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Re: movmem pattern and missed alignment
On Tue, Oct 9, 2018 at 8:41 AM Eric Botcazou <ebotcazou@adacore.com> wrote:
>
> > It's not quite obvious what SSE has to do with this - any hint please?
>
> SSE introduced alignment constraints into the non-strict-alignment target x86
> so people didn't really want to play by the rules of strict-alignment targets.
Yeah. We've walked back and forth for that very issue though. We now require
all targest to play by the same rules -- if you have a *(double *) access then
that has to be aligned according to double.
We couldn't realistically walk back and rely on alignment of addresses based
on their type (like C would allow us to do) because we've thrown away types
on addresses. See also the thread about string-length warning stuff where
we've posted testcases that show you can get arbitrarily typed addresses
into your strlen() calls for example by means of CSE. The middle-end is
simply not prepared to preserve that information.
It was repeatedly suggested that we _could_ derive alignment info from
function parameter types since we rely on precise typing there for example
for points-to analysis (albeit only for restrict qualification processing and
for DECL_BY_REFERENCE "pointers"). That would fix the simple testcase
that was presented here.
> > (according to my quick check this changed between gcc-4.5 and gcc-4.6)
>
> Possibly indeed, I remembered GCC 4.5 as being the turning point.
It was really changing over several releases, but yes.
Richard.
>
> --
> Eric Botcazou