[Bug libc/18712] New: ARM <string.h> incompatible with -O2 -Werror=packed -Wsystem-headers

[Bug libc/18712] New: ARM <string.h> incompatible with -O2 -Werror=packed -Wsystem-headers

[roland at gnu dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18712

            Bug ID: 18712
           Summary: ARM <string.h> incompatible with -O2 -Werror=packed
                    -Wsystem-headers
           Product: glibc
           Version: 2.19
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: roland at gnu dot org
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

This is a bit dubious, since you need -Wsystem-headers to see it in an actual
installation.  But we strive to have warning-free headers even without the
system-header suppression.

        $ arm-linux-gnueabihf-gcc -c -o /dev/null -xc <(echo '#include
<string.h>') -O2 -Werror=packed -Wsystem-headers
        In file included from /usr/arm-linux-gnueabihf/include/string.h:635:0,
                         from /dev/fd/63:1:
        /usr/arm-linux-gnueabihf/include/bits/string2.h:79:1: error: packed
attribute causes inefficient alignment [-Werror=packed]
         __STRING2_COPY_TYPE (2);
         ^
        /usr/arm-linux-gnueabihf/include/bits/string2.h:80:1: error: packed
attribute causes inefficient alignment [-Werror=packed]
         __STRING2_COPY_TYPE (3);
         ^
        /usr/arm-linux-gnueabihf/include/bits/string2.h:81:1: error: packed
attribute causes inefficient alignment [-Werror=packed]
         __STRING2_COPY_TYPE (4);
         ^
        /usr/arm-linux-gnueabihf/include/bits/string2.h:82:1: error: packed
attribute causes inefficient alignment [-Werror=packed]
         __STRING2_COPY_TYPE (5);
         ^
        /usr/arm-linux-gnueabihf/include/bits/string2.h:83:1: error: packed
attribute causes inefficient alignment [-Werror=packed]
         __STRING2_COPY_TYPE (6);
         ^
        /usr/arm-linux-gnueabihf/include/bits/string2.h:84:1: error: packed
attribute causes inefficient alignment [-Werror=packed]
         __STRING2_COPY_TYPE (7);
         ^
        /usr/arm-linux-gnueabihf/include/bits/string2.h:85:1: error: packed
attribute causes inefficient alignment [-Werror=packed]
         __STRING2_COPY_TYPE (8);
         ^
        cc1: some warnings being treated as errors

I could not figure out a way with __extension__ to avoid this warning.  If
there is no way to do it, that should probably be a GCC bug.  But perhaps there
is a way to get the same effect (looks like this is only used for
constant-folded cases) without using __attribute__ ((packed)).  Or perhaps
these optimizations can just be turned off for new-enough GCC if it does all
that constant folding itself.

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[Bug libc/18712] ARM <string.h> incompatible with -O2 -Werror=packed -Wsystem-headers

[roland at gnu dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18712

Roland McGrath <roland at gnu dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at sourceware dot org   |jsm28 at gcc dot gnu.org

--- Comment #1 from Roland McGrath <roland at gnu dot org> ---
This exists in trunk and at least as far back as 2.19.

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[Bug libc/18712] bits/string2.h incompatible with -O2 -Werror=packed -Wsystem-headers

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18712

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|jsm28 at gcc dot gnu.org           |unassigned at gcc dot gnu.org
            Summary|ARM <string.h> incompatible |bits/string2.h incompatible
                   |with -O2 -Werror=packed     |with -O2 -Werror=packed
                   |-Wsystem-headers            |-Wsystem-headers

--- Comment #2 from Joseph Myers <jsm28 at gcc dot gnu.org> ---
This is nothing to do with ARM; exactly the same appears on other
strict-alignment configurations (e.g. MIPS), while non-strict-alignment
configurations (e.g. most Power processors) produce "error: packed attribute is
unnecessary".  And bits/string2.h is architecture-independent.

I don't see *why* you get the "inefficient" warning, instead of "unnecessary"
in all cases (packed should make no difference to these structures, for normal
ABIs as opposed to e.g. old-ABI ARM).  But if the warnings are bogus, that's a
GCC issue.

I'm not sure if the code using these types is active at all for current GCC
other than in the _FORCE_INLINES case.

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[Bug string/18712] bits/string2.h incompatible with -O2 -Werror=packed -Wsystem-headers

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18712

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|libc                        |string

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