https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88709
Ramana Radhakrishnan <ramana at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |ramana at gcc dot gnu.org
--- Comment #13 from Ramana Radhakrishnan <ramana at gcc dot gnu.org> ---
(In reply to Richard Earnshaw from comment #9)
> (In reply to Jakub Jelinek from comment #7)
> > (In reply to Christophe Lyon from comment #6)
> > > I've noticed that the new test store_merging_29.c fails on
> > > arm-none-eabi --with-cpu cortex-a9
> > > FAIL: gcc.dg/store_merging_29.c scan-tree-dump store-merging "New sequence
> > > of 3 stores to replace old one of 6 stores"
> >
> > That is because target-supports.exp lies on arm, even for -mcpu=cortex-a9
> > STRICT_ALIGNMENT is 1 on arm (it is 1 unconditionally), but
> > check_effective_target_non_strict_align returns true on arm anyway.
> > I've already added hacks for it in r256783 in another testcase though, guess
> > I'll do something similar now, but I must say I'm not very excited about
> > that.
>
> Support for misaligned accesses is a three(.5!)-valued problem:
>
> 1) There's no support in the architecture at all
> 2) There's some support with a limited set of instructions
> 3) There's full support: any memory access can handle any alignment.
> 3.5) There's full support: but some accesses may be very slow
>
> I would think that these days most CPU architectures actually fall into
> either 1 or 2. Many architectures have limitations, for example on atomic
> accesses that are unaligned.
>
> STRICT_ALIGNMENT only covers, in reality case 3. I'm not even sure if it
> would be defined on a machine with case 3.5.
>
> I think the real problem here is that it's not clear what question this
> target-supports macro is really asking - does the CPU have the capability to
> do (some) unaligned acceses? or can it arbitrarily support casts from
> unaligned pointers to standard types?
I agree - it sounds like this should be put in a comment next to the
target-supports query.
