[InstSimplify] Fold insertelement vec, poison, idx into vec

[aqjune]

Summary

This is a simple patch that adds folding from insertelement vec, poison, idx into vec.

Alive2 proof: https://alive2.llvm.org/ce/z/2y2vbC

[nikic]

llvm/lib/Analysis/InstructionSimplify.cpp

4402

You can also switch this to use isGuaranteedNotToBePoison() to be more precise.

[aqjune]

Address comment

[aqjune]

llvm/lib/Analysis/InstructionSimplify.cpp

4402

Made D94053 to make the function distinguish constant vector with undef elements but not poison. :)

[nikic]

It looks like there are some test failures.

[aqjune]

Oh yes, I'll look into them

[aqjune]

Resolve failures
A few of those was regressions. It was because ConstantVector::get(Arr) was unable to understand Poison array. I updated those.

If the diffs look okay, I'll split this into two commits (one with the ConstantVector and another one with the InstSimplify) when landing.

[nikic]

LGTM. Nice catch on the ConstantVector issue.

[aqjune]

llvm/test/Transforms/SLPVectorizer/X86/alternate-int.ll

428 ↗

(On Diff #315027)

I think something is happening here, I imagine some transformation is passing undef to SimplifyInsertElementInst instead of poison (because sdiv _, undef is simplified to poison) I will look into this later.

llvm/lib/Analysis/InstructionSimplify.cpp

 //===- InstructionSimplify.cpp - Fold instruction operands ----------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
if (isa<FixedVectorType>(Vec->getType()) &&
         CI->uge(cast<FixedVectorType>(Vec->getType())->getNumElements()))
       return PoisonValue::get(Vec->getType());
   }
 
   // If index is undef, it might be out of bounds (see above case)
   if (Q.isUndefValue(Idx))
     return PoisonValue::get(Vec->getType());
 
-  // If the scalar is undef, and there is no risk of propagating poison from the
-  // vector value, simplify to the vector value.
-  if (Q.isUndefValue(Val) &&
-      isGuaranteedNotToBeUndefOrPoison(Vec))
+  // If the scalar is poison, or it is undef and there is no risk of
+  // propagating poison from the vector value, simplify to the vector value.
+  if (isa<PoisonValue>(Val) ||
+      (Q.isUndefValue(Val) && isGuaranteedNotToBeUndefOrPoison(Vec)))
     return Vec;
 
   // If we are extracting a value from a vector, then inserting it into the same
   // place, that's the input vector:
   // insertelt Vec, (extractelt Vec, Idx), Idx --> Vec
   if (match(Val, m_ExtractElt(m_Specific(Vec), m_Specific(Idx))))
     return Vec;
 

llvm/test/Transforms/InstSimplify/insertelement.ll

 ; CHECK-NEXT:    ret <4 x i32> poison
 ;
   %I = insertelement <4 x i32> %A, i32 5, i64 poison
   ret <4 x i32> %I
 }
 
 define <4 x i32> @elem_poison(<4 x i32> %A) {
 ; CHECK-LABEL: @elem_poison(
-; CHECK-NEXT:    [[B:%.*]] = insertelement <4 x i32> [[A:%.*]], i32 poison, i32 1
-; CHECK-NEXT:    ret <4 x i32> [[B]]
+; CHECK-NEXT:    ret <4 x i32> [[A:%.*]]
 ;
   %B = insertelement <4 x i32> %A, i32 poison, i32 1
   ret <4 x i32> %B
 }
 
 ; The undef may be replacing a poison value, so it is not safe to just return 'A'.
 
 define <4 x i32> @PR1286(<4 x i32> %A) {