[cfe-dev] [RFC] Additional frontend timers in -ftime-report

[Brian Gesiak via cfe-dev]

Hello all,

I'd like to hear your thoughts on expanding the amount of information
provided by `clang -ftime-report`.

I noticed that `gcc -ftime-report` provides a breakdown of how much
time GCC spends doing preprocessing, parsing, template instantiation,
and more. `clang -ftime-report`, on the other hand, gives an
incredibly detailed breakdown of how long each LLVM pass takes, but
does not provide much detail on front-end tasks such as preprocessing
and template instantiation.

Here's an example of output from `clang -ftime-report` and `gcc
-ftime-report`:
https://gist.github.com/modocache/d74833818107ed50d11387a5a4e3fb72

As a result, when attempting to diagnose slow compile times with
Clang, my understanding is that users have two choices:

1. Use external profiling tools on a `clang` executable that does not
have its symbols stripped in order to determine what the bottleneck
is.
2. Compile the same code with `gcc -ftime-report` to determine which
phase is taking a long time. This workflow suits users who aren't
interested in improving Clang's performance, and are happy to simply
rewrite their source code so that it compiles faster.

I think it would be an improvement if Clang output more front-end
information in `clang -ftime-report`. For example, I've sent up a diff
that adds timers related to preprocessing:
https://reviews.llvm.org/D36492. Would anyone be interested in
reviewing it?

In addition, if anyone has thoughts on the general idea of adding more
timers to `clang -ftime-report`, please let me know. Assuming there's
interest in better resembling `gcc -ftime-report` behavior, I would be
happy to add timers for preprocessing, parsing, template
instantiation, and more.

Thanks!

- Brian Gesiak