Ch8b

Intermediate Code Generation Part II Chapter 6 (1 st ed Chapter 8) COP5621 Compiler Construction Copyright Robert van Engelen, Florida State University, 2007-2009

Advanced Intermediate Code Generation Techniques Reusing temporary names Addressing array elements Translating logical and relational expressions Translating short-circuit Boolean expressions and flow-of-control statements with backpatching lists Translating procedure calls

Reusing Temporary Names Evaluate E 1 into t1 Evaluate E 2 into t2 t3 := t1 + t2 E 1 + E 2 Modify newtemp () to use a “stack”: Keep a counter c , initialized to 0 Decrement counter on each use of a $ i in a three-address statement newtemp () returns temporary $ c++ (that is, c is post incremented ) If t1 no longer used, can reuse t1 instead of using new temp t3 generate

Reusing Temporary Names (cont’d) x := a * b + c * d - e * f $ 0 := a * b $ 1 := c * d $ 0 := $0 + $1 $ 1 := e * f $ 0 := $0 - $1 x := $0 0 0 -> 0 -> 1 1 -> 1 -> 2 2 -> 0 -> 1 1 -> 1 -> 2 2 -> 0 -> 1 1 -> 0 -> 0 c -> c decr -> c incr Statement

Addressing Array Elements: One-Dimensional Arrays = base A + ( i - low ) * w = i * w + c where c = base A - low * w with low = 10; w = 4 A : array [10..20] of integer; … := A[i] t1 := c // c = base A - 10 * 4 t2 := i * 4 t3 := t1[t2] … := t3

Addressing Array Elements: Multi-Dimensional Arrays A : array [1..2,1..3] of integer; low 1 = 1, low 2 = 1, n 1 = 2 , n 2 = 3 , w = 4 A[1,1] A[1,2] A[1,3] A[2,1] A[2,2] A[2,3] Row-major A[1,1] A[2,1] A[1,2] A[2,2] A[1,3] A[2,3] Column-major base A base A

Addressing Array Elements: Multi-Dimensional Arrays = base A + (( i 1 - low 1 ) * n 2 + i 2 - low 2 ) * w = (( i 1 * n 2 ) + i 2 ) * w + c where c = base A - (( low 1 * n 2 ) + low 2 ) * w with low 1 = 1; low 2 = 1; n 2 = 3; w = 4 A : array [1..2,1..3] of integer; (Row-major) … := A[i,j] t1 := i * 3 t1 := t1 + j t2 := c // c = base A - (1 * 3 + 1) * 4 t3 := t1 * 4 t4 := t2[t3] … := t4

Addressing Array Elements: Grammar S  L := E E  E + E | ( E ) | L L  Elist ] | id Elist  Elist , E | id [ E Synthesized attributes: E .place name of temp holding value of E Elist .array array name Elist .place name of temp holding index value Elist .ndim number of array dimensions L .place lvalue (=name of temp) L .offset index into array (=name of temp) null indicates non-array simple id

Addressing Array Elements S  L := E { if L .offset = null then emit ( L .place ‘:=’ E .place) else emit ( L .place[ L .offset] ‘:=’ E .place) } E  E 1 + E 2 { E .place := newtemp (); emit ( E .place ‘:=’ E 1 .place ‘+’ E 2 .place) } E  ( E 1 ) { E .place := E 1 .place } E  L { if L .offset = null then E .place := L .place else E .place := newtemp (); emit ( E .place ‘:=’ L .place[ L .offset] }

Addressing Array Elements L  Elist ] { L .place := newtemp (); L .offset := newtemp (); emit ( L .place ‘:=’ c ( Elist .array); emit ( L .offset ‘:=’ Elist .place ‘*’ width ( Elist .array)) } L  id { L .place := id .place; L .offset := null } Elist  Elist 1 , E { t := newtemp (); m := Elist 1 .ndim + 1; emit ( t ‘:=’ Elist 1 .place ‘*’ limit ( Elist 1 .array, m )); emit ( t ‘:=’ t ‘+’ E .place); Elist .array := Elist 1 .array; Elist .place := t ; Elist .ndim := m } Elist  id [ E { Elist .array := id .place; Elist .place := E .place; Elist .ndim := 1 }

Translating Logical and Relational Expressions a or b and not c t1 := not c t2 := b and t1 t3 := a or t2 if a < b goto L1 t1 := 0 goto L2 L1: t1 := 1 L2: a < b

Backpatch Operations with Lists makelist ( i ) creates a new list containing three-address location i , returns a pointer to the list merge ( p 1 , p 2 ) concatenates lists pointed to by p 1 and p 2 , returns a pointer to the concatenates list backpatch ( p , i ) inserts i as the target label for each of the statements in the list pointed to by p

Backpatching with Lists: Example a < b or c < d and e < f 100: if a < b goto _ 101: goto _ 102: if c < d goto _ 103: goto _ 104: if e < f goto _ 105: goto _ 100: if a < b goto TRUE 101: goto 102 102: if c < d goto 104 103: goto FALSE 104: if e < f goto TRUE 105: goto FALSE backpatch

Backpatching with Lists: Translation Scheme M   { M .quad := nextquad () } E  E 1 or M E 2 { backpatch ( E 1 .falselist, M .quad); E .truelist := merge ( E 1 .truelist, E 2 .truelist); E .falselist := E 2 .falselist } E  E 1 and M E 2 { backpatch ( E 1 .truelist, M .quad); E .truelist := E 2 .truelist; E .falselist := merge ( E 1 .falselist, E 2 .falselist); } E  not E 1 { E .truelist := E 1 .falselist; E .falselist := E 1 .truelist } E  ( E 1 ) { E .truelist := E 1 .truelist; E .falselist := E 1 .falselist }

Backpatching with Lists: Translation Scheme (cont’d) E  id 1 relop id 2 { E .truelist := makelist ( nextquad ()); E .falselist := makelist ( nextquad () + 1); emit (‘ if ’ id 1 .place relop .op id 2 .place ‘ goto _ ’); emit (‘ goto _ ’) } E  true { E .truelist := makelist ( nextquad ()); E .falselist := nil; emit (‘ goto _ ’) } E  false { E .falselist := makelist ( nextquad ()); E .truelist := nil; emit (‘ goto _ ’) }

Flow-of-Control Statements and Backpatching: Grammar S  if E then S | if E then S else S | while E do S | begin L end | A L  L ; S | S Synthesized attributes: S .nextlist backpatch list for jumps to the next statement after S (or nil) L .nextlist backpatch list for jumps to the next statement after L (or nil) S 1 ; S 2 ; S 3 ; S 4 ; S 5 … backpatch ( S 1 .nextlist, 200) backpatch ( S 2 .nextlist, 300) backpatch ( S 3 .nextlist, 400) backpatch ( S 4 .nextlist, 500) 100: Code for S1 200: Code for S2 300: Code for S3 400: Code for S4 500: Code for S5 Jumps out of S 1

Flow-of-Control Statements and Backpatching S  A { S .nextlist := nil } S  begin L end { S .nextlist := L .nextlist } S  if E then M S 1 { backpatch ( E .truelist, M .quad); S .nextlist := merge ( E .falselist, S 1 .nextlist) } L  L 1 ; M S { backpatch ( L 1 .nextlist, M .quad); L .nextlist := S .nextlist; } L  S { L .nextlist := S .nextlist; } M   { M .quad := nextquad () } A  … Non-compound statements, e.g. assignment, function call

Flow-of-Control Statements and Backpatching (cont’d) S  if E then M 1 S 1 N else M 2 S 2 { backpatch ( E .truelist, M 1 .quad); backpatch ( E .falselist, M 2 .quad); S .nextlist := merge ( S 1 .nextlist, merge ( N .nextlist, S 2 .nextlist)) } S  while M 1 E do M 2 S 1 { backpatch ( S 1 ,nextlist, M 1 .quad); backpatch ( E .truelist, M 2 .quad); S .nextlist := E .falselist; emit (‘ goto M 1 .quad’) } N   { N .nextlist := makelist ( nextquad ()); emit (‘ goto _ ’) }

Translating Procedure Calls S  call id ( Elist ) Elist  Elist , E | E call foo(a+1, b, 7) t1 := a + 1 t2 := 7 param t1 param b param t2 call foo 3

Translating Procedure Calls S  call id ( Elist ) { for each item p on queue do emit (‘ param ’ p ); emit (‘ call ’ id .place | queue |) } Elist  Elist , E { append E .place to the end of queue } Elist  E { initialize queue to contain only E .place }

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