[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

diff --git a/docs/LangRef.rst b/docs/LangRef.rst
index 363847af0a8a877311fe5bae506076d906c827cd..b452ec03041317185393009da022af3c920585c5 100644 (file)
--- 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 84dace5db760073dfbc439bc8f38cd1db458cf20..b7099bec3807a4391b2a776adcf36f49f1e41e2f 100644 (file)
--- 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<UndefValue>(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 a1ca6999f865fffa7d4745c344e500ad009307eb..c26380eaa71be3216b60b09948c8c7bd3849d9f1 100644 (file)
--- 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> <i32 0, i32 undef, i32 6, i32 undef>
+; CHECK-NEXT:    [[SEL:%.*]] = shufflevector <4 x i32> %a, <4 x i32> %b, <4 x i32> <i32 0, i32 5, i32 6, i32 3>
+; CHECK-NEXT:    ret <4 x i32> [[SEL]]
+;
+  %sel = select <4 x i1> <i1 true, i1 false, i1 false, i1 true>, <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> <i1 true, i1 undef, i1 false, i1 undef>, <4 x i32> %a, <4 x i32> %b
 ; CHECK-NEXT:    ret <4 x i32> [[SEL]]
 ;
   %sel = select <4 x i1> <i1 true, i1 undef, i1 false, i1 undef>, <4 x i32> %a, <4 x i32> %b