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Currently undef is used as a don’t-care vector when constructing a vector using a series of insertelement. However, this is problematic because undef isn’t undefined enough. Especially, a sequence of insertelement can be optimized to shufflevector, but using undef as its placeholder makes shufflevector a poison-blocking instruction because undef cannot be optimized to poison. This makes a few straightforward optimizations incorrect, such as: ``` ; https://bugs.llvm.org/show_bug.cgi?id=44185 define <4 x float> @insert_not_undef_shuffle_translate_commute(float %x, <4 x float> %y, <4 x float> %q) { %xv = insertelement <4 x float> %q, float %x, i32 2 %r = shufflevector <4 x float> %y, <4 x float> %xv, <4 x i32> { 0, 6, 2, undef } ret <4 x float> %r ; %r[3] is undef } => define <4 x float> @insert_not_undef_shuffle_translate_commute(float %x, <4 x float> %y, <4 x float> %q) { %r = insertelement <4 x float> %y, float %x, i32 1 ret <4 x float> %r ; %r[3] = %y[3], incorrect if %y[3] = poison } Transformation doesn't verify! ERROR: Target is more poisonous than source ``` I’d like to suggest 1. Using poison as insertelement’s placeholder value (IRBuilder::CreateVectorSplat should be patched too) 2. Updating shufflevector’s semantics to return poison element if mask is undef Note that poison is currently lowered into UNDEF in SelDag, so codegen part is okay. m_Undef() matches PoisonValue as well, so existing optimizations will still fire. The only concern is hidden miscompilations that will go incorrect when poison constant is given. A conservative way is copying all tests having `insertelement undef` & replacing it with `insertelement poison` & run Alive2 on it, but it will create many tests and people won’t like it. :( Instead, I’ll simply locally maintain the tests and run Alive2. If there is any bug found, I’ll report it. Relevant links: https://bugs.llvm.org/show_bug.cgi?id=43958 , http://lists.llvm.org/pipermail/llvm-dev/2019-November/137242.html Reviewed By: nikic Differential Revision: https://reviews.llvm.org/D93586
35 lines
1.6 KiB
LLVM
35 lines
1.6 KiB
LLVM
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
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; RUN: opt < %s -instcombine -S | grep "insertelement <4 x float> poison"
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; Instcombine should be able to prove that none of the
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; insertelement's first operand's elements are needed.
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define internal void @""(i8*) {
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; <label>:1
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bitcast i8* %0 to i32* ; <i32*>:2 [#uses=1]
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load i32, i32* %2, align 1 ; <i32>:3 [#uses=1]
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getelementptr i8, i8* %0, i32 4 ; <i8*>:4 [#uses=1]
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bitcast i8* %4 to i32* ; <i32*>:5 [#uses=1]
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load i32, i32* %5, align 1 ; <i32>:6 [#uses=1]
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br label %7
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; <label>:7 ; preds = %9, %1
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%.01 = phi <4 x float> [ undef, %1 ], [ %12, %9 ] ; <<4 x float>> [#uses=1]
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%.0 = phi i32 [ %3, %1 ], [ %15, %9 ] ; <i32> [#uses=3]
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icmp slt i32 %.0, %6 ; <i1>:8 [#uses=1]
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br i1 %8, label %9, label %16
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; <label>:9 ; preds = %7
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sitofp i32 %.0 to float ; <float>:10 [#uses=1]
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insertelement <4 x float> %.01, float %10, i32 0 ; <<4 x float>>:11 [#uses=1]
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shufflevector <4 x float> %11, <4 x float> undef, <4 x i32> zeroinitializer ; <<4 x float>>:12 [#uses=2]
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getelementptr i8, i8* %0, i32 48 ; <i8*>:13 [#uses=1]
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bitcast i8* %13 to <4 x float>* ; <<4 x float>*>:14 [#uses=1]
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store <4 x float> %12, <4 x float>* %14, align 16
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add i32 %.0, 2 ; <i32>:15 [#uses=1]
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br label %7
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; <label>:16 ; preds = %7
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ret void
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}
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