http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46854
Michael Meissner <meissner at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2010.12.09 17:56:39
CC| |meissner at gcc dot gnu.org
Ever Confirmed|0 |1
--- Comment #2 from Michael Meissner <meissner at gcc dot gnu.org> 2010-12-09
17:56:39 UTC ---
Note, -O2 generates mostly the code you want, except that it looks the address
of the string twice:
Here is the code generated with a 4.4.4 based compiler (the compiler happens to
be the IBM advance toolchain, version 3.0-1) using -O2 -m32 (-O1/-O3 generate
the same code):
test:
mr. 0,3
mtctr 0
beq 0,.L10
lis 3,.lanch...@ha
la 3,.lanch...@l(3)
.p2align 4,,15
.L8:
lbzu 0,1(3)
cmpwi 7,0,0
bne 7,.L8
bdnz .L8
blr
.L10:
lis 3,.lanch...@ha
la 3,.lanch...@l(3)
blr
The SLES 11SP1 system compiler, which is based on GCC 4.3.4 generates the same
code.
However, the GCC 4.6 trunk seems to have regressed slightly with -O2 or -O3, in
that it does not track that the lbzu updates the pointer, but maintains its own
copy:
mr. 0,3
mtctr 0
beq- 0,.L5
lis 3,.lanch...@ha
la 3,.lanch...@l(3)
.L4:
mr 9,3
.L3:
lbzu 0,1(9)
addi 3,3,1
cmpwi 7,0,0
bne+ 7,.L3
bdnz .L4
blr
.L5:
lis 3,.lanch...@ha
la 3,.lanch...@l(3)
blr
Trunk with -Os does generate the two comparisons:
mr 9,3
lis 3,.lanch...@ha
la 3,.lanch...@l(3)
b .L2
.L5:
mr 11,3
addi 3,3,1
lbz 0,1(11)
cmpwi 7,0,0
bne+ 7,.L5
addi 9,9,-1
.L2:
cmpwi 7,9,0
bne+ 7,.L5
blr
So, there are two bugs in this. One that -Os generates larger code than -O2,
and the code regression for GCC 4.6.
