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 <shac...@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
>
> _______________________________________________
> Linux-il mailing list
> Linux-il@cs.huji.ac.il
> http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
>
>
--
Joan Crawford - "I, Joan Crawford, I believe in the dollar.
Everything I earn, I spend." -
http://www.brainyquote.com/quotes/authors/j/joan_crawford.html
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