Re: [PATCH 1/1] mingw: fix CPU reporting in `git version --build-options`

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Eric,

On Fri, 8 Feb 2019, Eric Sunshine wrote:

> On Thu, Feb 7, 2019 at 5:46 AM Johannes Schindelin via GitGitGadget
> <gitgitgadget@gmail.com> wrote:
> > We cannot rely on `uname -m` in Git for Windows' SDK to tell us what
> > architecture we are compiling for, as we can compile both 32-bit and
> > 64-bit `git.exe` from a 64-bit SDK, but the `uname -m` in that SDK will
> > always report `x86_64`.
> >
> > So let's go back to our original design. And make it explicitly
> > Windows-specific.
> 
> b22894049f (version --build-options: also report host CPU, 2017-12-15)
> took this sort of case into consideration by introducing Makefile
> variable HOST_CPU (which takes precedence over `uname -m`), with the
> intention that, when cross-compiling, your build environment should
> (somehow) set HOST_CPU to the canonical name of the CPU on which the
> built Git will run (for instance, "x86_64" or "i686"). It would be
> nice to employ this mechanism to solve this issue rather than
> (re-)introducing a manually-maintained list of CPU names.

Heh, this is also manually-maintained, but I agree that it is cleaner.

> Can you say a few words (here in the email thread) about how the Git
> for Windows SDK is instructed to build for one architecture or the
> other?

To cross-compile a 32-bit Git in a 64-bit Git for Windows SDK, use this
incantation:

	MSYSTEM=MINGW32 PATH=/mingw32/bin:$PATH make

> With such information, perhaps we can figure out how to get the build
> environment itself to set HOST_CPU automatically so we don't have to
> resort to and worry about maintenance costs of a hard-coded CPU name
> list.

Indeed, we can set HOST_CPU in the same conditionals as prefix (which is
/mingw32 for 32-bit and /mingw64 for 64-bit) in config.mak.uname.

Patch incoming,
Dscho

> > Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> > ---
> > diff --git a/compat/mingw.h b/compat/mingw.h
> > @@ -6,6 +6,25 @@ typedef _sigset_t sigset_t;
> > +#ifdef __MINGW64_VERSION_MAJOR
> > +/*
> > + * In Git for Windows, we cannot rely on `uname -m` to report the correct
> > + * architecture: /usr/bin/uname.exe will report the architecture with which the
> > + * current MSYS2 runtime was built, not the architecture for which we are
> > + * currently compiling (both 32-bit and 64-bit `git.exe` is built in the 64-bit
> > + * Git for Windows SDK).
> > + */
> > +#undef GIT_HOST_CPU
> > +/* This was figured out by looking at `cpp -dM </dev/null`'s output */
> > +#if defined(__x86_64__)
> > +#define GIT_HOST_CPU "x86_64"
> > +#elif defined(__i686__)
> > +#define GIT_HOST_CPU "i686"
> > +#else
> > +#error "Unknown architecture"
> > +#endif
> > +#endif
>