[Bug target/38824] [4.4 Regression] performance regression of sse code from 4.2/4.3

["xuepeng dot guo at intel dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #17 from xuepeng dot guo at intel dot com  2009-02-09 09:16 -------
Below is a loop in the case in its original form(compiled by GCC 4.4):

_Z7bench_1PfS_fj:
.LFB2309:
        shrl    $2, %edx
        shufps  $0, %xmm0, %xmm0
        subl    $1, %edx
        xorl    %eax, %eax
        addq    $1, %rdx
        salq    $4, %rdx
        .p2align 4,,10
        .p2align 3
.L11:
        movaps  %xmm0, %xmm1       
        addps   (%rsi,%rax), %xmm1
        movaps  %xmm1, (%rdi,%rax)
        addq    $16, %rax
        cmpq    %rdx, %rax
        jne     .L11
        rep
        ret

The time is:

[xguo2@shgcc-10 38824]$ g++ 44.s -o orig.out
[xguo2@shgcc-10 38824]$ time ./orig.out

real    0m1.878s
user    0m1.877s
sys     0m0.000s
[xguo2@shgcc-10 38824]$ time ./orig.out

real    0m1.879s
user    0m1.879s
sys     0m0.001s
[xguo2@shgcc-10 38824]$ time ./orig.out

real    0m1.873s
user    0m1.872s
sys     0m0.001s

After adding two nop:

.L11:
        movaps  %xmm0, %xmm1
        nop
        nop
        addps   (%rsi,%rax), %xmm1
        movaps  %xmm1, (%rdi,%rax)
        addq    $16, %rax
        cmpq    %rdx, %rax
        jne     .L11
        rep
        ret

The time is:
[xguo2@shgcc-10 38824]$ g++ 44.s -o 2nop.out
[xguo2@shgcc-10 38824]$ time ./2nop.out

real    0m1.762s
user    0m1.762s
sys     0m0.000s
[xguo2@shgcc-10 38824]$ time ./2nop.out

real    0m1.762s
user    0m1.762s
sys     0m0.000s
[xguo2@shgcc-10 38824]$ time ./2nop.out

real    0m1.762s
user    0m1.761s
sys     0m0.000s

I suspect that the code layout maybe hurt the performance.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38824