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

[Richard Smith via llvm-dev]

As I recall, the reason we chose to emit this via an intrinsic from Clang
rather than directly as profile information was to catch cases where the
builtin is *not* the immediate operand of a control flow construct, but
would be after some simple optimization. For instance:

  bool b = __builtin_expect(n > 5, true);
  if (b) { ... }
  // ...
  if (b) { ... }

or

  if (x && __builtin_expect(y, true)) { ... }

It sounds like subsequent changes to LLVM have already made most of these
cases not work, so some of the motivation for the current approach may have
evaporated. What optimizations do we still perform before lowering the
expect intrinsics?

On Fri, Apr 22, 2016 at 1:50 PM, Sanjay Patel via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> I, of course, thought the ~100 lines added by D19299 was a reasonable
> trade for the ~800 lines removed in D19300.
>
> David Li (and anyone else following along), do you still like those
> patches after hearing this objection or should I abandon?
>
>
> On Fri, Apr 22, 2016 at 11:55 AM, Reid Kleckner <rnk at google.com> wrote:
>
>> Sorry, I didn't realize that was the clang side.
>>
>> I think it's kind of ugly that the frontend has to pattern match and
>> lower this intrinsic in three places: if, switch, and general case. The
>> existing intrinsic is nice because it directly represents what the user can
>> write, and lets the optimizer decide how to best represent it. As David Li
>> mentioned, we may want to do value profiling in the future, and if we
>> remove this intrinsic, we will have to come and revisit this code.
>>
>> I think a single early pass to lower this intrinsic is a low cost to pay
>> for that simplicity.
>>
>> On Fri, Apr 22, 2016 at 10:39 AM, Sanjay Patel <spatel at rotateright.com>
>> wrote:
>>
>>> 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
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>>
>>>>
>>>
>>
>
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