Firefox can't blindly use AVX without checking for its presence or it will crash on these types of systems.

EDIT: I should have clarified, the way the author of the article phrased it at first, I got the impression Clang did better auto-vectorization with Firefox code than GCC.

The closest I can find is Valve's hardware survey, which says that 87% of users are running on computers that support AVX. https://store.steampowered.com/hwsurvey (click on "Other Settings" on the bottom to expand).

Firefox is presumably lower than this, although I don't know by how much. Does Mozilla collect statistics on the capabilities of the computers that Firefox is being run on?

> Moreover GCC -O2 defaults are (in my opinion unfortunately) still not enabling vectorization and unrolling which may have noticeable effects on benchmarks.

... which makes it sound like Clang does a better job at this with Firefox code.

- first how you set -O2 defaults in your compiler. This is a delicate problem since you need to find right balance of code size, compile time, robustness of generated code (do not trigger undefined effect in super evil ways) and of course runtime. In benchmarks I have found that Clang has bit of edge for runtime which is mostly vectorization (on x86-64)

- selection of minimal ISA you support. For GCC x86-64 is still the original Opteron, but distributions can easily (and some do) decide for better. Indeed AVX is big win, but for general purpose distribution this is still too agressive. You can provide AVX optimized libraries where it depends

- selection of CPU tunning (i.e. generic/intel)

So I consider it mistake that GCC traded vectorization over compile time speed+reliablity for -O2 because it can make important difference in common workloads this days (not 10 years ago, say).

It is also clearly a bug for GCC to produce AVX instruction when not explicitly asked for :)

I also do testing on Zen, Core and some PowerPC. For the firefox machine I use Buldozer box because I don't care it spends long nights running builds & benchmarks and I think this particular problem is not very CPU specific.

Yes, this seems like one reasonable approach. The current approach of compiling to a "least common denominator" and then updating this denominator every decade or so seems insufficient. Instead of interpreting the absence of a "-march=" flag to mean optimize for nothing, maybe it can mean that multiple optimizations are automatically compiled and the appropriate one is selected at runtime. Alternatively, maybe we need to move away from the idea of compiling a single binary that runs on all platforms, and encourage greater use of platform specific compilations.

And some of the Boolean operations can be quite useful on integers. So even for integers, I was definitely wrong to characterize AVX as 128-bit.

This leads to issues like you have on Mac: When using safari, the lower consumption is super low. With Chrome you don't get nearly the runtime (when playing video or listing to music).

As 'hubicka' mentions in another comment, GCC does have "multi-versioning" capability, but it doesn't use it by default. Instead, one needs to mark individual functions with GCC specific attributes, asking for versions with different capabilities to be created: https://lwn.net/Articles/691932/. This isn't necessarily a bad approach, but the fact that it seems be used rarely makes me wonder if some other more default approach that works with unmodified code might be an improvement.