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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