[DA] Don't propagate from unreachable blocks

Summary: Fixes crash that could occur when a divergent terminator has an unreachable parent.

Reviewers: rampitec, nhaehnle, arsenm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73323

lib/Analysis/DivergenceAnalysis.cpp

@@ -301,6 +301,10 @@ void DivergenceAnalysis::propagateBranchDivergence(const Instruction &Term) {
 
   markDivergent(Term);
 
+  // Don't propagate divergence from unreachable blocks.
+  if (!DT.isReachableFromEntry(Term.getParent()))
+    return;
+
   const auto *BranchLoop = LI.getLoopFor(Term.getParent());
 
   // whether there is a divergent loop exit from BranchLoop (if any)

test/Analysis/DivergenceAnalysis/AMDGPU/unreachable-loop-block.ll

@@ -0,0 +1,17 @@
+; RUN: opt %s -mtriple amdgcn-- -analyze -divergence -use-gpu-divergence-analysis | FileCheck %s
+
+; CHECK: DIVERGENT:  %tmp = cmpxchg volatile
+define amdgpu_kernel void @unreachable_loop(i32 %tidx) #0 {
+entry:
+  unreachable
+
+unreachable_loop:                                        ; preds = %do.body.i, %if.then11
+  %tmp = cmpxchg volatile i32 addrspace(1)* null, i32 0, i32 0 seq_cst seq_cst
+  %cmp.i = extractvalue { i32, i1 } %tmp, 1
+  br i1 %cmp.i, label %unreachable_loop, label %end
+
+end:                                      ; preds = %do.body.i51, %atomicAdd_g_f.exit
+  unreachable
+}
+
+attributes #0 = { norecurse nounwind }