[Libav-user] a little performance/optimisation headbreaker :)

[Claudio Freire]

On Fri, Feb 15, 2013 at 12:48 PM, "René J.V. Bertin"
<rjvbertin at gmail.com> wrote:
> On Feb 15, 2013, at 16:33, Claudio Freire wrote:
>
>> gcc, which tends to inhibit many of its other optimizations. Why don't
>> you try gcc's vector primitives instead?
>
> Which ones?

Well, you specify "memory" so it will inhibit all redundant load
optimizations, and probably play havoc with aliasing too, so it will
hurt code performance all around it most certainly.

> As in the few lines with intrinsics for MSVC, which also compile under gcc but shows no speed dis/advantage with gcc ?

SSE intrinsics (and automatic vectorization in fact) perform a lot
more poorly if you don't use __builtin_assume_aligned[0]


[0] http://gcc.gnu.org/projects/tree-ssa/vectorization.html#assume-aligned