[Bug rtl-optimization/46854] PowerPC optimization regression

[joakim.tjernlund at transmode dot se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46854

--- Comment #3 from joakim.tjernlund at transmode dot se <joakim.tjernlund at transmode dot se> 2010-12-09 18:21:50 UTC ---
(In reply to comment #2)
> 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,.LANCHOR0@ha
>         la 3,.LANCHOR0@l(3)
>         .p2align 4,,15
> .L8:
>         lbzu 0,1(3)
>         cmpwi 7,0,0
>         bne 7,.L8
>         bdnz .L8
>         blr
> .L10:
>         lis 3,.LANCHOR0@ha
>         la 3,.LANCHOR0@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

I have seen more similar mistakes such as not using lwzu/stwu at all. Will
add a copy of a mail I sent earlier about that.

> copy:
> 
>         mr. 0,3
>         mtctr 0
>         beq- 0,.L5
>         lis 3,.LANCHOR0@ha
>         la 3,.LANCHOR0@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,.LANCHOR0@ha
>         la 3,.LANCHOR0@l(3)
>         blr
> 
> Trunk with -Os does generate the two comparisons:
> 
>         mr 9,3
>         lis 3,.LANCHOR0@ha
>         la 3,.LANCHOR0@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.

And gcc 4.4.4/4.4.5. I suspect this started much earlier though.