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Date: Tue, 11 Jun 2002 05:06:00 -0000
From: fshvaige@cisco.com
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Subject: optimization/6990: -O do better optimization than -O3/-O2/-Os on jump-to-jump for PowerPC
X-SW-Source: 2002-06/txt/msg00238.txt.bz2
List-Id: <gcc-prs.sourceware.org>


>Number:         6990
>Category:       optimization
>Synopsis:       -O do better optimization than -O3/-O2/-Os on jump-to-jump for PowerPC
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          pessimizes-code
>Submitter-Id:   net
>Arrival-Date:   Tue Jun 11 05:06:01 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     fshvaige@cisco.com
>Release:        3.1 20020510
>Organization:
>Environment:
--with-gcc-version-trigger=/home/install/gcc-3.1-20020510/gcc/version.c --host=i686-pc-linux-gnu --with-newlib --enable-target-optspace --target=ppc-eabi --prefix=/home/crossGCC/ppc-eabi --with-local-prefix=/home/crossGCC/ppc-eabi --program-prefix=ppc-eabi- --enable-languages=c,c++ -v --norecursion
>Description:
File memmove.c with function memmove():

void *memmove (void *dst, const void *src, unsigned n) {
  void * tmp = dst;
  char * d = (char*)dst;
  char * s = (char*)src;
  if (n) { 
    if (d > s) {
      d += n;           
      s += n;           
      l1: //do-while loop bug in gcc 3.1 #6984
        * --d = * --s;          
      if (--n) goto l1; 
    } else {    
      --d;              
      --s;              
      l2: //do-while loop bug in gcc 3.1 #6984
        * ++d = * ++s;          
      if (--n) goto l2; 
    }           
  }     
  return tmp;
}

Compilation with -O produces code:

memmove:
  mtctr 5
  cmpwi 0,5,0 
  beqlr- 0
  cmplw 0,3,4 
  ble- 0,.L3 
  add 9,3,5 
  add 4,4,5 
.L4:
  lbzu 0,-1(4)
  stbu 0,-1(9)
  bdnz .L4
  blr   
.L3:
  addi 9,3,-1
  addi 4,4,-1
.L7:
  lbzu 0,1(4)
  stbu 0,1(9)
  bdnz .L7
  blr

Compilation with -O3 produces code:

memmove:
  cmpwi 0,5,0 
  mtctr 5
  beqlr- 0
  cmplw 0,3,4 
  ble- 0,.L3 
  add 9,3,5 
  add 4,4,5 
.L4:
  lbzu 5,-1(4)
  stbu 5,-1(9)
  bdnz .L4
.L2:
  blr   
.L3:
  addi 9,3,-1
  addi 4,4,-1
.L7:
  lbzu 5,1(4)
  stbu 5,1(9)
  bdnz .L7
  b .L2 

Note last instruction 'b .L2' - it is jump to another jump 'blr' after label '.L2'.
-O managed to produce 'blr' for last instruction.
>How-To-Repeat:
Command lines looks like:
ppc-eabi-gcc -S -O memmove.c -Wall
ppc-eabi-gcc -S -O3 memmove.c -Wall
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted: