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

Sanjay Patel via cfe-dev cfe-dev at lists.llvm.org
Fri Apr 22 10:39:54 PDT 2016

Hi Reid -

The intent of D19299 is to remove all Clang refs to llvm.expect. Do you see
any holes after applying that patch?

On Fri, Apr 22, 2016 at 11:36 AM, Reid Kleckner <rnk at google.com> wrote:

> 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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>> llvm-dev at lists.llvm.org
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