Boxing is very slow on x86_64. Doing a complicated box operation just to write that Value to the stack is silly if the type is known and the payload is 32-bit. This saves about 5.6ms locally on SunSpider (407.5 -> 401.9), but I'm running with very old hardware, so it's entirely likely that nothing at all may happen on AWFY. Who knows. It's a good idea anyway. (I tried V8, but it's even less stable on this Pentium 4 than it is on the i7, making it impossible to get any useful information out of it.)

Comment on attachment 657198 [details] [diff] [review] patch Review of attachment 657198 [details] [diff] [review]: ----------------------------------------------------------------- ::: js/src/ion/x64/MacroAssembler-x64.h @@ +137,4 @@ > void storeValue(const Value &val, const T &dest) { > jsval_layout jv = JSVAL_TO_IMPL(val); > movq(ImmWord(jv.asBits), ScratchReg); > + if (!val.isPrimitive()) This should be val.isMarkable() since strings are primitive