[cfe-dev] [llvm-dev] [RFC] remove the llvm.expect intrinsic

Reid Kleckner via cfe-dev cfe-dev at lists.llvm.org
Fri Apr 22 10:36:59 PDT 2016

Clang still appears to use llvm.expect. I think if you can show that it's
trivial to update clang with a patch, then yeah, moving back down to one
representation for this sounds reasonable.

On Fri, Apr 22, 2016 at 9:20 AM, Sanjay Patel via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I've proposed removing the llvm.expect intrinsic:
> http://reviews.llvm.org/D19300
> The motivation for this change is in:
> http://reviews.llvm.org/D19299
> For reference:
> 1. We created an intrinsic that's only reason for existing is to improve
> perf, but the intrinsic can harm optimization by interfering with
> transforms in other passes.
> 2. To solve that, we created a pass to always transform the intrinsic into
> metadata at a very early stage in LLVM. But now every program is paying a
> compile-time tax (running the LowerExpectIntrinsic pass) for a feature that
> is rarely used.
> A possible front-end replacement transformation for a source-level
> "builtin_expect()" is in D19299: I think a front-end can always directly
> create metadata for this kind of programmer hint rather than using an
> intermediate representation (the intrinsic). Likewise, if there's some
> out-of-tree IR pass that is creating an llvm.expect, it should be able to
> create branch weight metadata directly instead.
> Please let me know if you see any problems with this proposal or the
> patches.
> For reference, here's the original post-commit review thread for
> llvm.expect:
> https://marc.info/?l=llvm-commits&m=130997676129580&w=4
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