[PATCH] [codegen] Don't emit implicit unreachable at the end of a non-empty block

[John McCall]

On Mar 16, 2013, at 10:14 AM, Argyrios Kyrtzidis <kyrtzidis at apple.com> wrote:
> The attached patch adds a check so that, if we flow off the end of a value-returning function, the implicit unreachable will only be emitted if the returning block is empty.
> For example:
> 
> int baz(int x) {
>  switch (x) {
>    case 1: return 10;
>    case 2: return 20;
>  }
>  // implicit unreachable is inserted
> }
> 
> int baz2(int x) {
>  switch (x) {
>    case 1: return 10;
>    case 2: return 20;
>  }
>  foo();
>  // no unreachable
> }
> 
> As a consequence, functions that have no returns, will not get the implicit unreachable:
> 
> int baz2(int x) {
>  foo();
>  // no unreachable
> }
> 
> 
> I believe this allows taking advantage of the optimization opportunities that the implicit unreachable intended, in a safer manner.

Hmm.  There's a pretty large spectrum of ways in which something could end up with a non-empty return block while still conceptually falling off the end in a way we'd like to optimize.  We were talking about maybe re-using the same logic that we'd use for -Wreturn-type;  what's the value in doing this intermediate stage?

Also, if we do decide to do this, please don't use empty();  that can lead to us generating semantically different code based on the presence of debug info.  I believe you can instead ask whether getFirstNonPHIOrDbgOrLifetime() returns null.

John.