[llvm-dev] RFC: Atomic LL/SC loops in LLVM revisited

[Tim Northover via llvm-dev]

On Thu, 14 Jun 2018 at 13:45, Alex Bradbury <asb at lowrisc.org> wrote:
> Oh I see what you're saying, it's the fact that by bypassing
> instruction selection we're missing cases where an ADDI could be
> selected rather than an ADD, which would potentially free up a
> register and save the instruction generated for materialising the
> constant.

Yes.

> I don't like to see the compiler generate code that's
> obviously dumber than what a human would write, but in this case do we
> really think there would be any sort of measurable impact on
> performance?

It's certainly going to be marginal, but then so is the benefit of
late expansion.

There's also the barrier that this genuinely is a place where people
are willing to hand-code assembly, and go rooting around in the
compiler's output to check we're doing what they expect even if they
don't use assembly. The assembly is possibly mostly for historical
reasons, but it's still out there and we want to convince people to
move away from it.

I think I'm going to try to implement that verifier pass you mentioned
and run it across an iOS build.

Cheers.

Tim.