Hello,

During highly parallel builds on fast hosts, we have experienced
sporadic bootstrap failures on libquadmath like

  In file included from ../../../src/libquadmath/printf/printf_fp.c:39:0:
  ../../../src/libquadmath/printf/quadmath-printf.h:24:20: fatal error: .../build/./gcc/include-fixed/limits.h: No such file or directory
  #include <limits.h>

A pretty clear sign of a race condition caused by some inaccuracy in the
dependency statements.

Investigation led us to suspect this piece in the toplevel Makefile.in:

  all-libcc1: maybe-all-gcc

which differs from all the other dependencies on maybe-all-gcc in that it's
unconditional whereas the other ones are conditioned on @if gcc-no-bootstrap.

(Thanks to Nico Roche, cc'ed for the worked involved in finding this out)

Our understanding is that it's incorrect to have dependencies on maybe-all-gcc
in the bootstrap case; that this should be a dependency on stage_current
instead.

This patch is a proposal to address this by first removing the following
statement in Makefile.def:

  dependencies = { module=all-libcc1; on=all-gcc; };

(which emits the dependency unconditionally), then refining the expansion
of "all" targets in Makefile.tpl so they include a possible dep conditioned by
gcc-no-bootstrap, on demand for "host_module"s that ask for it by way of a new
"depgcc" parameter.

We have been using this in-house for months now. The sporadic failures
have disappeared since then and we haven't observed any related fallout
so far.

Bootstrapped and regression tested on x86_64-linux.

OK to commit ?

Thanks in advance for your feedback,

With Kind Regards,

Olivier

2017-06-13  Olivier Hainque  <hainque@adacore.com>

	* Makefile.def (host_modules): Set depgcc to true for libcc1,
	meaning need of a dep on stage_current if gcc-bootstrap and on
	maybe-all-gcc otherwise.
	(dependencies) Remove unconditional dependency on all-gcc.
    
	* Makefile.tpl ("all" targets): Handle depgcc.
	* Makefile.in: Regenerate