[DivergenceAnalysis] Add methods for querying divergence at use

Summary:
The existing isDivergent(Value) methods query whether a value is
divergent at its definition. However even if a value is uniform at its
definition, a use of it in another basic block can be divergent because
of divergent control flow between the def and the use.

This patch adds new isDivergent(Use) methods to DivergenceAnalysis,
LegacyDivergenceAnalysis and GPUDivergenceAnalysis.

This might allow D63953 or other similar workarounds to be removed.

Reviewers: alex-t, nhaehnle, arsenm, rtaylor, rampitec, simoll, jingyue

Reviewed By: nhaehnle

Subscribers: jfb, jvesely, wdng, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65141

llvm-svn: 367218

1 files changed, 22 insertions, 7 deletions

diff --git a/llvm/lib/Analysis/LegacyDivergenceAnalysis.cpp b/llvm/lib/Analysis/LegacyDivergenceAnalysis.cpp
index 52212e1c42a..2fd8b8d6e1d 100644
--- a/llvm/lib/Analysis/LegacyDivergenceAnalysis.cpp
+++ b/llvm/lib/Analysis/LegacyDivergenceAnalysis.cpp
@@ -93,8 +93,9 @@ namespace {
 class DivergencePropagator {
 public:
   DivergencePropagator(Function &F, TargetTransformInfo &TTI, DominatorTree &DT,
-                       PostDominatorTree &PDT, DenseSet<const Value *> &DV)
-      : F(F), TTI(TTI), DT(DT), PDT(PDT), DV(DV) {}
+                       PostDominatorTree &PDT, DenseSet<const Value *> &DV,
+                       DenseSet<const Use *> &DU)
+      : F(F), TTI(TTI), DT(DT), PDT(PDT), DV(DV), DU(DU) {}
   void populateWithSourcesOfDivergence();
   void propagate();
 
@@ -118,11 +119,14 @@ private:
   PostDominatorTree &PDT;
   std::vector<Value *> Worklist; // Stack for DFS.
   DenseSet<const Value *> &DV;   // Stores all divergent values.
+  DenseSet<const Use *> &DU;   // Stores divergent uses of possibly uniform
+                               // values.
 };
 
 void DivergencePropagator::populateWithSourcesOfDivergence() {
   Worklist.clear();
   DV.clear();
+  DU.clear();
   for (auto &I : instructions(F)) {
     if (TTI.isSourceOfDivergence(&I)) {
       Worklist.push_back(&I);
@@ -197,8 +201,10 @@ void DivergencePropagator::exploreSyncDependency(Instruction *TI) {
   // dominators of TI until it is outside the influence region.
   BasicBlock *InfluencedBB = ThisBB;
   while (InfluenceRegion.count(InfluencedBB)) {
-    for (auto &I : *InfluencedBB)
-      findUsersOutsideInfluenceRegion(I, InfluenceRegion);
+    for (auto &I : *InfluencedBB) {
+      if (!DV.count(&I))
+        findUsersOutsideInfluenceRegion(I, InfluenceRegion);
+    }
     DomTreeNode *IDomNode = DT.getNode(InfluencedBB)->getIDom();
     if (IDomNode == nullptr)
       break;
@@ -208,9 +214,10 @@ void DivergencePropagator::exploreSyncDependency(Instruction *TI) {
 
 void DivergencePropagator::findUsersOutsideInfluenceRegion(
     Instruction &I, const DenseSet<BasicBlock *> &InfluenceRegion) {
-  for (User *U : I.users()) {
-    Instruction *UserInst = cast<Instruction>(U);
+  for (Use &Use : I.uses()) {
+    Instruction *UserInst = cast<Instruction>(Use.getUser());
     if (!InfluenceRegion.count(UserInst->getParent())) {
+      DU.insert(&Use);
       if (DV.insert(UserInst).second)
         Worklist.push_back(UserInst);
     }
@@ -320,6 +327,7 @@ bool LegacyDivergenceAnalysis::runOnFunction(Function &F) {
     return false;
 
   DivergentValues.clear();
+  DivergentUses.clear();
   gpuDA = nullptr;
 
   auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
@@ -332,7 +340,7 @@ bool LegacyDivergenceAnalysis::runOnFunction(Function &F) {
 
   } else {
     // run LLVM's existing DivergenceAnalysis
-    DivergencePropagator DP(F, TTI, DT, PDT, DivergentValues);
+    DivergencePropagator DP(F, TTI, DT, PDT, DivergentValues, DivergentUses);
     DP.populateWithSourcesOfDivergence();
     DP.propagate();
   }
@@ -351,6 +359,13 @@ bool LegacyDivergenceAnalysis::isDivergent(const Value *V) const {
   return DivergentValues.count(V);
 }
 
+bool LegacyDivergenceAnalysis::isDivergentUse(const Use *U) const {
+  if (gpuDA) {
+    return gpuDA->isDivergentUse(*U);
+  }
+  return DivergentValues.count(U->get()) || DivergentUses.count(U);
+}
+
 void LegacyDivergenceAnalysis::print(raw_ostream &OS, const Module *) const {
   if ((!gpuDA || !gpuDA->hasDivergence()) && DivergentValues.empty())
     return;