Re: STRICT_ALIGNMENT is too strict for m68k

[Kamil Iskra <kiskra@ernie.icslab.agh.edu.pl>]

On Mon, 18 May 1998, Jim Wilson wrote:

> 	Seems to me that there should be a separate macro for controling
> 	the alignment of members in structures, leaving STRICT_ALIGNMENT
> 	for indicating if unaligned accesses to memory will trap.
> Actually, there is already a second macro in use by expmed.c/expr.c called
> SLOW_UNALIGNED_ACCESS, which defaults to STRICT_ALIGNMENT.  He could perhaps
> make m68k.h define this macro, and modify other places if necessary to
> use SLOW_UNALIGNED_ACCESS instead of STRICT_ALIGNMENT.  That would avoid
> the ABI issue without creating a new macro.

According to tm.texi:

@item SLOW_UNALIGNED_ACCESS
Define this macro to be the value 1 if unaligned accesses have a cost
many times greater than aligned accesses, for example if they are
emulated in a trap handler.

Please, let's not repeat the STRICT_ALIGNMENT problem by modifying
functionality of yet another simple macro in a confusing way.

IMHO, there is little use for SLOW_UNALIGNED_ACCESS, the way it is
currently defined, since:

1. with -m68000, unaligned access will simply crash.
2. with -m68020, unaligned access will not crash and will not be emulated
by a trap handler: it will merely run slightly slower.

The macro that SHOULD apply in this case is STRICT_ALIGNMENT. As pointed
out by the Cygnus people (thanks for the testcase, Jim: I observed the
BLKmode <-> ?Imode differences, but I wasn't aware of their consequences),
it can't be used, since (contrary to the way it is currently documented)
it is ABI unsafe.

Therefore, I think that what we need is a new macro, like
USE_STRICT_ALIGNMENT_ABI or sth, that, when defined and non-zero simulates
the ABI changes imposed by STRICT_ALIGNMENT (mode, alignment, register
arguments), but does not force unaligned accesses to be made scattered
into byte-accesses.

If done carefully, this would allow us to alter m68k.h in ABI-compatible
manner.

Does this sound OK for you guys? I am willing to contribute the necessary
changes.

/ Kamil Iskra    AmigaOS  Linux/i386  Linux/m68k               \
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