[llvm-commits] [llvm] r164866 - in /llvm/trunk: lib/Transforms/Scalar/CodeGenPrepare.cpp test/CodeGen/X86/2012-09-28-CGPBug.ll

[Rafael Espíndola]

>> - Turn the dead blockaddrs into undef. I think that could work but I'm not really sure about the implications.
>
> Overly aggressive, given that we've already made some minor
> concessions that blockaddr does something useful even without an
> associated indirectbr.

I think the main issue is that we have to cleanup the spec to say what
are the guarantees we provide and update the passes to match. Right
now the spec says one thing, but CodeGenPrepare.cpp and/or SimplifyCFG
don't agree with it.

Currently the spec says that the values is non-null and defines what
happens when used in indirectbr. Being strict about the non-null bit
is probably enough to say that it is illegal to replace it with undef
(as it can be 0). From your proposal I assume you would be OK with

* The blockaddress points somewhere in the current function.
* A unused (not just indirectbr-unused) one can be deleted.
* No guarantees are given about a indirectbr-unused label other than
it pointing to the current function.

BTW, was there any discussion/rationale for the current semantics when
indirectbr was introduced? Is it available somewhere?

> -Eli

Cheers,
Rafael