[Libav-user] gcc auto-vectorisation

Claudio Freire klaussfreire at gmail.com
Tue Feb 26 02:21:11 CET 2013

On Mon, Feb 25, 2013 at 7:34 PM, "René J.V. Bertin" <rjvbertin at gmail.com> wrote:
> On Feb 25, 2013, at 20:19, Claudio Freire wrote:
>> That's because __builtin_assume_aligned isn't being called (most
>> likely, didn't check). That results in **far** sub-optimal
>> vectorization. I don't know about the failing tests though.
> I doubt that call (or rather, token?) is required on OS X, where memory allocations (and stack alignment) are aligned. I know of a case where the absence of the token didn't prevent a very substantial performance gain, but haven't checked if that's always the case.

I wouldn't assume. Even if they are in effect aligned, if the compiler
doesn't know it (ie, if malloc doesn't mark them as such),
vectorization will still assume out-of-alignment access.

Architecture-mandated and SSE/2/3/MMX/Whatever alignment requirements
tend to be different.

You can write a very simple test case to check it out.

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