[llvm-dev] [lld] avoid emitting PLT entries for ifuncs

[Mark Johnston via llvm-dev]

On Tue, Aug 21, 2018 at 11:14:02PM +0200, Joerg Sonnenberger wrote:
> On Tue, Aug 21, 2018 at 09:47:41AM -0400, Mark Johnston via llvm-dev wrote:
> > We've recently started using ifuncs in the x86(_64) FreeBSD kernel.
> > Currently lld will emit a PLT entry for each ifunc, so ifunc calls are
> > more expensive that those of regular functions.
> 
> If you rewrite the PLT entry to be a plain jump whenever possible, the
> difference should be pretty small. Have you considered that?

I considered it, but don't like it as much: for each supported CPU
architecture we would need code to find the PLT entry referencing the
GOT entry being relocated, verify that the entry contains the
instruction(s) that we expect, and write the plain jump.  For my
approach the kernel linker will just do the right thing for each CPU
architecture without requiring any magic.  Having support in the static
linker means that the optimization is less fragile, and we do not incur
the cost of the extra jump.  It may be that other projects can benefit
as well, and as I mentioned, I think there are other use-cases for
similar functionality.