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

[Joel Rosdahl via ccache]

On Tue, 25 Jun 2019 at 00:28, Malfettone, Kris via ccache
 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
.

-- Joel

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

[Bernd Petrovitsch via ccache]

On 25/06/2019 00:28, Malfettone, Kris via ccache 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.

TTBOMK ccache ignores mtime/ctime/... (as make/ninja/... use that anyway
before) but only the preprocessed content matters.

IIUC, the `make clean` removes all *.o and all generated .h/.c/.hpp/.cpp
files.
The rm $(find -name "*.o") removes only *.o.

So the analysis above is at least incomplete.

Are you really sure the newly generated .h/.c/.hpp/.cpp files are
identical (enough)?
No static variables or expanded preprocessor #defines with the build
date+time or similar in there?

[ Fullquote deleted ]
MfG,
Bernd
-- 
Bernd Petrovitsch  Email : be...@petrovitsch.priv.at
 LUGA : http://www.luga.at

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

[Malfettone, Kris via ccache]

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:j...@rosdahl.net]
Sent: Tuesday, June 25, 2019 3:03 PM
To: Malfettone, Kris 
Cc: ccache@lists.samba.org
Subject: Re: [ccache] ccache on generated C++ files...

On Tue, 25 Jun 2019 at 00:28, Malfettone, Kris via ccache
 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
.

-- Joel



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