[LV] Fix PR34523 - avoid generating redundant selects

When converting a PHI into a series of 'select' instructions to combine the
incoming values together according their edge masks, initialize the first
value to the incoming value In0 of the first predecessor, instead of
generating a redundant assignment 'select(Cond[0], In0, In0)'. The latter
fails when the Cond[0] mask is null, representing a full mask, which can
happen only when there's a single incoming value.

No functional changes intended nor expected other than surviving null Cond[0]'s.

This fix follows D35725, which introduced using null to represent full masks.

Differential Revision: https://reviews.llvm.org/D37619

llvm-svn: 313119

1 files changed, 3 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index 5267a2a3f19..487a7fb72f0 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -4526,10 +4526,10 @@ void InnerLoopVectorizer::widenPHIInstruction(Instruction *PN, unsigned UF,
 
       for (unsigned Part = 0; Part < UF; ++Part) {
         Value *In0 = getOrCreateVectorValue(P->getIncomingValue(In), Part);
-        // We might have single edge PHIs (blocks) - use an identity
-        // 'select' for the first PHI operand.
+        assert((Cond[Part] || NumIncoming == 1) &&
+               "Multiple predecessors with one predecessor having a full mask");
         if (In == 0)
-          Entry[Part] = Builder.CreateSelect(Cond[Part], In0, In0);
+          Entry[Part] = In0; // Initialize with the first incoming value.
         else
           // Select between the current value and the previous incoming edge
           // based on the incoming mask.