[LLVMdev] SIMD for sdiv <2 x i64>

Philip Reames
Fri, 24 Jul 2015 17:16:09 -0000

On 07/24/2015 03:42 AM, Benjamin Kramer wrote:
>> On 24.07.2015, at 08:06, zhi chen <zchenhn at gmail.com> wrote:
>>
>> It seems that that it's hard to vectorize int64 in LLVM. For
example, LLVM 3.4 generates very complicated code for the following IR. I am
running on a Haswell processor. Is it because there is no alternative AVX/2
instructions for int64? The same thing also happens to zext <2 x i32>
-> <2 x i64> and trunc <2 x i64> -> <2 x i32>. Any ideas
to optimize these instructions? Thanks.
>>
>> %sub.ptr.sub.i6.i.i.i.i = sub <2 x i64>
%sub.ptr.lhs.cast.i4.i.i.i.i, %sub.ptr.rhs.cast.i5.i.i.i.i
>> %sub.ptr.div.i7.i.i.i.i = sdiv <2 x i64> %sub.ptr.sub.i6.i.i.i.i,
<i64 24, i64 24>
>>
>> Assembly:
>>      vpsubq  %xmm6, %xmm5, %xmm5
>>      vmovq   %xmm5, %rax
>>      movabsq $3074457345618258603, %rbx # imm = 0x2AAAAAAAAAAAAAAB
>>      imulq   %rbx
>>      movq    %rdx, %rcx
>>      movq    %rcx, %rax
>>      shrq    $63, %rax
>>      shrq    $2, %rcx
>>      addl    %eax, %ecx
>>      vpextrq $1, %xmm5, %rax
>>      imulq   %rbx
>>      movq    %rdx, %rax
>>      shrq    $63, %rax
>>      shrq    $2, %rdx
>>      addl    %eax, %edx
>>      movslq  %edx, %rax
>>      vmovq   %rax, %xmm5
>>      movslq  %ecx, %rax
>>      vmovq   %rax, %xmm6
>>      vpunpcklqdq %xmm5, %xmm6, %xmm5 # xmm5 = xmm6[0],xmm5[0]
> AVX2 doesn't have integer vector division instructions and LLVM lowers
divides by constants into (128 bit) multiplies. However, AVX2 doesn't have a
way to get to the upper 64 bits of a 64x64->128 bit multiply either, so LLVM
uses the scalar imulq instruction to do that. There's not much room to
optimize here given the limitations of AVX2.
>
> You seem to be subtracting pointers though, so if you can guarantee that
the pointers are aligned you could set the exact bit on your 'sdiv'
instruction. That should give better code.
Depending on what you're using the result of the divide for, there might
be optimizations which could be applied as well.  Can you give a
slightly larger context for your source IR?  (1-2 level of uses/defs out
from the instructions would help)
>
> - Ben
>
>
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