[cfe-dev] [llvm-dev] Vector trunc code generation difference between llvm-3.9 and 4.0
Sanjay Patel via cfe-dev
cfe-dev at lists.llvm.org
Thu Mar 9 07:26:29 PST 2017
Thanks, Akira.
I don't know enough about vectors in the front-end to be much use here.
cc'ing authors/reviewers of some of the patches that might be related:
https://reviews.llvm.org/rL284579
https://reviews.llvm.org/rL281669
https://reviews.llvm.org/rL278501
On Wed, Mar 8, 2017 at 8:28 PM, Akira Hatanaka <ahatanaka at apple.com> wrote:
> There were several patches (r278501 was the first) that fixed vector shift
> bugs. I don’t think the IR changes were intentional.
>
> I’m not sure if it’s the right solution, but inserting an integral cast
> before the CK_VectorSplat cast in checkVectorShift makes IRGen emit the
> trunc before the splat.
>
> On Mar 8, 2017, at 7:21 AM, Sanjay Patel via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> The regression for the reported case should be avoided after:
> https://reviews.llvm.org/rL297232
> https://reviews.llvm.org/rL297242
> https://reviews.llvm.org/rL297280
>
> It would still be good to understand if the clang change was intentional
> or if that was a side effect that can be limited.
>
> On Sat, Feb 18, 2017 at 9:11 AM, Sanjay Patel <spatel at rotateright.com>
> wrote:
>
>> Yes, there is an IR difference between clang 3.9.1 and clang trunk before
>> any IR transforms are done:
>> https://godbolt.org/g/FuBqIb
>>
>> We can't solve this problem (moving a trunc ahead of other vector ops) in
>> general in IR because we take a conservative approach to vector transforms
>> in IR. That means the burden for solving the general problem falls on the
>> front-end or the back-end. If you can bisect to find the clang commit where
>> this changed, that would be very helpful.
>>
>> However, I think we can handle a very specific case (a too fat splat) in
>> IR in instcombine, and it will resolve this exact example. This will take a
>> couple of patches to restore your example. Here's a proposal for the first
>> one:
>> https://reviews.llvm.org/D30123
>>
>>
>> On Sat, Feb 18, 2017 at 12:33 AM, Saurabh Verma <
>> saurabh.verma at movidius.com> wrote:
>>
>>> 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
>>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>>
>>>>
>>>
>>
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