optimization/6990: -O do better optimization than -O3/-O2/-Os on jump-to-jump for PowerPC
fshvaige@cisco.com
fshvaige@cisco.com
Tue Jun 11 07:02:00 GMT 2002
>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:
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