[Bug middle-end/41475] New: common variables cannot be expected to be aligned

[Bug middle-end/41475] New: common variables cannot be expected to be aligned

[mikulas at artax dot karlin dot mff dot cuni dot cz]

Hi.

When I compile these two sources, one with -O3 -march=pentium3 and the other
with -Os, the linker warns about nonmatching alignments and the program crashes
because of misaligned SSE accesses.

/usr/bin/ld: Warning: alignment 4 of symbol `array' in commonalign2.o is
smaller than 32 in commonalign1.o
/usr/bin/ld: Warning: alignment 4 of symbol `array2' in commonalign2.o is
smaller than 32 in commonalign1.o

The reson is that gcc aligns arrays in common section to 32 bytes and expects
that they will be aligned so. This expectation is wrong, the common entries may
be resolved pointing to data section in another module that doesn't meet the
alignment (it may be compiled with different compiler or with the same compiler
with different flags (-Os)).

For extern arrays, gcc correctly assumes that they are aligned to their ABI
standard (4 bytes) and generates appropriate SSE code; for common arrays it
should expect 4-byte alignment too.


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           Summary: common variables cannot be expected to be aligned
           Product: gcc
           Version: 4.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: mikulas at artax dot karlin dot mff dot cuni dot cz
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41475

[Bug middle-end/41475] common variables cannot be expected to be aligned

[mikulas at artax dot karlin dot mff dot cuni dot cz]

------- Comment #1 from mikulas at artax dot karlin dot mff dot cuni dot cz  2009-09-26 04:05 -------
Created an attachment (id=18653)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=18653&action=view)
the first file

The first file of a two-file program. Compile with gcc -c -O3 -march=pentium3.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41475

[Bug middle-end/41475] common variables cannot be expected to be aligned

[mikulas at artax dot karlin dot mff dot cuni dot cz]

------- Comment #2 from mikulas at artax dot karlin dot mff dot cuni dot cz  2009-09-26 04:06 -------
Created an attachment (id=18654)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=18654&action=view)
the second file

The second file. Compile with gcc -c -Os. Then, link both object files together
and run it.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41475

[Bug middle-end/41475] common variables cannot be expected to be aligned

[mikulas at artax dot karlin dot mff dot cuni dot cz]

------- Comment #3 from mikulas at artax dot karlin dot mff dot cuni dot cz  2009-09-26 04:14 -------
BTW. this bug is in both gcc 4.3 and 4.4


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41475

[Bug middle-end/41475] common variables cannot be expected to be aligned

[rguenth at gcc dot gnu dot org]

------- Comment #4 from rguenth at gcc dot gnu dot org  2009-09-26 11:23 -------
Isn't this a linker issue in that it doesn't use the biggest alignment?


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41475

[Bug middle-end/41475] common variables cannot be expected to be aligned

[hjl dot tools at gmail dot com]

------- Comment #5 from hjl dot tools at gmail dot com  2009-09-26 14:14 -------
(In reply to comment #4)
> Isn't this a linker issue in that it doesn't use the biggest alignment?
> 

Linker can't change alignment on non-common symbols. I think it is wrong
to give a different alignment for the same symbol in different files.


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hjl dot tools at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hjl dot tools at gmail dot
                   |                            |com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41475

[Bug middle-end/41475] common variables cannot be expected to be aligned

[rguenth at gcc dot gnu dot org]

------- Comment #6 from rguenth at gcc dot gnu dot org  2009-09-26 14:37 -------
The bug is about common symbols.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41475

[Bug middle-end/41475] common variables cannot be expected to be aligned

[mikulas at artax dot karlin dot mff dot cuni dot cz]

------- Comment #7 from mikulas at artax dot karlin dot mff dot cuni dot cz  2009-09-26 15:27 -------
Richard Guenther: the bug caused by common symbol (in file commonalign1.o) with
the same name as data section symbol (in file commonalign2.o). In this case,
the linker redirects the common symbol to the symbol in the data section. But
the linker cannot change the content of the data section, so it cannot make
both symbols array and array2 aligned. Linker is innocent in this and giving a
warning is the best thing it can do.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41475

[Bug middle-end/41475] common variables cannot be expected to be aligned

[pinskia at gcc dot gnu dot org]

------- Comment #8 from pinskia at gcc dot gnu dot org  2009-09-26 19:29 -------
This code is undefined I think (and really it is not valid C90/C99 code).


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41475

[Bug middle-end/41475] common variables cannot be expected to be aligned

[mikulas at artax dot karlin dot mff dot cuni dot cz]

------- Comment #9 from mikulas at artax dot karlin dot mff dot cuni dot cz  2009-09-27 09:51 -------
The common linker definitions were made to exactly to make code like this work
and share the array between two object.

So if you think it is undefined, don't support it (make -fno-common default and
remove -fcommon at all). Or, if you want to support it, support it correctly.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41475