You're right, sorry. I concentrate on integer operations, which are mostly restricted to 128-bit on AVX, and had forgotten that it also supported 256-bit floating point. I'm not sure which (if either) would be most relevant to a web browser. I hope this error doesn't distract too much from my overall point.

AVX does not add anything to integer operation, like SSE. 128-bit integer is SSE2, which is part of amd64.

Well, AVX does explicitly add 256-bit integer loads and stores: https://software.intel.com/sites/landingpage/IntrinsicsGuide....

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.

Also AVX has shuffles which are interesting for integer code.