preprocessor cache/direct mode doesn't produce -MF output

[glandium]

STR:

touch foo.h
echo '#include "foo.h"' > foo.c
sccache gcc -o foo.o -c foo.c -MD -MF foo.pp

On the first run, sccache calls the preprocessor, which produces the foo.pp file, with contents:

foo.o: foo.c /usr/include/stdc-predef.h foo.h

Typically, this would be used by a build system to know when to rebuild foo.o. Without this file, the build system wouldn't know to rebuild foo.o when e.g. foo.h changes.

Now, clean your build directory and try again:

rm foo.o foo.pp
sccache gcc -o foo.o -c foo.c -MD -MF foo.pp

Now here's the problem: since the preprocessor wasn't called, and since the preprocessor cache doesn't store what would normally be in foo.pp, foo.pp is not created.

[glandium]

preprocessor cache doesn't store what would normally be in foo.pp

well, technically, it kind of does, in a different form, and it doesn't know how to recreate foo.pp from that.

[glandium]

#2195 addresses this in the same way #2173 addressed #2122: by disabling preprocessor cache when it's broken.

This issue and #2122 will likely need common changes.

For contributors of Gecko without super-fast hardware it'd be nice if this could get fixed.

[ahartmetz]

Most things seem to be in place to fix this. If it still doesn't work, I see two possible problems:

• -MF implies -E (i.e. only preprocess, don't compile) and I don't know how sccache deals with preprocessing only invocations of the compiler
• gleaning $filename from -MF $filename as opposed to -MD $filename might not be supported

In the former case, dumping dependency info while compiling (basically using -MD instead of -MF) seems to be the strictly superior option - at least I don't know of any downsides. Would it be possible to improve the Gecko build system like that?