[InstCombine] Teach SimplifyDemandedVectorElts how to handle ConstantVector select masks with ConstantExpr elements (PR24922)

If the mask of a select instruction is a ConstantVector, method
SimplifyDemandedVectorElts iterates over the mask elements to identify which
values are selected from the select inputs.

Before this patch, method SimplifyDemandedVectorElts always used method
Constant::isNullValue() to check if a value in the mask was zero. Unfortunately
that method always returns false when called on a ConstantExpr.

This patch fixes the problem in SimplifyDemandedVectorElts by adding an explicit
check for ConstantExpr values. Now, if a value in the mask is a ConstantExpr, we
avoid calling isNullValue() on it.

Fixes PR24922.

Differential Revision: http://reviews.llvm.org/D13219

llvm-svn: 249390

1 files changed, 7 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index a1fbda7..743d514 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -1057,7 +1057,13 @@ Value *InstCombiner::SimplifyDemandedVectorElts(Value *V, APInt DemandedElts,
     APInt LeftDemanded(DemandedElts), RightDemanded(DemandedElts);
     if (ConstantVector* CV = dyn_cast<ConstantVector>(I->getOperand(0))) {
       for (unsigned i = 0; i < VWidth; i++) {
-        if (CV->getAggregateElement(i)->isNullValue())
+        Constant *CElt = CV->getAggregateElement(i);
+        // Method isNullValue always returns false when called on a
+        // ConstantExpr. If CElt is a ConstantExpr then skip it in order to
+        // to avoid propagating incorrect information.
+        if (isa<ConstantExpr>(CElt))
+          continue;
+        if (CElt->isNullValue())
           LeftDemanded.clearBit(i);
         else
           RightDemanded.clearBit(i);