Saurabh Verma via llvm-dev
2017-Feb-18 07:33 UTC
[llvm-dev] Vector trunc code generation difference between llvm-3.9 and 4.0
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 >> >> >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170218/c360a10b/attachment.html>
Sanjay Patel via llvm-dev
2017-Feb-18 16:11 UTC
[llvm-dev] Vector trunc code generation difference between llvm-3.9 and 4.0
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 >>> >>> >> >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170218/18284a9a/attachment.html>
Sanjay Patel via llvm-dev
2017-Mar-08 15:21 UTC
[llvm-dev] Vector trunc code generation difference between llvm-3.9 and 4.0
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 >>>> >>>> >>> >> >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170308/eb62cb6a/attachment.html>