From 2c5adb18ba8739f7a9171fe9600714170947522b Mon Sep 17 00:00:00 2001 From: Sanjay Patel Date: Wed, 12 Apr 2017 18:39:53 +0000 Subject: [PATCH] [InstCombine] fix wrong undef handling when converting select to shuffle As discussed in: https://bugs.llvm.org/show_bug.cgi?id=32486 ...the canonicalization of vector select to shufflevector does not hold up when undef elements are present in the condition vector. Try to make the undef handling clear in the code and the LangRef. Differential Revision: https://reviews.llvm.org/D31980 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300092 91177308-0d34-0410-b5e6-96231b3b80d8 --- docs/LangRef.rst | 7 ++++--- .../InstCombine/InstCombineSelect.cpp | 6 ++++-- test/Transforms/InstCombine/select.ll | 17 ++++++++++++++--- 3 files changed, 22 insertions(+), 8 deletions(-) diff --git a/docs/LangRef.rst b/docs/LangRef.rst index 363847af0a8..b452ec03041 100644 --- a/docs/LangRef.rst +++ b/docs/LangRef.rst @@ -7071,9 +7071,10 @@ Semantics: The elements of the two input vectors are numbered from left to right across both of the vectors. The shuffle mask operand specifies, for each element of the result vector, which element of the two input vectors the -result element gets. The element selector may be undef (meaning "don't -care") and the second operand may be undef if performing a shuffle from -only one vector. +result element gets. If the shuffle mask is undef, the result vector is +undef. If any element of the mask operand is undef, that element of the +result is undef. If the shuffle mask selects an undef element from one +of the input vectors, the resulting element is undef. Example: """""""" diff --git a/lib/Transforms/InstCombine/InstCombineSelect.cpp b/lib/Transforms/InstCombine/InstCombineSelect.cpp index 84dace5db76..b7099bec380 100644 --- a/lib/Transforms/InstCombine/InstCombineSelect.cpp +++ b/lib/Transforms/InstCombine/InstCombineSelect.cpp @@ -1053,8 +1053,10 @@ static Instruction *canonicalizeSelectToShuffle(SelectInst &SI) { // If the select condition element is false, choose from the 2nd vector. Mask.push_back(ConstantInt::get(Int32Ty, i + NumElts)); } else if (isa(Elt)) { - // If the select condition element is undef, the shuffle mask is undef. - Mask.push_back(UndefValue::get(Int32Ty)); + // Undef in a select condition (choose one of the operands) does not mean + // the same thing as undef in a shuffle mask (any value is acceptable), so + // give up. + return nullptr; } else { // Bail out on a constant expression. return nullptr; diff --git a/test/Transforms/InstCombine/select.ll b/test/Transforms/InstCombine/select.ll index a1ca6999f86..c26380eaa71 100644 --- a/test/Transforms/InstCombine/select.ll +++ b/test/Transforms/InstCombine/select.ll @@ -1298,11 +1298,22 @@ define <2 x i32> @select_icmp_slt0_xor_vec(<2 x i32> %x) { ret <2 x i32> %x.xor } -; Make sure that undef elements of the select condition are translated into undef elements of the shuffle mask. - define <4 x i32> @canonicalize_to_shuffle(<4 x i32> %a, <4 x i32> %b) { ; CHECK-LABEL: @canonicalize_to_shuffle( -; CHECK-NEXT: [[SEL:%.*]] = shufflevector <4 x i32> %a, <4 x i32> %b, <4 x i32> +; CHECK-NEXT: [[SEL:%.*]] = shufflevector <4 x i32> %a, <4 x i32> %b, <4 x i32> +; CHECK-NEXT: ret <4 x i32> [[SEL]] +; + %sel = select <4 x i1> , <4 x i32> %a, <4 x i32> %b + ret <4 x i32> %sel +} + +; Undef elements of the select condition may not be translated into undef elements of a shuffle mask +; because undef in a shuffle mask means we can return anything, not just one of the selected values. +; https://bugs.llvm.org/show_bug.cgi?id=32486 + +define <4 x i32> @undef_elts_in_condition(<4 x i32> %a, <4 x i32> %b) { +; CHECK-LABEL: @undef_elts_in_condition( +; CHECK-NEXT: [[SEL:%.*]] = select <4 x i1> , <4 x i32> %a, <4 x i32> %b ; CHECK-NEXT: ret <4 x i32> [[SEL]] ; %sel = select <4 x i1> , <4 x i32> %a, <4 x i32> %b -- 2.40.0