[llvm-dev] InstructionSimplify: adding a hook for shufflevector instructions

[Rackover, Zvi via llvm-dev]

Thanks, Sanjay, that makes sense. The opportunity for improving instcombining splat sounds promising.

Another question about shuffle simplification. This is a testcase from test/Transforms/InstCombine/vec_shuffle.ll:

define <4 x i32> @test10(<4 x i32> %tmp5) nounwind {
   %tmp6 = shufflevector <4 x i32> %tmp5, <4 x i32> undef, <4 x i32> <i32 1, i32 undef, i32 undef, i32 undef>
   %tmp7 = shufflevector <4 x i32> %tmp6, <4 x i32> undef, <4 x i32> zeroinitializer
   ret <4 x i32> %tmp7
}

opt –instcombine will combine to:
define <4 x i32> @test10(<4 x i32> %tmp5) nounwind {
   %tmp7 = shufflevector <4 x i32> %tmp5, <4 x i32> undef, <4 x i32> <i32 1, i32 1, i32 1, i32 1>
   ret <4 x i32> %tmp7
}

Would it be ok to simplify the original function to the following?
define <4 x i32> @test10(<4 x i32> %tmp5) nounwind {
   %tmp7 = shufflevector <4 x i32> %tmp5, <4 x i32> undef, <4 x i32> <i32 1, i32 undef, i32 undef, i32 undef>
   ret <4 x i32> %tmp7
}

If the function is required to return a splat value, then I believe the answer is no, because the undef indices allow returning a value that is not a splat.

Thanks, Zvi
From: Sanjay Patel [mailto:spatel at rotateright.com]
Sent: Thursday, March 30, 2017 18:31
To: Rackover, Zvi <zvi.rackover at intel.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: InstructionSimplify: adding a hook for shufflevector instructions

My grasp of LLVM history isn't great, but I think these are missing because there wasn't much need for vector optimization in IR because there just weren't that many vector opportunities in IR. Ie, the vectorizers are relatively new, and hand-written vector code (eg, SSE intrinsics in source) generally went straight to the backend as target-specific IR intrinsics.

Now that we're vectorizing more aggressively (and plan to do even more) and we're converting target-specific vector source to generic vector IR whenever possible, it makes sense to add these kinds of optimizations.
One frequently visible sign of scalar privilege in instcombine is the use of "m_ConstantInt". In many cases, this can be converted to "m_APInt" without much effort, and the transform will auto-magically apply to splat vector constants too.



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