[llvm] 3969d2c - [InstCombine] Disable select known bits fold for vectors

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2024-07-03T09:56:48+02:00
New Revision: 3969d2c3b5f42e4a180f5205efa780b0f950d733

URL: https://github.com/llvm/llvm-project/commit/3969d2c3b5f42e4a180f5205efa780b0f950d733
DIFF: https://github.com/llvm/llvm-project/commit/3969d2c3b5f42e4a180f5205efa780b0f950d733.diff

LOG: [InstCombine] Disable select known bits fold for vectors

This is not safe if the simplification ends up looking through
lane-crossing operations. For now, we don't have a good way to
limit this in computeKnownBits(), so just disable vector handling
entirely.

Fixes https://github.com/llvm/llvm-project/issues/97475.

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
    llvm/test/Transforms/InstCombine/select-binop-cmp.ll

Removed: 
    


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diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 736013395e8c2..394dfca262e13 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -4049,7 +4049,9 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
   if (CondVal->getType() == SI.getType() && isKnownInversion(FalseVal, TrueVal))
     return BinaryOperator::CreateXor(CondVal, FalseVal);
 
-  if (SelType->isIntOrIntVectorTy() &&
+  // For vectors, this transform is only safe if the simplification does not
+  // look through any lane-crossing operations. For now, limit to scalars only.
+  if (SelType->isIntegerTy() &&
       (!isa<Constant>(TrueVal) || !isa<Constant>(FalseVal))) {
     // Try to simplify select arms based on KnownBits implied by the condition.
     CondContext CC(CondVal);

diff  --git a/llvm/test/Transforms/InstCombine/select-binop-cmp.ll b/llvm/test/Transforms/InstCombine/select-binop-cmp.ll
index 9ee2bc57c3b87..1fa0c09a9e987 100644
--- a/llvm/test/Transforms/InstCombine/select-binop-cmp.ll
+++ b/llvm/test/Transforms/InstCombine/select-binop-cmp.ll
@@ -571,7 +571,10 @@ define <2 x i8> @select_xor_icmp_vec_bad(<2 x i8> %x, <2 x i8> %y, <2 x i8> %z)
 
 define <2 x i32> @vec_select_no_equivalence(<2 x i32> %x) {
 ; CHECK-LABEL: @vec_select_no_equivalence(
-; CHECK-NEXT:    ret <2 x i32> [[X:%.*]]
+; CHECK-NEXT:    [[X10:%.*]] = shufflevector <2 x i32> [[X:%.*]], <2 x i32> poison, <2 x i32> <i32 1, i32 0>
+; CHECK-NEXT:    [[COND:%.*]] = icmp eq <2 x i32> [[X]], zeroinitializer
+; CHECK-NEXT:    [[S:%.*]] = select <2 x i1> [[COND]], <2 x i32> [[X10]], <2 x i32> [[X]]
+; CHECK-NEXT:    ret <2 x i32> [[S]]
 ;
   %x10 = shufflevector <2 x i32> %x, <2 x i32> undef, <2 x i32> <i32 1, i32 0>
   %cond = icmp eq <2 x i32> %x, zeroinitializer