Jakub Jelinek - Re: s390{,x} ABI incompatibility between gcc 4.0 and 4.1 (original) (raw)

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• From: Jakub Jelinek
• To: Joern RENNECKE
• Cc: Richard Henderson , Ulrich Weigand , gcc at gcc dot gnu dot org
• Date: Wed, 30 Nov 2005 05:27:25 -0500
• Subject: Re: s390{,x} ABI incompatibility between gcc 4.0 and 4.1
• References: <20051128144923.GK31785@devserv.devel.redhat.com> <438C5B4C.7030007@st.com> <20051129140018.GY31785@devserv.devel.redhat.com> <438CCFB5.6070007@st.com>
• Reply-to: Jakub Jelinek

On Tue, Nov 29, 2005 at 10:01:25PM +0000, Joern RENNECKE wrote:

If we use MIN (tree_low_cst (TYPE_SIZE (type), 0), BIGGEST_ALIGNMENT) here, I'm afraid that would be much bigger ABI incompatibility. Currently, say typedef char attribute((vector_size (64))) v64qi; is 64 bytes aligned on most arches, even when BIGGEST_ALIGNMENT is much smaller. GCC 4.0.x on s390{,x} aligned vector_size 1/2/4/8/64/128/... types to their size, just vector_size 16 and 32 has been 8 bytes aligned (BIGGEST_ALIGNMENT).

That sounds very strange. Is there a rationale for that, or is this a SNAFU?

It is just a side-effect of the 3.4/4.0 code - if there was a supported integer mode on the target for the requested size, then alignment of that mode was used (and mode alignments are at most BIGGEST_ALIGNMENT). If no integer mode was supported for that size, it would use the earlier alignment, i.e. vector_size. Unfortunately, while say vector_size (64) etc. vectors are IMHO very unlikely, vector_size (16) will occurr in user programs from time to time and thus the ABI incompatibility might affect existing programs. We can document the incompatibility as a feature (after all, getting rid of that alignment anomaly on s390{,x} wouldn't be a bad thing I guess).

Not capping to BIGGEST_ALIGNMENT might have issues with some object formats though, if they don't support ridiculously big aligments.

We have MAX_OFILE_ALIGNMENT for that.

But is it used for vector type alignments?

Jakub

• Follow-Ups:
  • Re: s390{,x} ABI incompatibility between gcc 4.0 and 4.1
    * From: Joern RENNECKE
  • Re: s390{,x} ABI incompatibility between gcc 4.0 and 4.1
    * From: Paolo Bonzini
• References:
  • s390{,x} ABI incompatibility between gcc 4.0 and 4.1
    * From: Jakub Jelinek
  • Re: s390{,x} ABI incompatibility between gcc 4.0 and 4.1
    * From: Joern RENNECKE
  • Re: s390{,x} ABI incompatibility between gcc 4.0 and 4.1
    * From: Jakub Jelinek
  • Re: s390{,x} ABI incompatibility between gcc 4.0 and 4.1
    * From: Joern RENNECKE