For both SpiderMonkey and v8, this is counting the number of calls to the MacroAssembler. SpiderMonkey commonly use the prefix "masm", while v8 uses the macro "__ " to alias the MacroAssembler.
The MacroAssembler, is basically what is used to produce assembly code in both JavaScript engines.
$ git clone https://github.com/v8/v8.git $ cd v8 $ find . -name '*.S'
There's still some macro assembler in the built-in's but it's emitted rather than being assembler.