Re: does ccache include source code modified time into hash?

Hi Makson,

Since the manual wasn't clear enough, what kind of information do you think
would be good to add to the "How ccache works" section? Suggestions are
very welcome.

-- Joel

On 12 November 2014 02:26, Makson Lee <[email protected]> wrote:

> Hi Guys,
>
> Yes, it did help, thanks for the detailed explanation.
>
> 2014-11-12 1:33 GMT+08:00 Andrew Stubbs <[email protected]>:
>
> > On 11/11/14 15:11, Makson Lee wrote:
> >
> >> i am not sure that whether ccache will re-compile source code if we just
> >> change the modified time of it, can not find much about the details of
> >> "how
> >> ccache works", in the manual, it just says that ccache will hash the
> >> source
> >> code, but what kinds of information of source code are hashed?
> >>
> >
> > Ccache has two modes: "direct mode" and "preprocessor mode". It tries
> > both, in that order.
> >
> > Preprocessor mode is the slower, older mode. This does not pay any
> > attention to the file stats. It simply runs the compiler preprocessor and
> > sees if the MD4 hash of the output matches something it has seen before.
> In
> > theory, you can even edit the comments and macro text, to some degree,
> and
> > still get a cache hit.
> >
> > Direct mode is much faster, but less tolerant of irrelevant changes. In
> > this mode, ccache keeps a cache ("manifest") of every source file used
> by a
> > compile, including times, size, and an MD4 hash. It then tries to match
> > each file and determine if the source matches without calling the
> compiler
> > at all. Ordinarily, it uses only the MD4 hash to determine this, so
> > changing the mtime will have no effect.
> >
> > However, if you set "CACHE_SLOPPINESS=file_stat_matches" then direct mode
> > will ignore the contents of the source files, and determine similarity
> > using only the ctime, mtime, and size attributes. In this case, changing
> > the mtime will cause a direct mode cache miss. Ccache will then fall-back
> > to preprocessor mode, and you should still get a cache hit that way,
> albeit
> > slower. The cached manifest will be updated, and the next time you build
> > you will get a direct mode cache hit again, unless the mtime changes
> again.
> >
> > Hope that helps
> >
> > Andrew
> >
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