ABI voilation

[Andrew Haley]

Dorit Nuzman writes:
 > > Tim Prince writes:
 > >  > kanishk rastogi wrote:
 > >  > > hi all,
 > >  > > I wanted to know if when i specify -O[0-3] flag to gcc does gcc
 > >  > > optimize the code even violating ABI for that specific arch.
 > >  > > If yes can i specify gcc not to violate the ABI ....
 > >  > > I am mostly concerned witht he ABI specification which tells how to
 > >  > > call another function..
 > >  >
 > >  > Only the -Os changes the value of -mpreferred-stack-boundary, breaking
 > >  > the ability to support vectorization or other alignment dependent code
 > >  > in the callee.
 > >
 > > -Os breaks vectorization?  That would be A Bug, for sure.  Do you have
 > > references, Bugzilla entries, for this?
 > 
 > I don't think -mpreferred-stack-boundary breaks vectorization, but it can
 > affect vectorization because vectorization is sensitive to alignment, and
 > the ability to force the alignment of arrays on the stack currently depends
 > on the preferred stack boundary. You can see
 > http://gcc.gnu.org/ml/gcc-patches/2007-10/msg00202.html for more details.

Geer Bosch asked "Can't we just explicitly align the stack frame to
128-bits during the function prologue, when we know that there are
locals that require such alignment?"  This seems so obvious that I
can't understand why it hasn't been done.

Andrew.