asm.js is now validated and compiled to WebAssembly
If V8 encounters asm.js code it now tries to validate it. Valid asm.js code is then transpiled to WebAssembly. According to V8:undefined:’:undefined:s performance evaluations, this generally boosts throughput performance. Due to the added validation step, isolated regressions in startup performance might happen.
Read the full article: https://v8project.blogspot.jp/2017/08/v8-release-61.html