modocache planned changes to this revision.
modocache added inline comments.
================
Comment at: lib/Lex/Preprocessor.cpp:746
void Preprocessor::Lex(Token &Result) {
+ llvm::TimeRegion(PPOpts->getTimer());
+
----------------
erik.pilkington wrote:
> Doesn't this just start a timer and immediately end the timer? Shouldn't we
> do: `llvm::TimeRegion LexTime(PPOpts->getTimer())` so that the dtor gets
> called when this function returns and we track the time spent in this
> function?
>
> Also: this is a pretty hot function, and it looks like TimeRegion does some
> non-trivial work if time is being tracked. Have you tried testing this on a
> big c++ file with and without this patch and seeing what the difference in
> compile time looks like?
Ah, yes you're right! Sorry about that. Actually keeping the timer alive for
the duration of the method also reveals that the method is called recursively,
which causes an assert, because timers can't be started twice.
Another spot in Clang works around this with a "reference counted" timer:
https://github.com/llvm-mirror/clang/blob/6ac9c51ede0a50cca13dd4ac03562c036f7a3f48/lib/CodeGen/CodeGenAction.cpp#L130-L134.
I have a more generic version of this "reference counting timer" that I've
been using for some of the other timers I've been adding; maybe I'll use it
here as well.
https://reviews.llvm.org/D36492
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