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

["solar-gcc at openwall dot com" <gcc-bugzilla@gcc.gnu.org>]

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

--- Comment #12 from Alexander Peslyak <solar-gcc at openwall dot com> ---
(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?

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?

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