The result of a few experiments looking at the assembler produced by gcc for eval_frame():

* on PCs, reading the arguments as an unsigned short instead of two bytes is a good win.

* oparg is more "local" with this patch: its value doesn't need to be saved across an iteration of the main loop, allowing it to live in a register only.

* added an explicit "case STOP_CODE:" so that the switch starts at 0 instead of 1 -- that's one instruction less with gcc.

* it seems not to pay off to move reading the argument at the start of each case of an operation that expects one, even though it removes the unpredictable branch "if (HAS_ARG(op))".

This patch should be timed on other platforms to make sure that it doesn't slow things down.  If it does, then only reading the arg as an unsigned short could be checked in -- it is compilation-conditional over the fact that shorts are 2 bytes in little endian order.

By the way, anyone knows why 'stack_pointer' isn't a 'register' local?  I bet it would make a difference on PowerPC, for example, with compilers that care about this keyword.