[PATCH] D35384: PGOInstrumentation: Move profile matching warnings to remarks

[David Blaikie via llvm-commits]

Ping (Chandler and/or David)

On Fri, Jul 14, 2017 at 7:15 PM David Blaikie <dblaikie at gmail.com> wrote:

> On Thu, Jul 13, 2017 at 10:53 PM Xinliang David Li <davidxl at google.com>
> wrote:
>
>> On Thu, Jul 13, 2017 at 10:41 PM, Chandler Carruth <chandlerc at google.com>
>> wrote:
>>
>>> On Thu, Jul 13, 2017 at 7:29 PM Xinliang David Li via llvm-commits <
>>> llvm-commits at lists.llvm.org> wrote:
>>>
>>>> On Thu, Jul 13, 2017 at 4:17 PM, David Blaikie <dblaikie at gmail.com>
>>>> wrote:
>>>>
>>>>>
>>>>>
>>>>> On Thu, Jul 13, 2017 at 2:03 PM David Li via Phabricator <
>>>>> reviews at reviews.llvm.org> wrote:
>>>>>
>>>>>> davidxl added a comment.
>>>>>>
>>>>>> Blindly downgrade the warning into remarks can be bad -- profile
>>>>>> mismatch problems can go undetected. People may spend more time diagnosing
>>>>>> performance regressions due to missed warnings. Worse, we may regress in
>>>>>> compiler without noticing.
>>>>>>
>>>>>
>>>>> Any other ideas?
>>>>>
>>>>> The diagnostic handling callbacks through to Clang aren't granular
>>>>> enough to demote only this warning to -Wno-error
>>>>>
>>>>
>>>> Can this be fixed?
>>>>
>>>
> Maybe, though I imagine to do that would mean plumbing this diagnostic as
> a first class/separately identified one in the callback system, which I
> would guess/assume we try not to do because it increases the
> coupling/surface area between LLVM and Clang and the optimizers and
> diagnostic infrastructure.
>
> In any case, even if we could, we'd still be left in the same place of
> telling users basically "this diagnostic isn't high enough quality to
> -Werror it, you should turn it back into a warning-not-error" and at that
> point, we know from experience, people tend to to ignore them...
>
>
>>
>>>>
>>>>
>>>>> (also as a warning it undermines the confidence as warnings as
>>>>> actionable things that Clang has helped provide users).
>>>>>
>>>>
>>>>
>>>> Warnings like this are usually actionable by users or compiler
>>>> developers.
>>>>
>>>
>>> Warnings are really to indicate a bug in user code.
>>>
>>> For example, we use some things like this to warn users when inline
>>> assembly is likely to have a bug, because that is user code that can be
>>> fixed.
>>>
>>> A profile mismatch seems much less like "user code has a bug" and so
>>> doesn't seem to fit any well in the "warning" category.
>>>
>>
>> It is not user code bug, but still be a user bug in how PGO is used.
>>
>
> Is it, though? It sounds like this is expected to some degree (& sounds
> like internally we encourage users to -Wno-error to get around this? Which
> really seems to undermine confidence in Clang's warning quality) - because
> the intent is to apply profiles from previous versions of the program to
> new versions of the program, so things will mismatch, right? And there's
> nothing the user's really meant to do in that case - maybe if it were over
> some threshold.
>
>
>> The warning happens also to be a useful for catch potential compiler bug.
>>
>
>> David
>>
>>
>>>
>>> On the other hand, remarks were specifically designed to be non-bug
>>> important information from optimization passes that users may need to act
>>> on.
>>>
>>
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