Hi Linus, On Mon, 13 Sep 2021 18:29:26 -0700 Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Mon, Sep 13, 2021 at 5:58 PM Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx> wrote: > > > > > I have no idea why it then complains about removal of the GCC4 macros. > > > > Me neither :-( > > Ooh. > > So I'm looking at gcc sources, just to see if "maybe this thing is > somehow conditional". > > And bingo. > > In cpp_init_special_builtins(), gcc does > > if (b->value == BT_HAS_ATTRIBUTE > && (CPP_OPTION (pfile, lang) == CLK_ASM > || pfile->cb.has_attribute == NULL)) > continue; > > which basically says that if we're pre-processing an ASM file, the > magical pre-processor symbol for __has_attribute is not defined. > > I'm not sure what that 'pfile->cb.has_attribute == NULL' thing means, > but the libcpp/ChangeLog file also mentions this: > > (cpp_init_special_builtins): Don't initialize __has_attribute > or __has_cpp_attribute if CLK_ASM or pfile->cb.has_attribute is NULL. > > So this is a very very special magical thing: if building an *.S file, > __has_attribute magically goes away. > > And sure enough, that's exactly what is going on. It's during that > build of arch/powerpc/boot/crt0.S, and the reason this hits on powerpc > is that in arch/powerpc/boot/Makefile we have > > -include $(srctree)/include/linux/compiler_attributes.h > > as part of BOOTCFLAGS, and then it does > > BOOTAFLAGS := -D__ASSEMBLY__ $(BOOTCFLAGS) -nostdinc > > to also include that header file when building ASM files. > > And our old GCC4 code silently hid this all, and made it work, because > for a *.S file you'd then (completely illogically) get those fake > gcc-4 attribute macros. > > Now, do I know *why* that ppc Makefile it does that? No. Neither do I > know why the gcc people decided to just make ASM preprocessor so > special. > > But at least I understand how the odd error happens. Its good to know there is a reason :-) > This was too damn subtle. When you have to go read the compiler > sources to figure things like this out, you know you are too deep. > > The fix should be pretty simple: remove almost all of BOOTCFLAGS from > BOOTAFLAGS. > > But sadly, "almost all" isn't "all". There's the include path stuff, > there's the ABI and endianness, and there's the bit size ones. > > So I think the fix is either > > (a) remove that > > -include $(srctree)/include/linux/compiler_attributes.h > > thing entirely, and add it as required to the C files. > > OR > > (b) something like this ENTIRELY UNTESTED ATTACHED patch > > I will leave it to the powerpc people to make the right choice. That patch works for me - for the ppc64_defconfig build at least. -- Cheers, Stephen Rothwell
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