Index: llvm/trunk/include/llvm/IR/Instruction.h
===================================================================
--- llvm/trunk/include/llvm/IR/Instruction.h
+++ llvm/trunk/include/llvm/IR/Instruction.h
@@ -535,6 +535,14 @@
   /// matters, isSafeToSpeculativelyExecute may be more appropriate.
   bool mayHaveSideEffects() const { return mayWriteToMemory() || mayThrow(); }
 
+  /// Return true if the instruction can be removed if the result is unused.
+  ///
+  /// When constant folding some instructions cannot be removed even if their
+  /// results are unused. Specifically terminator instructions and calls that
+  /// may have side effects cannot be removed without semantically changing the
+  /// generated program.
+  bool isSafeToRemove() const;
+  
   /// Return true if the instruction is a variety of EH-block.
   bool isEHPad() const {
     switch (getOpcode()) {
Index: llvm/trunk/lib/IR/Instruction.cpp
===================================================================
--- llvm/trunk/lib/IR/Instruction.cpp
+++ llvm/trunk/lib/IR/Instruction.cpp
@@ -589,6 +589,11 @@
   return isa<ResumeInst>(this);
 }
 
+bool Instruction::isSafeToRemove() const {
+  return (!isa<CallInst>(this) || !this->mayHaveSideEffects()) &&
+         !isa<TerminatorInst>(this);
+}
+
 bool Instruction::isAssociative() const {
   unsigned Opcode = getOpcode();
   if (isAssociative(Opcode))
Index: llvm/trunk/lib/Transforms/Scalar/SCCP.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/SCCP.cpp
+++ llvm/trunk/lib/Transforms/Scalar/SCCP.cpp
@@ -1900,7 +1900,7 @@
         if (Inst->getType()->isVoidTy())
           continue;
         if (tryToReplaceWithConstant(Solver, Inst)) {
-          if (!isa<CallInst>(Inst) && !isa<TerminatorInst>(Inst))
+          if (Inst->isSafeToRemove())
             Inst->eraseFromParent();
           // Hey, we just changed something!
           MadeChanges = true;
Index: llvm/trunk/test/Transforms/IPConstantProp/remove-call-inst.ll
===================================================================
--- llvm/trunk/test/Transforms/IPConstantProp/remove-call-inst.ll
+++ llvm/trunk/test/Transforms/IPConstantProp/remove-call-inst.ll
@@ -0,0 +1,33 @@
+; RUN: opt < %s -S -ipsccp | FileCheck %s
+; PR5596
+
+; IPSCCP should propagate the 0 argument, eliminate the switch, and propagate
+; the result.
+
+; CHECK: define i32 @main() #0 {
+; CHECK-NEXT: entry:
+; CHECK-NOT: call
+; CHECK-NEXT: ret i32 123
+
+define i32 @main() noreturn nounwind {
+entry:
+  %call2 = tail call i32 @wwrite(i64 0) nounwind
+  ret i32 %call2
+}
+
+define internal i32 @wwrite(i64 %i) nounwind readnone {
+entry:
+  switch i64 %i, label %sw.default [
+    i64 3, label %return
+    i64 10, label %return
+  ]
+
+sw.default:
+  ret i32 123
+
+return:
+  ret i32 0
+}
+
+; CHECK: attributes #0 = { noreturn nounwind }
+; CHECK: attributes #1 = { nounwind readnone }
Index: llvm/trunk/test/Transforms/IPConstantProp/user-with-multiple-uses.ll
===================================================================
--- llvm/trunk/test/Transforms/IPConstantProp/user-with-multiple-uses.ll
+++ llvm/trunk/test/Transforms/IPConstantProp/user-with-multiple-uses.ll
@@ -15,7 +15,7 @@
   ret i32 %call2
 }
 
-define internal i32 @wwrite(i64 %i) nounwind readnone {
+define internal i32 @wwrite(i64 %i) nounwind {
 entry:
   switch i64 %i, label %sw.default [
     i64 3, label %return
@@ -30,5 +30,4 @@
 }
 
 ; CHECK: attributes #0 = { noreturn nounwind }
-; CHECK: attributes #1 = { nounwind readnone }
-; CHECK: attributes [[NUW]] = { nounwind }
+; CHECK: attributes #1 = { nounwind }