[PATCH] D28207: Add second fast path for DiagnosticsEngine::GetDiagStatePointForLoc

[Richard Smith via Phabricator via cfe-commits]

rsmith added inline comments.


================
Comment at: lib/Basic/Diagnostic.cpp:179
+
+  // 2nd most frequent case: L is before the first diag state change.
+  FullSourceLoc FirstStateChangePos = DiagStatePoints[1].Loc;
----------------
djasper wrote:
> rsmith wrote:
> > It's surprising to me that this would be particularly frequent.
> > 
> > I suspect what you're actually seeing is a consequence of a bug in how we manage `DiagStatePoint`s with modules. It looks like `ASTReader::InitializeContext` is called once per top-level PCM file that we load, and its call to `ReadPragmaDiagnosticMappings` adds entries to the `DiagStatePoints` list regardless of whether they've already been added. So, we'll end up with duplicates in the `DiagStatePoints` list, and it won't even be in translation unit order.
> > 
> > Can you take a look at the `DiagStatePoints` list for a translation unit where you see a performance problem and check whether it seems reasonable?
> I looked at the DiagStatePoints and they do look somewhat sane but suspicious.
> 
> The translation unit I am looking at has (AFAICT) only includes that get translated to modules. DiagStatePoints only has entries for the translation unit itself, not a single one coming from any of the modules. So DiagStatePoints looks like:
> 
>   [0]: <<invalid>>  (for the command line)
>   [1]: myfile.cc:100
>   [2]: myfile.cc:100
>   ...
>   [2500]: myfile.cc:2800
> 
> And because of that, the new fast path is hit every single time when a source location coming from a module is queried. There are always two entries for a line of myfile.cc which always seem to denote entering and exiting a macro.
> 
> So, specific questions:
> - Should there by DiagStatePoints from modules?
> - Should there really be a DiagStatePoint entry (or actually two) for every macro invocation in myfile.cc?
Yes, there should be `DiagStatePoints` from modules, but support for that seems to basically be unimplemented at this point.

I believe the `DiagStatePoints` for macros are coming from a `_Pragma("clang diagnostic ...")` within the macro expansion. Doesn't look like there's any other way we'd create them.


https://reviews.llvm.org/D28207