[InstCombine] Set correct insertion point for selects generated while folding phis

Summary:
When we fold vector constants that are operands of phi's that feed into select,
we need to set the correct insertion point for the *new* selects that get generated.
The correct insertion point is the incoming block for the phi.
Such cases can occur with patch r298845, which fixed folding of
vector constants, but the new selects could be inserted incorrectly (as the added
test case shows).

Reviewers: majnemer, spatel, sanjoy

Reviewed by: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34162

llvm-svn: 305591

1 files changed, 11 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 65e6d2e3590..a2dd27d282a 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -939,9 +939,19 @@ Instruction *InstCombiner::foldOpIntoPhi(Instruction &I, PHINode *PN) {
       // `TrueVInPred`.
       if (InC && !isa<ConstantExpr>(InC) && isa<ConstantInt>(InC))
         InV = InC->isNullValue() ? FalseVInPred : TrueVInPred;
-      else
+      else {
+        // Generate the select in the same block as PN's current incoming block.
+        // Note: ThisBB need not be the NonConstBB because vector constants
+        // which are constants by definition are handled here.
+        // FIXME: This can lead to an increase in IR generation because we might
+        // generate selects for vector constant phi operand, that could not be
+        // folded to TrueVInPred or FalseVInPred as done for ConstantInt. For
+        // non-vector phis, this transformation was always profitable because
+        // the select would be generated exactly once in the NonConstBB.
+        Builder->SetInsertPoint(ThisBB->getTerminator());
         InV = Builder->CreateSelect(PN->getIncomingValue(i),
                                     TrueVInPred, FalseVInPred, "phitmp");
+      }
       NewPN->addIncoming(InV, ThisBB);
     }
   } else if (CmpInst *CI = dyn_cast<CmpInst>(&I)) {