[Bug tree-optimization/51017] GCC 4.6 performance regression (vs. 4.4/4.5), PRE increases register pressure

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51017

--- Comment #15 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Alexander Peslyak from comment #12)
> (In reply to Richard Biener from comment #11)
> > I wonder if you could share the exact CPU type you are using?
> 
> This is on (dual) Xeon E5420 (using only one core for these benchmarks), but
> there was similar slowdown with GCC 4.6 on other Core 2'ish CPUs as well
> (such as desktop Core 2 Duo CPUs). You might not call these "modern".
> 
> > Note that we have to use movups because [...]
> 
> Thank you for looking into this. I still have a question, though: does this
> mean you're treating older GCC's behavior, where it dared to use movaps
> anyway, a bug?

If you used intrinsics for aligned loads then no.

> I was under impression that with most SSE*/AVX* intrinsics (except for those
> explicitly defined to do unaligned loads/stores) natural alignment is
> assumed and is supposed to be provided by the programmer. Not only with GCC,
> but with compilers for x86(-64) in general. I thought this was part of the
> contract: I use intrinsics and I guarantee alignment. (Things would
> certainly not work for me at least with older GCC if I assumed the compiler
> would use unaligned loads whenever it was unsure of alignment.) Was I wrong,
> or has this changed (in GCC? or in some compiler-neutral specification?), or
> is GCC wrong in not assuming alignment now?

GCC was changed to be more permissive to broken programs and also intrinsics
were changed to map to plain C code in some cases (thus they are not visible
as intrinsics to the compiler).

> Is there a command-line option to ask GCC to assume alignment, like it did
> before?

No.