x86 and SIMD extensions

The approach I’m taking for the tinderbox is to not use any special optimisation flag in it, even though I could use it to test stuff that can get tricky, such as -ftracer. It is important to note that it doesn’t even set any particular architecture, meaning that the compiler is told to use the default i686 architecture without any SIMD extension enabled.

Why is this relevant? Well, there seems to be some trouble with this idea. A number of packages, both multimedia and not, seem to rely to the presence of some of those instructions. The problem is not when they actually insert literal ASM code using said extensions, as that is passing right through the compiler and sent directly to the assembler program, which is always supporting the whole set of extensions, at least for what concerns x86, for what I can tell (other architectures seem to have more constrained assemblers).

The problem is that a C compiler such as GCC provides an “higher-level” interface to the processor’s: intrinsics. Intrinsics allow to write C code that calls instructions directly on the C objects, without using volatile ASM literals, and reusing similar syntax between MMX/SSE or other extensions. This is not really a common way to implement SIMD-based optimisations, as most of the developers involved in hard optimisation, such as in libav, prefer writing their own code directly, tweaking it depending on the processor’s design and not just on the extension used. As you can see in libav’s sources, the most recent optimisations have been written directly in assembly source files compiled with yasm rather than in C files.

Unfortunately, this is hindering my tinderboxing efforts, as it is going to stop me from installing in the container some of the packages, which in turn means that their dependencies can’t be installed. Now it is true that many times this situation is mostly something that the developer didn’t consider, and can be fixed, but in a number of projects there is no intention to remove the blocker, as they refuse to support older CPUs without said SIMD, mostly because of performance reasons.

I’m not sure how to proceed here: I could either change the tinderbox’s flags so that SSE is enabled, which would reflect your average modern system, or I could keep filing bugs for those packages until we sort out a way to deal with them. Suggestions are definitely welcome.