[cfe-dev] [llvm-dev] Vector trunc code generation difference between llvm-3.9 and 4.0

[Saurabh Verma via cfe-dev]

Thanks Sanjay. Interestingly for me, disable-llvm-optmzns did not make a
difference in the way the shift was handled. Does the initial IR generated
for you show this difference when the option is passed?

Best regards
Saurabh


On 17 February 2017 at 19:03, Sanjay Patel <spatel at rotateright.com> wrote:

> I think this is caused by a front-end change (cc'ing clang-dev) because
> the IR with "-Xclang -disable-llvm-optzns" shows the difference.
>
> But independently of that, there's a missing IR canonicalization -
> instcombine doesn't currently do anything with either version.
>
> And the version where we trunc later survives through the backend and
> produces worse code even for x86 with AVX2:
> before:
>     vmovd    %edi, %xmm1
>     vpmovzxwq    %xmm1, %xmm1
>     vpsraw    %xmm1, %xmm0, %xmm0
>     retq
>
> after:
>     vmovd    %edi, %xmm1
>     vpbroadcastd    %xmm1, %ymm1
>     vmovdqa    LCPI1_0(%rip), %ymm2
>     vpshufb    %ymm2, %ymm1, %ymm1
>     vpermq    $232, %ymm1, %ymm1
>     vpmovzxwd    %xmm1, %ymm1
>     vpmovsxwd    %xmm0, %ymm0
>     vpsravd    %ymm1, %ymm0, %ymm0
>     vpshufb    %ymm2, %ymm0, %ymm0
>     vpermq    $232, %ymm0, %ymm0
>     vzeroupper
>
>
> So this example may have won the bug lottery by exposing all of front-,
> middle-, back-end bugs. :)
>
>
>
> On Fri, Feb 17, 2017 at 9:38 AM, Saurabh Verma via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Correction in the C snippet:
>>
>> typedef signed short v8i16_t   __attribute__((ext_vector_type(8)));
>>
>> v8i16_t foo (v8i16_t a, int n)
>> {
>>    return a >> n;
>> }
>>
>> Best regards
>> Saurabh
>>
>>
>>
>> On 17 February 2017 at 16:21, Saurabh Verma <saurabh.verma at movidius.com>
>> wrote:
>>
>>> Hello,
>>>
>>> We are investigating a difference in code generation for vector splat
>>> instructions between llvm-3.9 and llvm-4.0, which could lead to a
>>> performance regression for our target. Here is the C snippet
>>>
>>> typedef signed v8i16_t __attribute__((ext_vector_type(8)))
>>>
>>> v8i16_t foo (v8i16 a, int n)
>>> {
>>>    return result = a >> n;
>>> }
>>>
>>> With llvm-3.9, the generated sequence does a trunc followed by splat,
>>> but with llvm-4.0 it is reversed to a splat to a bigger vector followed by
>>> a v8i32->v8i16 trunc. Is this by design? The earlier code sequence is
>>> definitely better for our target, but are there known scenarios where the
>>> new sequence would lead to better code?
>>>
>>> Here are the instruction sequences generated in the two cases:
>>>
>>> With llvm 3.9:
>>>
>>> define <8 x i16> @foo(<8 x i16>, i32) #0 {
>>>   %3 = trunc i32 %1 to i16
>>>   %4 = insertelement <8 x i16> undef, i16 %3, i32 0
>>>   %5 = shufflevector <8 x i16> %4, <8 x i16> undef, <8 x i32>
>>> zeroinitializer
>>>   %6 = ashr <8 x i16> %0, %5
>>>   ret <8 x i16> %6
>>> }
>>>
>>>
>>> With llvm 4.0:
>>>
>>> define <8 x i16> @foo(<8 x i16>, i32) #0 {
>>>   %3 = insertelement <8 x i32> undef, i32 %1, i32 0
>>>   %4 = shufflevector <8 x i32> %3, <8 x i32> undef, <8 x i32>
>>> zeroinitializer
>>>   %5 = trunc <8 x i32> %4 to <8 x i16>
>>>   %6 = ashr <8 x i16> %0, %5
>>>   ret <8 x i16> %6
>>> }
>>>
>>> Best regards
>>> Saurabh Verma
>>>
>>
>>
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>>
>
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