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From: "whaley at cs dot utsa dot edu" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug fortran/38496] New: Gcc misaligns arrays when stack is forced follow the x8632 ABI Date: Thu, 11 Dec 2008 22:54:00 -0000 [thread overview] Message-ID: <bug-38496-12761@http.gcc.gnu.org/bugzilla/> (raw) I notice that gcc does not follow the 32-bit ABI for the x86, in that it forces the stack alignment to 16 bytes rather than the ABI-required 4 bytes. This is a problem when interacting with compilers that are ABI-compliant. So, I threw the -mpreferred-stack-boundary=2 on gcc/gfortran/g77 to restore ABI compliance, and then I notice that gfortran/g77 generates double precision arrays that are not aligned to 8-byte boundaries, which breaks pretty much all interoperability as well (and also means you will get cache splits all the time, and so decrease your performance). The -malign-double flag does not fix this. So I now have the choice of not interoperating due to ABI violation, or not interoperating with other compilers due to lack of data alignment. I read the gcc manual on this point, and it talks like you have to choose: either you force the stack to always be aligned to 16-byte boundary, or gcc is unable to guarantee alignment for any stack value. However, this is a false choice: given an unaligned stack it is trivial for a function to force a particular alignment (eg., 16-byte) for some values it is storing on the stack. I can provide code and pointers, if that is helpful. So, I think it is a bad idea for gcc to violate the ABI for something that is trivial to do without violating the ABI: not only do you rule out interoperability by not following the ABI, but even in strict gcc code, you lose a lot of performance when people need to follow the ABI. However, if the preferred stack trick worked, I could work around, but since it actually makes arrays not be aligned to a multiple of their base size, I cannot find a workaround for this misfeature. Any ideas? Thanks, Clint -- Summary: Gcc misaligns arrays when stack is forced follow the x8632 ABI Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: fortran AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: whaley at cs dot utsa dot edu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38496
next reply other threads:[~2008-12-11 22:54 UTC|newest] Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top 2008-12-11 22:54 whaley at cs dot utsa dot edu [this message] 2008-12-11 23:03 ` [Bug fortran/38496] " whaley at cs dot utsa dot edu 2008-12-11 23:05 ` [Bug target/38496] " pinskia at gcc dot gnu dot org 2008-12-11 23:06 ` pinskia at gcc dot gnu dot org 2008-12-11 23:26 ` whaley at cs dot utsa dot edu 2008-12-11 23:29 ` pinskia at gcc dot gnu dot org 2008-12-11 23:44 ` whaley at cs dot utsa dot edu 2008-12-12 0:02 ` joseph at codesourcery dot com 2008-12-12 0:53 ` whaley at cs dot utsa dot edu 2008-12-12 1:06 ` hjl dot tools at gmail dot com 2008-12-12 1:26 ` joseph at codesourcery dot com 2008-12-12 1:50 ` whaley at cs dot utsa dot edu 2008-12-15 0:23 ` joseph at codesourcery dot com 2008-12-15 14:54 ` whaley at cs dot utsa dot edu 2008-12-15 18:19 ` joseph at codesourcery dot com 2008-12-15 21:34 ` whaley at cs dot utsa dot edu 2008-12-15 21:39 ` Andrew Thomas Pinski 2008-12-15 21:40 ` pinskia at gmail dot com 2008-12-15 22:03 ` whaley at cs dot utsa dot edu 2008-12-15 23:02 ` Andrew Pinski 2008-12-15 23:03 ` pinskia at gmail dot com 2008-12-15 23:40 ` whaley at cs dot utsa dot edu 2008-12-16 0:10 ` joseph at codesourcery dot com 2009-03-18 13:25 ` sliwa at cft dot edu dot pl 2009-07-23 13:14 ` jakub at gcc dot gnu dot org 2009-07-23 14:35 ` mikulas at artax dot karlin dot mff dot cuni dot cz 2009-07-24 12:16 ` rguenth at gcc dot gnu dot org 2009-07-24 17:05 ` whaley at cs dot utsa dot edu 2009-07-31 1:18 ` mikulas at artax dot karlin dot mff dot cuni dot cz 2009-10-23 9:45 ` rguenth at gcc dot gnu dot org
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