Re: [PATCH] Makefile: make COMPUTE_HEADER_DEPENDENCIES=auto work with DEVOPTS=pedantic

On Wed, Sep 22, 2021 at 1:17 PM Jeff King [off-list ref] wrote:

On Wed, Sep 22, 2021 at 11:28:38AM -0700, Junio C Hamano wrote:

quoted

quoted

 ifeq ($(COMPUTE_HEADER_DEPENDENCIES),auto)
-dep_check = $(shell $(CC) $(ALL_CFLAGS) \
+dep_check = $(shell $(CC) \
    -c -MF /dev/null -MQ /dev/null -MMD -MP \
    -x c /dev/null -o /dev/null 2>&1; \
    echo $$?)

I am all for this simplification.  It takes us back to the state
before 1816bf26 (Makefile: Improve compiler header dependency check,
2011-08-30).  But I think that is more or less orthogonal to the
"you are not supposed to feed an empty compilation unit" issue.

Hmm, my suggestion was off the cuff without digging to see whether we
used to do something similar. ;)

I do worry a bit that we'd be regressing the case that commit tried to
fix. OTOH, I'm not sure I understand its commit message. It talks about
things in CFLAGS being a problem, but it looks like the original (and my
proposal here) would not look at CFLAGS at all? If people are putting
stuff into CC that will break when used without CFLAGS, then I feel like
the answer might be "don't do that". Or are there common situations
where $(CC) is not expected to behave sensibly on its own?

Yes, the problem was ONLY with the gcc that came in macOS before they
moved to clang and that had a frankenstein set of options which will
prevent people building BOTH fat binaries and doing this header
dependency computation at the same time.

Ironically, modern clang makes his use case even uglier (running on
top of Ævar's fix) :

$ make CFLAGS="-arch x86_64 -arch arm64"
fatal error: /Library/Developer/CommandLineTools/usr/bin/lipo: can't
create temporary output file: /dev/null.lipo (Operation not permitted)
clang: error: lipo command failed with exit code 1 (use -v to see invocation)
fatal error: /Library/Developer/CommandLineTools/usr/bin/lipo: can't
create temporary output file: /dev/null.lipo (Operation not permitted)
clang: error: lipo command failed with exit code 1 (use -v to see invocation)
Non-zero 1 exit with COMPUTE_HEADER_DEPENDENCIES=auto, set it to "yes" or "no" t

so the test fails, and COMPUTE_HEADER_DEPENDENCIES is disabled, but it
shouldn't and works fine if the test is overridden with
COMPUTE_HEADER_DEPENDENCIES=yes

so by removing the CFLAGS from that test, we will actually be fixing
this use case as well IMHO.

Carlo