[LLVMdev] Does nounwind have semantics?

Andrew Trick atrick at apple.com
Sun Jul 21 17:56:32 PDT 2013

Does 'nounwind' have semantics that inform optimization passes? It seems to in some cases, but not consistently. For example...

int32_t foo(int32_t* ptr) {
  int i = 0;
  int result;
  do {
    result = *ptr;
  } while (i++ < *ptr);
  return result;

Say we have a front end that declares bar as...

declare void @bar(i32*) readonly;

So 'bar' is 'readonly' and 'may-unwind'.

When LICM tries to hoist the load it interprets the 'may-unwind' as "MayThrow" in LICM-language and bails. However, when it tries to sink the call itself it sees the 'readonly', assumes no side effects and sinks it below the loads. Hmm...

There doesn't appear to be a way to declare a function that is guaranteed not to write to memory in a way that affects the caller, but may have another well-defined side effect like aborting the program. This is interesting, because that is the way runtime checks for safe languages would like to be defined. I'm perfectly happy telling front ends to generate control flow for well-defined traps, since I like lots of basic blocks in my IR. But I'm still curious how others deal with this.


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