[SimplifyDemandedVec] Strengthen handling all undef lanes (particularly GEPs)

A change of two parts:
1) A generic enhancement for all callers of SDVE to exploit the fact that if all lanes are undef, the result is undef.
2) A GEP specific piece to strengthen/fix the vector index undef element handling, and call into the generic infrastructure when visiting the GEP.

The result is that we replace a vector gep with at least one undef in each lane with a undef.  We can also do the same for vector intrinsics.  Once the masked.load patch (D57372) has landed, I'll update to include call tests as well.

Differential Revision: https://reviews.llvm.org/D57468

llvm-svn: 356293

1 files changed, 13 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index d5cc7c5bf7e..3462149a002 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1557,6 +1557,19 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
   if (Value *V = SimplifyGEPInst(GEPEltType, Ops, SQ.getWithInstruction(&GEP)))
     return replaceInstUsesWith(GEP, V);
 
+  // For vector geps, use the generic demanded vector support.
+  if (GEP.getType()->isVectorTy()) {
+    auto VWidth = GEP.getType()->getVectorNumElements();
+    APInt UndefElts(VWidth, 0);
+    APInt AllOnesEltMask(APInt::getAllOnesValue(VWidth));
+    if (Value *V = SimplifyDemandedVectorElts(&GEP, AllOnesEltMask,
+                                              UndefElts)) {
+      if (V != &GEP)
+        return replaceInstUsesWith(GEP, V);
+      return &GEP;
+    }
+  }
+
   Value *PtrOp = GEP.getOperand(0);
 
   // Eliminate unneeded casts for indices, and replace indices which displace