[LV] Process pointer IVs with PHINodes in collectLoopUniforms

This patch moves the processing of pointer induction variables in
collectLoopUniforms from the consecutive pointer phase of the analysis to the
phi node phase. Previously, if a pointer induction variable was used by both a
scalarized non-memory instruction as well as a vectorized memory instruction,
we would incorrectly identify the pointer as uniform. Pointer induction
variables should be treated the same as other phi nodes. That is, they are
uniform if all users of the induction variable and induction variable update
are uniform.

Differential Revision: https://reviews.llvm.org/D24511

llvm-svn: 281485

1 files changed, 22 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index a08c8cc145a..0b368d1df1d 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -5393,9 +5393,18 @@ void LoopVectorizationLegality::collectLoopUniforms() {
       if (!Ptr)
         continue;
 
+      // True if all users of Ptr are memory accesses that have Ptr as their
+      // pointer operand.
+      auto UsersAreMemAccesses = all_of(Ptr->users(), [&](User *U) -> bool {
+        return getPointerOperand(U) == Ptr;
+      });
+
       // Ensure the memory instruction will not be scalarized, making its
-      // pointer operand non-uniform.
-      if (memoryInstructionMustBeScalarized(&I))
+      // pointer operand non-uniform. If the pointer operand is used by some
+      // instruction other than a memory access, we're not going to check if
+      // that other instruction may be scalarized here. Thus, conservatively
+      // assume the pointer operand may be non-uniform.
+      if (!UsersAreMemAccesses || memoryInstructionMustBeScalarized(&I))
         PossibleNonUniformPtrs.insert(Ptr);
 
       // If the memory instruction will be vectorized and its pointer operand
@@ -5433,11 +5442,18 @@ void LoopVectorizationLegality::collectLoopUniforms() {
     }
   }
 
+  // Returns true if Ptr is the pointer operand of a memory access instruction
+  // I, and I is known to not require scalarization.
+  auto isVectorizedMemAccessUse = [&](Instruction *I, Value *Ptr) -> bool {
+    return getPointerOperand(I) == Ptr && !memoryInstructionMustBeScalarized(I);
+  };
+
   // For an instruction to be added into Worklist above, all its users inside
   // the loop should also be in Worklist. However, this condition cannot be
   // true for phi nodes that form a cyclic dependence. We must process phi
   // nodes separately. An induction variable will remain uniform if all users
   // of the induction variable and induction variable update remain uniform.
+  // The code below handles both pointer and non-pointer induction variables.
   for (auto &Induction : Inductions) {
     auto *Ind = Induction.first;
     auto *IndUpdate = cast<Instruction>(Ind->getIncomingValueForBlock(Latch));
@@ -5446,7 +5462,8 @@ void LoopVectorizationLegality::collectLoopUniforms() {
     // vectorization.
     auto UniformInd = all_of(Ind->users(), [&](User *U) -> bool {
       auto *I = cast<Instruction>(U);
-      return I == IndUpdate || !TheLoop->contains(I) || Worklist.count(I);
+      return I == IndUpdate || !TheLoop->contains(I) || Worklist.count(I) ||
+             isVectorizedMemAccessUse(I, Ind);
     });
     if (!UniformInd)
       continue;
@@ -5455,7 +5472,8 @@ void LoopVectorizationLegality::collectLoopUniforms() {
     // uniform after vectorization.
     auto UniformIndUpdate = all_of(IndUpdate->users(), [&](User *U) -> bool {
       auto *I = cast<Instruction>(U);
-      return I == Ind || !TheLoop->contains(I) || Worklist.count(I);
+      return I == Ind || !TheLoop->contains(I) || Worklist.count(I) ||
+             isVectorizedMemAccessUse(I, IndUpdate);
     });
     if (!UniformIndUpdate)
       continue;