[Bug 246630] stable/11 regression: base.txz reproducibility depends on number of cpu cores

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=246630

--- Comment #24 from Dimitry Andric <dim at FreeBSD.org> ---
(In reply to Alexandre Ganea from comment #23)

This is the tricky part! I've only been able to reproduce this problem until
now within the FreeBSD build system. That is, the result of one particular file
being compiled is dependent on whether make is running with or without -j (and
in some cases, the value of the -j option).

As of now, I can "fix" the problem by using #define CLANG_SPAWN_CC1 1 in
lib/clang/include/clang/Config/config.h, which effectively disables the
in-process cc1 by default. At least then you always get the same object file
output.

But as to *why* the result differs when CLANG_SPAWN_CC1=0, and being invoked
under different variations of make or make -j, it is not clear yet.

One thing which Bryan Drewery just mentioned on IRC is that make -j messes with
stderr, causing it to no longer be a tty. I think that must have some sort of
influence, but it's going to be tricky to debug (especially if you want to send
debug output to stderr :).

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