[FFmpeg-devel] [RFC] Loop unrolling in C code for 'vector_fmul_*' functions

Måns Rullgård mans
Fri Jan 11 01:33:04 CET 2008

Loren Merritt <lorenm at u.washington.edu> writes:

> On Fri, 11 Jan 2008, mark cox wrote:
>> On Jan 11, 2008 7:39 AM, Michael Niedermayer <michaelni at gmx.at> wrote:
>>> Add a assert((len & 7) == 0); and the compiler can know it.
>> That is a really interesting statement. are you saying that gcc will
>> optimize by adding such an assert? This is the first i have heard of this.
>> such code annotations could probably help in many places.
> No, he's saying that the information is then available to the compiler, 
> so the above advice about complaining to the gcc devs about suboptimal 
> code applies.
> But gcc certainly does apply some such optimizations, if not specifically 
> assert. e.g. if you dereference a pointer and then test its truth, the 
> test will be optimized away because it's a constant if the dereference 
> didn't crash.

Is it wise to assume that dereferencing a null pointer will crash?
This is certainly not true on many MMU-less systems.  Even some old
Unix versions (AIX, HP-UX or both, I forget which) allowed reading of
address 0.

M?ns Rullg?rd
mans at mansr.com

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