Fix PR2967 by not deleting volatile load/stores that occur before unreachable.

I don't really see this as being needed, but there is little harm from doing
it.

llvm-svn: 58385

lib/Transforms/Utils/SimplifyCFG.cpp

@@ -1932,7 +1932,18 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
     while (Unreachable != BB->begin()) {
       BasicBlock::iterator BBI = Unreachable;
       --BBI;
+      // Do not delete instructions that can have side effects, like calls
+      // (which may never return) and volatile loads and stores.
       if (isa<CallInst>(BBI)) break;
+
+      if (StoreInst *SI = dyn_cast<StoreInst>(BBI))
+        if (SI->isVolatile())
+          break;
+
+      if (LoadInst *LI = dyn_cast<LoadInst>(BBI))
+        if (LI->isVolatile())
+          break;
+
       // Delete this instruction
       BB->getInstList().erase(BBI);
       Changed = true;

test/Transforms/SimplifyCFG/trapping-load-unreachable.ll

@@ -0,0 +1,19 @@
+; RUN: llvm-as < %s | opt -simplifycfg | llvm-dis | grep {volatile load}
+; PR2967
+
+target datalayout =
+"e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32"
+target triple = "i386-pc-linux-gnu"
+
+define void @foo(i32 %x) nounwind {
+entry:
+        %0 = icmp eq i32 %x, 0          ; <i1> [#uses=1]
+        br i1 %0, label %bb, label %return
+
+bb:             ; preds = %entry
+        %1 = volatile load i32* null            ; <i32> [#uses=0]
+        unreachable
+        br label %return
+return:         ; preds = %entry
+        ret void
+}