Index: llvm/trunk/lib/Transforms/Scalar/GVN.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/GVN.cpp
+++ llvm/trunk/lib/Transforms/Scalar/GVN.cpp
@@ -2255,6 +2255,20 @@
   Instruction *PREInstr = nullptr;
 
   if (NumWithout != 0) {
+    if (!isSafeToSpeculativelyExecute(CurInst)) {
+      // It is only valid to insert a new instruction if the current instruction
+      // is always executed. An instruction with implicit control flow could
+      // prevent us from doing it. If we cannot speculate the execution, then
+      // PRE should be prohibited.
+      auto It = FirstImplicitControlFlowInsts.find(CurrentBlock);
+      if (It != FirstImplicitControlFlowInsts.end()) {
+        assert(It->second->getParent() == CurrentBlock &&
+               "Implicit control flow map broken?");
+        if (OI->dominates(It->second, CurInst))
+          return false;
+      }
+    }
+
     // Don't do PRE across indirect branch.
     if (isa<IndirectBrInst>(PREPred->getTerminator()))
       return false;
Index: llvm/trunk/test/Transforms/GVN/PRE/local-pre.ll
===================================================================
--- llvm/trunk/test/Transforms/GVN/PRE/local-pre.ll
+++ llvm/trunk/test/Transforms/GVN/PRE/local-pre.ll
@@ -1,6 +1,13 @@
 ; RUN: opt < %s -gvn -enable-pre -S | FileCheck %s
 
+declare void @may_exit() nounwind
+
+declare void @may_exit_1(i32) nounwind
+
 define i32 @main(i32 %p, i32 %q) {
+
+; CHECK-LABEL: @main(
+
 block1:
     %cmp = icmp eq i32 %p, %q 
 	br i1 %cmp, label %block2, label %block3
@@ -20,3 +27,121 @@
 ; CHECK: %b.pre-phi = phi i32 [ %.pre, %block3 ], [ %a, %block2 ]
 ; CHECK-NEXT: ret i32 %b.pre-phi
 }
+
+; Don't PRE across implicit control flow.
+define i32 @test2(i32 %p, i32 %q) {
+
+; CHECK-LABEL: @test2
+; CHECK: block1:
+
+block1:
+  %cmp = icmp eq i32 %p, %q
+  br i1 %cmp, label %block2, label %block3
+
+block2:
+ %a = sdiv i32 %p, %q
+ br label %block4
+
+block3:
+  br label %block4
+
+; CHECK: block4:
+; CHECK-NEXT: call void @may_exit(
+; CHECK-NEXT: %b = sdiv
+; CHECK-NEXT: ret i32 %b
+
+block4:
+  call void @may_exit() nounwind
+  %b = sdiv i32 %p, %q
+  ret i32 %b
+}
+
+; Don't PRE across implicit control flow.
+define i32 @test3(i32 %p, i32 %q, i1 %r) {
+
+; CHECK-LABEL: @test3
+; CHECK: block1:
+
+block1:
+  br i1 %r, label %block2, label %block3
+
+block2:
+ %a = sdiv i32 %p, %q
+ br label %block4
+
+block3:
+  br label %block4
+
+block4:
+
+; CHECK: block4:
+; CHECK-NEXT: phi i32
+; CHECK-NEXT: call void @may_exit_1(
+; CHECK-NEXT: %b = sdiv
+; CHECK-NEXT: ret i32 %b
+
+  %phi = phi i32 [ 0, %block3 ], [ %a, %block2 ]
+  call void @may_exit_1(i32 %phi) nounwind
+  %b = sdiv i32 %p, %q
+  ret i32 %b
+
+}
+
+; It's OK to PRE an instruction that is guaranteed to be safe to execute
+; speculatively.
+; TODO: Does it make any sense in this case?
+define i32 @test4(i32 %p, i32 %q) {
+
+; CHECK-LABEL: @test4
+; CHECK: block1:
+
+block1:
+  %cmp = icmp eq i32 %p, %q
+  br i1 %cmp, label %block2, label %block3
+
+block2:
+ %a = sdiv i32 %p, 6
+ br label %block4
+
+block3:
+  br label %block4
+
+; CHECK: block4:
+; CHECK-NEXT: %b.pre-phi = phi i32
+; CHECK-NEXT: call void @may_exit(
+; CHECK-NEXT: ret i32 %b
+
+block4:
+  call void @may_exit() nounwind
+  %b = sdiv i32 %p, 6
+  ret i32 %b
+}
+
+; It is OK to PRE across implicit control flow if we don't insert new
+; instructions.
+define i32 @test5(i1 %cond, i32 %p, i32 %q) {
+
+; CHECK-LABEL: @test5
+; CHECK: block1:
+
+block1:
+  br i1 %cond, label %block2, label %block3
+
+block2:
+ %a = sdiv i32 %p, %q
+ br label %block4
+
+block3:
+  %b = sdiv i32 %p, %q
+  br label %block4
+
+; CHECK: block4:
+; CHECK-NEXT: %c.pre-phi = phi i32 [ %b, %block3 ], [ %a, %block2 ]
+; CHECK-NEXT: call void @may_exit()
+; CHECK-NEXT: ret i32 %c.pre-phi
+
+block4:
+  call void @may_exit() nounwind
+  %c = sdiv i32 %p, %q
+  ret i32 %c
+}