[cfe-dev] clang 3.5 loop optimizer seems to jump in unintentional for simple loops

[Eric Christopher]

You'll want to carry on this discussion on llvmdev at cs.uiuc.edu rather
than cfe-dev. cfe-dev is largely concerned with the front end, llvmdev
is the back end and optimizers.

Thanks!

-eric

On Mon, Jul 21, 2014 at 10:04 PM, Dennis Luehring <dl.soluz at gmx.net> wrote:
> clang 3.5 loop optimizer seems to jump in unintentional for simple loops
>
> the very simple example
>
> ----
> const int SIZE = 3;
>
> int the_func(int* p_array)
> {
>   int dummy = 0;
> #if defined(ITER)
>   for(int* p = &p_array[0]; p < &p_array[SIZE]; ++p) dummy += *p;
> #else
>   for(int i = 0; i < SIZE; ++i) dummy += p_array[i];
> #endif
>   return dummy;
> }
>
> int main(int argc, char** argv)
> {
>   int* array = new int[SIZE];
>   for(int i = 0; i < SIZE; ++i){ array[i] = *argv[i]; }
>   int dummy = the_func(array);
>   delete[] array;
>   return dummy;
> }
> ----
>
> compiled with gcc 4.9.1 and clang 3.5
>
> clag -DITER -O2
> clag -DITER -O3
>
> gives:
>
> the_func:
>     leaq    12(%rdi), %rcx
>     leaq    4(%rdi), %rax
>     cmpq    %rax, %rcx
>     cmovaq    %rcx, %rax
>     movq    %rdi, %rsi
>     notq    %rsi
>     addq    %rax, %rsi
>     shrq    $2, %rsi
>     incq    %rsi
>     xorl    %edx, %edx
>     movabsq    $9223372036854775800, %rax # imm = 0x7FFFFFFFFFFFFFF8
>     andq    %rsi, %rax
>     pxor    %xmm0, %xmm0
>     je    .LBB0_1
> # BB#2:                                 # %vector.body.preheader
>     leaq    (%rdi,%rax,4), %r8
>     addq    $16, %rdi
>     movq    %rsi, %rdx
>     andq    $-8, %rdx
>     pxor    %xmm0, %xmm0
>     pxor    %xmm1, %xmm1
>     .align    16, 0x90
> .LBB0_3:                                # %vector.body
>                                         # =>This Inner Loop Header: Depth=1
>     movdqa    %xmm1, %xmm2
>     movdqa    %xmm0, %xmm3
>     movdqu    -16(%rdi), %xmm0
>     movdqu    (%rdi), %xmm1
>     paddd    %xmm3, %xmm0
>     paddd    %xmm2, %xmm1
>     addq    $32, %rdi
>     addq    $-8, %rdx
>     jne    .LBB0_3
> # BB#4:
>     movq    %r8, %rdi
>     movq    %rax, %rdx
>     jmp    .LBB0_5
> .LBB0_1:
>     pxor    %xmm1, %xmm1
> .LBB0_5:                                # %middle.block
>     paddd    %xmm1, %xmm0
>     movdqa    %xmm0, %xmm1
>     movhlps    %xmm1, %xmm1            # xmm1 = xmm1[1,1]
>     paddd    %xmm0, %xmm1
>     pshufd    $1, %xmm1, %xmm0        # xmm0 = xmm1[1,0,0,0]
>     paddd    %xmm1, %xmm0
>     movd    %xmm0, %eax
>     cmpq    %rdx, %rsi
>     je    .LBB0_7
>     .align    16, 0x90
> .LBB0_6:                                # %scalar.ph
>                                         # =>This Inner Loop Header: Depth=1
>     addl    (%rdi), %eax
>     addq    $4, %rdi
>     cmpq    %rcx, %rdi
>     jb    .LBB0_6
> .LBB0_7:                                # %._crit_edge
>     retq
>
> isn't that a little bit too long?
>
> other examples:
>
> clang -O2
> clang -O3
> gcc -O3
> gcc -DITER -O3
>
> gives:
>
> the_func:
>     movl    4(%rdi), %eax
>     addl    (%rdi), %eax
>     addl    8(%rdi), %eax
>     ret(q)
>
> looks good
>
> gcc -DITER -O2
>
> gives:
>
> the_func:
>     leaq    12(%rdi), %rdx
>     xorl    %eax, %eax
> .L2:
>     addl    (%rdi), %eax
>     addq    $4, %rdi
>     cmpq    %rdx, %rdi
>     jne    .L2
>     rep ret
>
> looks good
>
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