[Python-Dev] short fetch for NEXTARG macro (was: one byte byte code arguments)

[Cesare Di Mauro]

2011/1/31 Antoine Pitrou <solipsis at pitrou.net>

> On Mon, 31 Jan 2011 13:28:39 +0100
> "Jurjen N.E. Bos" <Jurjen.Bos at hetnet.nl> wrote:
> > I just did it: my first python source code hack.
> > I replaced the NEXTARG and PEEKARG macros in ceval.c using a cast to
> > short pointer, and lo and behold, a crude measurement indicates one
> > to two percent speed increase.
> > That isn't much, but it is virtually for free!
> >
> > Here are the macro's I used:
> > #define NEXTARG() (next_instr +=2, *(short*)&next_instr[-2])
> > #define PEEKARG() (*(short*)&next_instr[1])
>
> Some architectures forbid unaligned access, so this can't be used as-is.
>
> Regards
>
> Antoine.
>
>
WPython already addressed it (
http://code.google.com/p/wpython2/source/browse/Python/ceval.c?repo=wpython11):

#ifdef WORDS_BIGENDIAN
#define NEXTOPCODE() oparg = *next_instr++; \
opcode = oparg >> 8; oparg &= 0xff
#else
#define NEXTOPCODE() oparg = *next_instr++; \
opcode = oparg & 0xff; oparg >>= 8
#endif

Shorts alignament is also guaranted due to wordcodes (
http://wpython2.googlecode.com/files/Beyond%20Bytecode%20-%20A%20Wordcode-based%20Python.pdfpag.12).

Cesare
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