We've talked about versioning, and I think feature detection is probably the best approach. If the user agent publishes a list of supported asm.js features via a standard query mechanism (note that this is a feature list, not browser detection) -- e.g., ASM.SIMD === true -- then apps can negotiate content accordingly. This might entail compiling multiple versions of your codebase for various versions of asm.js, but that's an acceptable price to pay for VM evolution, IMO.