. type = array ( num . value , C 1 . type ); C → { C. width = num . value × C 1 . width ; } 15
Types and Their Widths (Example) t = B. type ; w = B. width ; } T → B { { T. type = C. type ; T. width = C. width ; } C → int { B. type = integer ; B. width = 4; } B → float { B. type = float ; B. width = 8; } B → { C. type = t ; C. width = w ; } C ǫ → [ num ] C 1 { C. type = array ( num . value , C 1 . type ); C C. width = num . value × C 1 . width ; } type = array (2 , array (3 , integer )) ❍ ❨ width =24 ❍ . .................. . T . ❍ . . ❍ . . . . . . . type = array (2 , array (3 , integer )) ③ . . t . . . . ❍ ❨ width =24 ❍ B type = integer . C ............ w ❍ . . . ❍ . . . . width =4 . . . type = array (3 , integer ) . . . width =12 ❍ ❨ ❍ [ 2 ] int . C ............ ❍ . . ❍ . . . . . type = integer . . width =4 [ 3 ] C . . ✸ . . ǫ 16
Sequences of Declarations → T id ; D | D ǫ Use offset as next available address → { offset = 0; } P D → T id ; { top . put ( id . lexeme , T. type , offset ); D offset = offset + T. width ; } D 1 → D ǫ 17
Fields in Records and Classes Example float x; record { float x; float y; } p; record { int tag; float x; float y; } q; x = p.x + q.x; → T id ; D | D ǫ record ′ { ′ D ′ } ′ → T • Fields are specified by sequence of declarations – Field names within record must be distinct – Relative address for field is relative to data area for that record 18
Fields in Records and Classes Stored in separate symbol table t Record type has form record ( t ) record ′ { ′ → { Env . push ( top ); top = new Env (); T Stack . push ( offset ); offset = 0; } D ′ } ′ { T. type = record ( top ); T. width = offset ; top = Env . pop (); offset = Stack . pop (); } 19
6.4 Translation of Expressions • Temporary names are created E → E 1 + E 2 yields t = E 1 + E 2 , e.g., t5 = t2 + t4 a = t5 • If expression is identifier, then no new temporary • Nonterminal E has two attributes: – E. addr – address that will hold value of E – E. code – three-address code sequence 20
Syntax-Directed Definition To produce three-address code for assignments Production Semantic Rules → id = E ; S. code = E. code || S gen ( top . get ( id . lexeme ) ′ = ′ E. addr ) → E 1 + E 2 E. addr = new Temp () E E. code = E 1 . code || E 2 . code || gen ( E. addr ′ = ′ E 1 . addr ′ + ′ E 2 . addr ) E. addr = new Temp () | − E 1 E. code = E 1 . code || gen ( E. addr ′ = ′ ′ minus ′ E 1 . addr ) | ( E 1 ) E. addr = E 1 . addr E. code = E 1 . code | id E. addr = top . get ( id . lexeme ) ′′ E. code = 21
Translation scheme To incrementally produce three-address code for assignments gen ( top . get ( id . lexeme ) ′ = ′ E. addr ); } → id = E ; { S → E 1 + E 2 { E. addr = new Temp (); E gen ( E. addr ′ = ′ E 1 . addr ′ + ′ E 2 . addr ); } | − E 1 { E. addr = new Temp (); gen ( E. addr ′ = ′ ′ minus ′ E 1 . addr ); } | ( E 1 ) { E. addr = E 1 . addr ; } id E. addr = top . get ( id . lexeme ); } | { 22
Addressing Array Elements • Array A [ n ] with elements at positions 0 , 1 , . . . , n − 1 • Let – w be width of array element – base be relative address of storage allocated for A (= A [0]) Element A [ i ] begins in location base + i × w • In two dimensions, let – w 1 be width of row, – w 2 be width of element of row Element A [ i ][ j ] begins in location base + i × w 1 + j × w 2 • In k dimensions base + i 1 ∗ w 1 + i 2 ∗ w 2 + · · · + i k ∗ w k 23
Translation of Array References L generates array name followed by sequence of index expressions L → L [ E ] | id [ E ] Three synthesized attributes • L. addr : temporary used to compute location in array • L. array : pointer to symbol-table entry for array name – L. array . base : base address of array • L. type : type of subarray generated by L – For type t : t. width – For array type t : t. elem 24
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