[SafeStack] Handle unreachable code with safe stack coloring.

[efriedma]

Summary

Instead of asserting that the function doesn't have any unreachable code, just ignore it for the purpose of computing liveness.

llvm/trunk/lib/CodeGen/SafeStackColoring.cpp

while (changed) {
 
     for (BasicBlock *BB : depth_first(&F)) {
       BlockLifetimeInfo &BlockInfo = BlockLiveness[BB];
 
       // Compute LiveIn by unioning together the LiveOut sets of all preds.
       BitVector LocalLiveIn;
       for (auto *PredBB : predecessors(BB)) {
         LivenessMap::const_iterator I = BlockLiveness.find(PredBB);
-        assert(I != BlockLiveness.end() && "Predecessor not found");
+        // If a predecessor is unreachable, ignore it.
+        if (I == BlockLiveness.end())
+          continue;
         LocalLiveIn |= I->second.LiveOut;
       }
 
       // Compute LiveOut by subtracting out lifetimes that end in this
       // block, then adding in lifetimes that begin in this block.  If
       // we have both BEGIN and END markers in the same basic block
       // then we know that the BEGIN marker comes after the END,
       // because we already handle the case where the BEGIN comes

llvm/trunk/test/Transforms/SafeStack/AArch64/unreachable.ll

+; RUN: opt -safe-stack -safe-stack-coloring -S -mtriple=aarch64-linux-android < %s -o - | FileCheck %s
+
+define void @foo() nounwind uwtable safestack {
+entry:
+; CHECK: %[[TP:.*]] = call i8* @llvm.thread.pointer()
+; CHECK: %[[SPA0:.*]] = getelementptr i8, i8* %[[TP]], i32 72
+; CHECK: %[[SPA:.*]] = bitcast i8* %[[SPA0]] to i8**
+; CHECK: %[[USP:.*]] = load i8*, i8** %[[SPA]]
+; CHECK: %[[USST:.*]] = getelementptr i8, i8* %[[USP]], i32 -16
+; CHECK: store i8* %[[USST]], i8** %[[SPA]]
+
+  %a = alloca i8, align 8
+  br label %ret
+
+ret:
+  ret void
+
+dead:
+  call void @Capture(i8* %a)
+  br label %ret
+}
+
+declare void @Capture(i8*)