[DA] Don't propagate from unreachable blocks

[kerbowa]

Summary

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

[rampitec]

LGTM

llvm/lib/Analysis/DivergenceAnalysis.cpp

bool DivergenceAnalysis::propagateJoinDivergence(const BasicBlock &JoinBlock,
   return false;
 }
 
 void DivergenceAnalysis::propagateBranchDivergence(const Instruction &Term) {
   LLVM_DEBUG(dbgs() << "propBranchDiv " << Term.getParent()->getName() << "\n");
 
   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)
   bool IsBranchLoopDivergent = false;
 
   // iterate over all blocks reachable by disjoint from Term within the loop
   // also iterates over loop exits that become divergent due to Term.
   for (const auto *JoinBlock : SDA.join_blocks(Term)) {

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

+; 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 }