amd64: improve -m32 support in machine/*.h

[brooks]

Summary

Install i386 headers in /usr/include/i386 and include their contents
in place of the amd64 headers where required. In most cases, rely on
the same header guards being used to cause the amd64 body to be
ignored.

With this it is possible to compile all libraries and csu bits for
i386 against an amd64 sysroot using -m32.

[brooks]

I'm looking for feedback on this approach.

Installing all the i386 headers is a little heavy, but easy to implement. Installing select files would be possible, if more work. Giving all the these headers the full include/x86 treatment would be more work, but maybe cleaner?

Note that I'm current missing make-delete-old support.

[jrtc27]

Relying on the include guards to avoid the need for an #else seems a bit gross... and tbh I'd rather these did get the x86 treatment, you could surely automate it

sys/amd64/include/asmacros.h

43

Why?..

[brooks]

In D34696#786246, @jrtc27 wrote:

Relying on the include guards to avoid the need for an #else seems a bit gross... and tbh I'd rather these did get the x86 treatment, you could surely automate it

I could just go with the big hammer and smoosh the files together automatically with some fixups of the copyright blocks as required. I fear there would then be a desire to dedup the cases and getting that wrong feels like a recipe for weird bugs. That being said, I've got a short list of files that should be consolidated since they are identical.

sys/amd64/include/asmacros.h

43

vdso32 is built with -m32 but expects to find things like the machine symlink in the kernel build directory and AFACT doesn't search the sysroot at all so it doesn't find the installed i386. It's been working all along with only the amd64 asmacros.h

[jhb]

Hmm, I would prefer the x86 approach, but it is indeed more work.

[brooks]

I've done x86-ification patches with a split of combining that that have high level of duplication and punting the i386 version to a new x86/i386 directory where there is little commonality. I'll post them, likely as a GitHub PR since it's a bunch of mostly boring changes I want to be separate commits for revertability.

[brooks]

PR with a better split https://github.com/freebsd/freebsd-src/pull/597