The absence of noreturn doesn't ensure mayReturn

There are two separate issues:
- LLVM doesn't consider infinite loops to be side effects: we happily
  hoist/sink above/below loops whose bounds are unknown.
- The absence of the noreturn attribute is insufficient for us to know
  if a function will definitely return.  Relying on noreturn in the
  middle-end for any property is an accident waiting to happen.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273762 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/Instruction.h

@@ -394,21 +394,13 @@ class Instruction : public User,
   /// Return true if this instruction may throw an exception.
   bool mayThrow() const;
 
-  /// Return true if this is a function that may return.
-  /// This is true for all normal instructions. The only exception
-  /// is functions that are marked with the 'noreturn' attribute.
-  ///
-  bool mayReturn() const;
-
   /// Return true if the instruction may have side effects.
   ///
   /// Note that this does not consider malloc and alloca to have side
   /// effects because the newly allocated memory is completely invisible to
   /// instructions which don't use the returned value.  For cases where this
   /// matters, isSafeToSpeculativelyExecute may be more appropriate.
-  bool mayHaveSideEffects() const {
-    return mayWriteToMemory() || mayThrow() || !mayReturn();
-  }
+  bool mayHaveSideEffects() const { return mayWriteToMemory() || mayThrow(); }
 
   /// Return true if the instruction is a variety of EH-block.
   bool isEHPad() const {

lib/IR/Instruction.cpp

@@ -545,12 +545,6 @@ bool Instruction::mayThrow() const {
   return isa<ResumeInst>(this);
 }
 
-bool Instruction::mayReturn() const {
-  if (const CallInst *CI = dyn_cast<CallInst>(this))
-    return !CI->doesNotReturn();
-  return true;
-}
-
 /// isAssociative - Return true if the instruction is associative:
 ///
 ///   Associative operators satisfy:  x op (y op z) === (x op y) op z