[PATCH] D81806: [DivergenceAnalysis] mark join of divergent loop exits

[Sameer Sahasrabuddhe via Phabricator via llvm-commits]

sameerds created this revision.
Herald added subscribers: llvm-commits, kerbowa, hiraditya, nhaehnle, jvesely.
Herald added a project: LLVM.

A block that is reachable along disjoint exits of a loop is also
divergent if any of the exits reaching that block is divergent. This
update was missed due to an early exit.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D81806

Files:
  llvm/lib/Analysis/DivergenceAnalysis.cpp
  llvm/test/Analysis/DivergenceAnalysis/AMDGPU/join-at-loop-exit.ll


Index: llvm/test/Analysis/DivergenceAnalysis/AMDGPU/join-at-loop-exit.ll
===================================================================
--- /dev/null
+++ llvm/test/Analysis/DivergenceAnalysis/AMDGPU/join-at-loop-exit.ll
@@ -0,0 +1,36 @@
+; RUN: opt -mtriple amdgcn-unknown-amdhsa -analyze -divergence -use-gpu-divergence-analysis %s | FileCheck %s
+
+; CHECK: bb3:
+; CHECK: DIVERGENT:       %Guard.bb4 = phi i1 [ true, %bb1 ], [ false, %bb2 ]
+; CHECK: DIVERGENT:       br i1 %Guard.bb4, label %bb4, label %bb5
+
+; Function Attrs: nounwind readnone speculatable
+declare i32 @llvm.amdgcn.workitem.id.x() #0
+
+define protected amdgpu_kernel void @test() {
+bb0:
+  %tid.x = call i32 @llvm.amdgcn.workitem.id.x()
+  %i5 = icmp eq i32 %tid.x, -1
+  br label %bb1
+
+bb1:                                              ; preds = %bb2, %bb0
+  %lsr.iv = phi i32 [ 7, %bb0 ], [ %lsr.iv.next, %bb2 ]
+  br i1 %i5, label %bb2, label %bb3
+
+bb2:                                              ; preds = %bb1
+  %lsr.iv.next = add nsw i32 %lsr.iv, -1
+  %i14 = icmp eq i32 %lsr.iv.next, 0
+  br i1 %i14, label %bb3, label %bb1
+
+bb3:                                              ; preds = %bb2, %bb1
+  %Guard.bb4 = phi i1 [ true, %bb1 ], [ false, %bb2 ]
+  br i1 %Guard.bb4, label %bb4, label %bb5
+
+bb4:                                              ; preds = %bb3
+  br label %bb5
+
+bb5:                                              ; preds = %bb3, %bb4
+  ret void
+}
+
+attributes #0 = { nounwind readnone speculatable }
Index: llvm/lib/Analysis/DivergenceAnalysis.cpp
===================================================================
--- llvm/lib/Analysis/DivergenceAnalysis.cpp
+++ llvm/lib/Analysis/DivergenceAnalysis.cpp
@@ -286,14 +286,11 @@
   // push non-divergent phi nodes in JoinBlock to the worklist
   pushPHINodes(JoinBlock);
 
-  // JoinBlock is a divergent loop exit
-  if (BranchLoop && !BranchLoop->contains(&JoinBlock)) {
-    return true;
-  }
-
-  // disjoint-paths divergent at JoinBlock
+  // disjoint-paths join at JoinBlock
   markBlockJoinDivergent(JoinBlock);
-  return false;
+
+  // JoinBlock is a divergent loop exit
+  return BranchLoop && !BranchLoop->contains(&JoinBlock);
 }
 
 void DivergenceAnalysis::propagateBranchDivergence(const Instruction &Term) {


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