[Bug target/48127] New: Program crashes reading a non-aligned variable

[Bug target/48127] New: Program crashes reading a non-aligned variable

[d.g.gorbachev at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48127

           Summary: Program crashes reading a non-aligned variable
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: d.g.gorbachev@gmail.com
            Target: i686-pc-linux-gnu


Created attachment 23657
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23657
Testcase

(See also: <http://sourceware.org/PR12578>.)

[Bug target/48127] Program crashes reading a non-aligned variable

[d.g.gorbachev at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48127

--- Comment #1 from Dmitry Gorbachev <d.g.gorbachev at gmail dot com> 2011-03-15 06:22:13 UTC ---
Works when changing the number of elements in baz to 8.

[Bug target/48127] Program crashes reading a non-aligned variable

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48127

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |matz at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-03-15 10:27:39 UTC ---
I think the testcase is invalid as you have two definitions of baz which
are not both common.

The vectorizer relies on being able to promote alignment of definitions
which I'm not sure it can do so for commons (it would rely on the linker
choosing the common with the largest alignment).

A testcase that works with -fno-common would be more convincing here ;)

[Bug target/48127] Program crashes reading a non-aligned variable

[d.g.gorbachev at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48127

--- Comment #3 from Dmitry Gorbachev <d.g.gorbachev at gmail dot com> 2011-03-15 18:00:23 UTC ---
I do not agree that the testcase is invalid. As the standart says:

  Common extensions

  Multiple external definitions

     There may be more than one external definition for the identifier of
  an object, with or without the explicit use of the keyword extern; if
  the definitions disagree, or more than one is initialized, the behavior
  is undefined.

According to the Binutils documentation,

  The linker turns a common symbol into a declaration, if there is a
  definition of the same variable.

Also, from Ian Lance Taylor's article [1]:

  5. If A is a strong definition in an object file:

    * If B is a common symbol, then we treat B as an undefined reference.

I think it would be better to fix this problem in the linker. Gold does not
even warn about it!

However, I don't understand why GCC makes baz[8] to have 32 bytes alignment,
while baz[4] has only 4 bytes alignment. It seems to be a GCC bug.

When baz is declared extern in main.c, the generated code is correct, but less
optimal.


1. http://www.airs.com/blog/archives/49

[Bug target/48127] Program crashes reading a non-aligned variable

[rguenther at suse dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48127

--- Comment #4 from rguenther at suse dot de <rguenther at suse dot de> 2011-03-16 09:40:35 UTC ---
On Tue, 15 Mar 2011, d.g.gorbachev at gmail dot com wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48127
> 
> --- Comment #3 from Dmitry Gorbachev <d.g.gorbachev at gmail dot com> 2011-03-15 18:00:23 UTC ---
> I do not agree that the testcase is invalid. As the standart says:
> 
>   Common extensions
> 
>   Multiple external definitions
> 
>      There may be more than one external definition for the identifier of
>   an object, with or without the explicit use of the keyword extern; if
>   the definitions disagree, or more than one is initialized, the behavior
>   is undefined.
> 
> According to the Binutils documentation,
> 
>   The linker turns a common symbol into a declaration, if there is a
>   definition of the same variable.
> 
> Also, from Ian Lance Taylor's article [1]:
> 
>   5. If A is a strong definition in an object file:
> 
>     * If B is a common symbol, then we treat B as an undefined reference.
> 
> I think it would be better to fix this problem in the linker. Gold does not
> even warn about it!
> 
> However, I don't understand why GCC makes baz[8] to have 32 bytes alignment,
> while baz[4] has only 4 bytes alignment. It seems to be a GCC bug.

It's an optimization to ensure accesses larger than 4 do not cross
cacheline boundaries.

Richard.

[Bug target/48127] Program crashes reading a non-aligned variable

[d.g.gorbachev at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48127

Dmitry Gorbachev <d.g.gorbachev at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
      Known to work|                            |4.8.4
         Resolution|---                         |FIXED

--- Comment #5 from Dmitry Gorbachev <d.g.gorbachev at gmail dot com> ---
GCC 4.8.4 (and recent 4.9, 4.10) do not generate SSE instructions for this
testcase.