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bug#44462: Problem with get_multilibs on macOS

From: Jacob Bachmeyer
Subject: bug#44462: Problem with get_multilibs on macOS
Date: Tue, 10 Nov 2020 18:48:54 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20090807 MultiZilla/ SeaMonkey/1.1.17 Mnenhy/

Fred Wright wrote:
On Sun, 8 Nov 2020, Jacob Bachmeyer wrote:
Fred Wright wrote:
On Thu, 5 Nov 2020, Jacob Bachmeyer wrote:
Fred Wright wrote:

I traced the results of get_multilibs (as used by the libffi tests) in many macOS versions, and even the "successful" cases seem to have questionably useful results:


The "/usr/." result returned on 10.4, 10.5, and 32-bit 10.6 is the same as what I see on Ubuntu 14.04, CentOS 7, and Fedora 25, though it's not clear to me what it's supposed to represent.

I have a suspicion that this feature is designed to support testing with an "experimental" compiler build that is not installed on the system and may be useless with system compilers generally, or with Apple's compilers specifically, if Apple does not use multilib.

Apple supports the concept of multi-architecture binaries, but not in the same way that multilib does it (AFAIK). Macs can have "universal" binaries, which are archives combining multiple per-architecture slices. This is applicable to object files, shared libraries, and executables. If the build setup allows it, it can be as simple as including multiple architecture options in the compile command. E.g.:

    cc -arch x86_64 -arch i386 -o hello hello.c

Under the hood, the compiler driver runs a separate compile/assemble for each architecture, and then combines the object files. The linker supports universal binaries directly.

With this arrangement, architecture-related conditionals in the source code work just fine, but what *doesn't* work is having architecture-related parameters in a configure script, which is unfortunately not as uncommon as it ought to be.

No, Autoconf pushes programs to respond to features instead of using known architectures. This approach has been very successful: as I understand, most programs using Autoconf needed no changes at all to port to RISC-V even if they were written long before RISC-V existed. Many programs, if they had previously been ported to any 64-bit architecture, needed no changes at all for x86_64. Autoconf has achieved something architecture #ifdefs cannot do: provide automatic portability to a new architecture that did not exist when the program was written. I consider Autoconf's approach here completely vindicated.

I'm not talking about checking for specific architectures; this is about checking architecture-related *properties*. For example, a configure check for sizeof(long) is incompatible with multi-architecture builds, while using LONG_BIT works just fine. But there are build procedures that do things like the former, for no good reason.

As far as I can tell, LONG_BIT as a preprocessor symbol is relatively new; newer than many programs that have demonstrated this level of Autoconf-assisted portability. On older systems, glibc provides CHAR_BIT and you can find the bit-length with CHAR_BIT*sizeof(long).

Since get_multilibs already has code to return an empty string in the "remote" case (where it assumes this function won't work), I just added code to unix.exp to set multitop to "" for all "darwin" targets, thereby short-circuiting almost all og get_multilibs. That certainly fixes the problem with the libffi tests, and doesn't change any non-Mac behavior, though I don't know if that's the ideal fix. The whole get_mutilibs function looks pretty ugly anyway, and it's generally recognized that relying on -dumpspecs is a bad idea.

It is most certainly not ideal. A better solution is probably to add a test to get_multilibs to return an empty string if the compiler is not GCC. Of course, if another compiler pretends to be GCC enough to pass that check, but does not actually implement -dumpspecs, that is not our bug.

The libffi test suite comes up with a "compiler_vendor" variable which seems to be able to distinguish clang from gcc, though I haven't looked at the details.

Fixing get_multilibs properly would probably mean making it both highly platform-specific and optional.

The get_multilibs procedure is *not* platform-specific; it is GCC-specific. I am still unsure how exactly it is used.

Since it seems to involve file paths, it may be specific to combinations of GCC and platforms.

But since neither of us seems to know very much about what get_multilibs is trying to do, it's hard to discuss it intelligently. :-)

This is the main reason that get_multilibs failing on MacOS X will be a known bug in 1.6.3.

This issue on Mac OS X will probably be a known bug in 1.6.3 and fixed in 1.6.4.

I primarily tested my patch against the 1.6.2 release, since the current master won't install from a non-git directory, and also has multiple failures in its own tests (even on Linux). The patch is nearly identical between the 1.6.2 and master cases, anyway.

Are we looking at the same current master? I have commit 3d62df24deedfb3c7c3e396a31b8ce431138eb49 here and all of the tests pass.

****These other problems are potential release blockers for 1.6.3.****
Can you file another bug report with the test failures and more information about these issues?

I looked into this more closely and it's probably related to the non-git issue. When running from a non-git directory, the configure script reports a "fatal" error, but then goes on to complete with a zero exit status and a more or less buildable setup, so you have to be paying close attention to the output to notice.

Actually it now looks like the two things are unrelated; I filed two separate bugs.

Thank you; PR44544 should now be fixed on master; I have responded accordingly at that report also. I am still looking into PR44545.

I can send the current patch, either as a bare email or as an attachment. AFAIK, Savannah doesn't have the pull request / merge request concept.

This will need to be fixed in libgloss.exp, not unix.exp. I am putting my foot down on fixing bugs in DejaGnu's own tree directly instead of hacking around them like that.

Well, OK, but there seem to be other similar hacks in unix.exp, and if the idea is that get_multilibs is completely useless on the Mac (which appears to be the case with the current implemenation, anyway), then disabling it in the target-related code doesn't seem unreasonable.

It is not completely useless even on MacOS X -- a user could install GCC and expect a testsuite to use it, particularly in the case of a cross-compiler for embedded development.

If it's a cross compiler, then by definition it can't run the resulting code on the host platform. But since get_multilibs already excludes all remote cases, it wouldn't be able to run it on a separate target platform, either.

Besides, aren't files like baseboards/unix.exp based on the *target* platform, not the host? If so, then it seems like disabling get_multilibs for the Mac there is exactly the right thing, at least until such time as get_multilibs can behave usefully for a Mac target.

Untangling cross-development configurations is an ongoing project.

GCC could (in theory) be installed on any platform. A user may build a native GCC for MacOS X and want to test or use it. In short, the solution needs to test the actual issue -- is $compiler GCC? -- instead of assuming that it is or is not based on the platform.

-- Jacob

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