[distcc] gcc bootstraps with distcc

[Zack Weinberg]

Martin Pool <mbp at sourcefrog.net> writes:

>> I don't understand.  On by default makes sense precisely because
>> there's no bad side effect to this feature.  If there *is* a bad
>> side effect which I'm not aware of, then my opinion may be different,
>> but I don't see how having this information in the .i file can
>> possibly interfere with anything.
>
> ccache is a cache of the mapping from [arguments, cpp output, some
> other stuff] to [object file, error output]. 
>
> Suppose we use ccache to build two identical trees in different
> directories.  Previously the second tree would have got 100% cache
> hits (roughly) but the directory in the second tree's debug info would
> have been wrong.
>
> With this change, the second tree will not get any cache hits, because
> the cpp output will contain the directory name.
>
> This makes the cache more accurate, which is generally desirable.
> However, if you never used the debug information, then you might be
> disappointed by the reduced hit rate.

Thank you for the clear explanation.  The conclusion I draw from it is
that we should turn this mode on when and only when it affects the
output of the compiler.  Would you, Martin, be willing to do some
research and figure out under exactly what conditions this makes a
difference?

Also, I agree with Neil, this has nothing to do with dependency
output, it shouldn't be an -M option.  If it affects only debug
information, let's make it -g(no-)pwd, otherwise -f(no-)pwd.

zw