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

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.