[ccache] ccache on generated C++ files...

[Malfettone, Kris]

I have confirmed that a few files can be different due to our generation code.  There are no volatile pieces such as preprocessor macros and such, just good non-deterministic print orders in our generation code.  I believe this should be the culprit but unfortunately will take a bit for me to cleanup to test again.  My mistake was "assuming" it was all consistent.

Thanks for all the time / info everybody.  CCache has always been a favorite tool of mine.

-Kris

-----Original Message-----
From: Joel Rosdahl [mailto:joel at rosdahl.net]
Sent: Tuesday, June 25, 2019 3:03 PM
To: Malfettone, Kris <Kris.Malfettone at msx.bala.susq.com>
Cc: ccache at lists.samba.org
Subject: Re: [ccache] ccache on generated C++ files...

On Tue, 25 Jun 2019 at 00:28, Malfettone, Kris via ccache
<ccache at lists.samba.org> wrote:
> So what I seem to be observing is only when I do a ninja -t clean or make
> clean. This removes all of my generated files (.h and .cpp files) and of
> course the normal build outputs. A subsequent run is then mostly cache
> misses. If I say manually do a rm $(find -name "*.o") then I get a 100% cache
> hit rate. I had assumed it was due to the fact that the generated files would
> have a newer mtime and ctime. My assumption must be wrong.

Maybe there is a timestamp in the generated files?

> Is there an easy way to print diagnostics on why a cache miss occurred?

Yes, assuming you use ccache 3.5 or newer, you can enable the debug mode and
compare the hashed input for two different builds. See "To debug why you don’t
get an expected cache hit for an object file [...]" on
<https://ccache.dev/manual/3.7.1.html#_cache_debugging>.

-- Joel

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