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SubjectGCC version requirement (was: Re: mips: math-emu: Fix compilation error ieee754.c)
On Wed, 11 Jun 2014, David Rientjes wrote:

> > > ieee754dp has bitfield member in struct without name. And this
> > > cause compilation error. This patch removes struct in ieee754dp
> > > declaration. So compilation error is fixed.
> > > Signed-off-by: Eunbong Song <eunb.song@samsung.com>
> >
> > What gcc version are you using?
> >
>
> make arch/mips/math-emu/ieee754.o for mips defconfig triggers the
> following on linux-next 30 times:
>
> arch/mips/math-emu/ieee754.c:45:2: error: unknown field 'sign' specified in initializer
> arch/mips/math-emu/ieee754.c:45:2: warning: missing braces around initializer
> arch/mips/math-emu/ieee754.c:45:2: warning: (near initialization for '__ieee754dp_spcvals[0].<anonymous>')
> arch/mips/math-emu/ieee754.c:45:2: error: unknown field 'bexp' specified in initializer
> arch/mips/math-emu/ieee754.c:45:2: warning: excess elements in union initializer
> arch/mips/math-emu/ieee754.c:45:2: warning: (near initialization for '__ieee754dp_spcvals[0]')
> arch/mips/math-emu/ieee754.c:45:2: error: unknown field 'mant' specified in initializer
> arch/mips/math-emu/ieee754.c:45:2: warning: excess elements in union initializer
> arch/mips/math-emu/ieee754.c:45:2: warning: (near initialization for '__ieee754dp_spcvals[0]')
>
> I'm using gcc 4.5.1 for mips. The patch makes all members part of the
> union so it's probably not what you want to fix it, though.

There's more recent breakage like this, e.g.:

mm/page_io.c: In function '__swap_writepage':
mm/page_io.c:277: error: unknown field 'bvec' specified in initializer
mm/page_io.c:278: warning: excess elements in struct initializer
mm/page_io.c:278: warning: (near initialization for 'from')

introduced with "bio_vec-backed iov_iter" (GCC 4.1.2 here). We still in
principle support GCC versions back to 3.2:

$ grep 'Gnu C' Documentation/Changes
o Gnu C 3.2 # gcc --version
$

so either this breakage has to be cleaned up or the requirement for the
minimum GCC version revisited.

This is a semi-standard language extension BTW, citing from the GCC
manual:

--------------------------------------------------------------------------
6 Extensions to the C Language Family
*************************************

GNU C provides several language features not found in ISO standard C.
(The `-pedantic' option directs GCC to print a warning message if any
of these features is used.) To test for the availability of these
features in conditional compilation, check for a predefined macro
`__GNUC__', which is always defined under GCC.

6.59 Unnamed struct/union fields within structs/unions
======================================================

As permitted by ISO C11 and for compatibility with other compilers, GCC
allows you to define a structure or union that contains, as fields,
structures and unions without names.
--------------------------------------------------------------------------

-- note the term "permitted" rather than "required".

We do make use of a few GCC language extensions, most notably inline
assembly, however in this case we merely save a couple of characters here
and there and this is IMO not worth breaking people's development
environments.

Maciej


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