[PATCH] D42216: Use New Module Metadata String "AvoidPLT" to avoid calls via PLT

[Rafael Avila de Espindola via llvm-commits]

Rafael Avila de Espindola <rafael.espindola at gmail.com> writes:

> My memory in these areas is fuzzy. Do we never create a GV for a call to
> an intrinsic or do we lose it at some point in the codegen path? If it is
> lost, could we track it a bit longer?
>
> For this patch, is there any other case when GV is null? If that only
> happens when calling an intrinsic shouldAssumeDSOLocal can use that to
> know that it is a call.
>
> Maybe it can always return false if GV is null?
>
> There are 3 ways of calling a function:
>
> call foo
> call foo at plt
> call *foo at got
>
> if shouldAssumeDSOLocal returns true, we are supposed to use the first
> one. If it returns false, one of the last two.
>
> Now, "call "foo" is almost identical to "call foo at plt" as the linker can
> convert plt calls if the target is local. If the only case when GV is
> null is a call to an intrinsic, it is safe to return false.

I am experimenting with returning false when GV is null.

The first problem I noticed is coff. For coff we always assume that a
null GV is local, which is a bit surprising. I guess the compiler
runtime is always static on windows?

This indicates that TargetMachine::shouldAssumeDSOLocal will never go
away completely, but if we can limit to just test the file type and
only when GV is null that would still be a good improvement.

I will report in a minute on the result of the experiment.

Cheers,
Rafael