optimization/7003: address of array loaded int register twice without any need

optimization/7003: address of array loaded int register twice without any need

[fshvaige]

>Number:         7003
>Category:       optimization
>Synopsis:       address of array loaded int register twice without any need
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          pessimizes-code
>Submitter-Id:   net
>Arrival-Date:   Wed Jun 12 05:26:01 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     fshvaige@cisco.com
>Release:        3.1 20020510 (prerelease)
>Organization:
>Environment:
../gcc-3.1/configure --with-gcc-version-trigger=/home/install/gcc-3.1/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 test3.c:

extern const char flags [256];

unsigned char * f (unsigned char * s) {
  while (flags[*++s]);
  while (!flags[*++s]);
  return s;
}

Produced code (PowerPC):

f:
  lis 4,flags@ha
  la 11,flags@l(4)
.L2:
  lbzu 5,1(3)
  lbzx 4,11,5
  cmpwi 0,4,0 
  bne+ 0,.L2 
  lis 6,flags@ha
  la 11,flags@l(6)
.L5:
  lbzu 8,1(3)
  lbzx 7,11,8
  cmpwi 0,7,0 
  beq+ 0,.L5 
  blr   

Note: address of 'flags' array loaded into register 11 twice. Why? Register 11 does not change from first load to second.
>How-To-Repeat:
ppc-eabi-gcc -S -O3 test3.c -Wall

See test3.s
>Fix:

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