[LLVMdev] SCCP and undef branches

[Chris Lattner]

On Wed, 7 Jun 2006, Domagoj Babic wrote:
> Here's something I don't understand... How come that UNDEF can
> appear as a branch condition at all? I just can't think of any ways.

Lots of different ways, e.g.:

fun() {
   _Bool C;
   if (C) { ... }
}

> Second, if it already can appear, isn't that a bug that should be
> reported by the compiler?

That's up to the front-end.  If you turn on enough warnings you should get 
one.

At the compiler level, they can occur due to unrealizable code paths (not 
necessarily bugs in the user code), so it's useful for the optimizer to 
take advantage of it.

-Chris

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