disabling loop unrolling in GCC

[Aviv Greenberg]

Just out of curiousity: why do you care about the resulting assembly?
It's a strong indication that you are doing something wrong :)

I would try to set i to volatile or to an extern to trick the compiler
to drop the optimization (if the flags don't work).

--Aviv

2009/12/21 Shachar Shemesh <shachar at shemesh.biz>:
> 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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> Linux-il at cs.huji.ac.il
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>
>



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