Demanded elements support for vector GEPs

GEPs can produce either scalar or vector results. If we're extracting only a subset of the vector lanes, simplifying the operands is helpful in eliminating redundant computation, and (eventually) allowing further optimizations

Differential Revision: https://reviews.llvm.org/D57177

llvm-svn: 352440

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index ab25c1c171a..b2863b0c865 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -1183,6 +1183,18 @@ Value *InstCombiner::SimplifyDemandedVectorElts(Value *V, APInt DemandedElts,
   switch (I->getOpcode()) {
   default: break;
 
+  case Instruction::GetElementPtr: {
+    // Conservatively track the demanded elements back through any vector
+    // operands we may have.  We know there must be at least one, or we
+    // wouldn't have a vector result to get here. Note that we intentionally
+    // merge the undef bits here since gepping with either an undef base or
+    // index results in undef. 
+    for (unsigned i = 0; i < I->getNumOperands(); i++)
+      if (I->getOperand(i)->getType()->isVectorTy())
+        simplifyAndSetOp(I, i, DemandedElts, UndefElts);
+
+    break;
+  }
   case Instruction::InsertElement: {
     // If this is a variable index, we don't know which element it overwrites.
     // demand exactly the same input as we produce.