[PATCH] D43578: -ftime-report switch support in Clang

[Kim Gräsman via Phabricator via cfe-commits]

kimgr added a comment.

In https://reviews.llvm.org/D43578#1067974, @rsmith wrote:

> In https://reviews.llvm.org/D43578#1067891, @kimgr wrote:
>
> > I disagreed up until the last paragraph :)
>
>
> Can you say which things you disagree with? There seem to be two important facts here:
>
> 1. LLVM's timing infrastructure cannot correctly report timings when one function containing a timing region calls another. That will happen all the time with this patch applied.
> 2. Clang's architecture means that if you time the amount of time spent in, say, the "ParseTemplate" function, you have not computed the amount of time spent parsing templates, because the parser calls into Sema, which might perform tasks that are only somewhat related to parsing the template (such as performing other instantiations), and likewise Sema calls into other layers (such as AST file deserialization and code generation).
>
>   The first might have been fixed in LLVM at this point, but I can't see any evidence of that in LLVM's Timer implementation. And the second seems even more fundamental. But if there's some way around that, which would allow us to produce (correct!) numbers for times spent parsing / whatever else, without, for instance, adding a correctness requirement that we annotate every Parser -> Sema entry point with a timer, then that's definitely something we should discuss.


Ah, I see. I read your original message as dismissive as in users don't need to know where time is spent.

I tried to make the case that users do care where time is spent, because it can give a hint as to what part of their physical/logical design they can work on to get better compile times.

But I think you were really saying that LLVM and Clang as designed can't provide anything perfect, only slightly misleading information?

Sorry for the misunderstanding.

- Kim


https://reviews.llvm.org/D43578