disabling loop unrolling in GCC

[Shachar Shemesh]

Hi all,

I'm trying, without success, to disable loop unrolling when compiling a 
program with -O3 with gcc (4.4, but I see the same problem with 4.3).

The program is the following one:
> volatile int v;
>
> void func()
> {
>     int i;
>
>     for( i=0; i<8; ++i ) {
>     v=0;
>     }
> }
I compile it with the following command line:

gcc -c -O3 test.c

An "objdump -S test.o" gives:
> test.o:     file format elf64-x86-64
>
> Disassembly of section .text:
>
> 0000000000000000 <func>:
>    0:   c7 05 00 00 00 00 00    movl   $0x0,0x0(%rip)        # a 
> <func+0xa>
>    7:   00 00 00
>    a:   c7 05 00 00 00 00 00    movl   $0x0,0x0(%rip)        # 14 
> <func+0x14>
>   11:   00 00 00
>   14:   c7 05 00 00 00 00 00    movl   $0x0,0x0(%rip)        # 1e 
> <func+0x1e>
>   1b:   00 00 00
>   1e:   c7 05 00 00 00 00 00    movl   $0x0,0x0(%rip)        # 28 
> <func+0x28>
>   25:   00 00 00
>   28:   c7 05 00 00 00 00 00    movl   $0x0,0x0(%rip)        # 32 
> <func+0x32>
>   2f:   00 00 00
>   32:   c7 05 00 00 00 00 00    movl   $0x0,0x0(%rip)        # 3c 
> <func+0x3c>
>   39:   00 00 00
>   3c:   c7 05 00 00 00 00 00    movl   $0x0,0x0(%rip)        # 46 
> <func+0x46>
>   43:   00 00 00
>   46:   c7 05 00 00 00 00 00    movl   $0x0,0x0(%rip)        # 50 
> <func+0x50>
>   4d:   00 00 00
>   50:   c3                      retq
If I compile with -O2, the results are:
> test.o:     file format elf64-x86-64
>
> Disassembly of section .text:
>
> 0000000000000000 <func>:
>    0:   31 c0                   xor    %eax,%eax
>    2:   66 0f 1f 44 00 00       nopw   0x0(%rax,%rax,1)
>    8:   83 c0 01                add    $0x1,%eax
>    b:   c7 05 00 00 00 00 00    movl   $0x0,0x0(%rip)        # 15 
> <func+0x15>
>   12:   00 00 00
>   15:   83 f8 08                cmp    $0x8,%eax
>   18:   75 ee                   jne    8 <func+0x8>
>   1a:   f3 c3                   repz retq
Where it gets worrying is when I try to cancel loop unrolling. I tried 
"-fno-unroll-loops" and "-fno-peel-loops", to no effect. I even tried 
messing with the --param option (max-unrolled-insns, max-unroll-times, 
max-peel-times) to no noticeable effect.

Even more worryingly, the documentation seems totally wrong. It claims 
(http://gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Optimize-Options.html#index-O3-632) 
that -O3 is equal to -O2 plus -finline-functions, -funswitch-loops, 
-fpredictive-commoning, -fgcse-after-reload and -ftree-vectorize. Trying 
to compile with -O2 and the additional optimization options does not, 
however, unroll the loop, which suggests that -O3 differs from -O2 in 
another way as well.

Help?

Shachar

-- 
Shachar Shemesh
Lingnu Open Source Consulting Ltd.
http://www.lingnu.com

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