Antoine, in my testing the "loss" of the HAS_ARG() optimisation in my
patch appears to have negligible cost on i386, but starts to look
significant on amd64.

On an Intel E8200 cpu running FreeBSD 7.1 amd64, with gcc 7.2.1 and the
3.1a2 sources, the computed goto version is ~8% faster (average time of
all rounds) for pybench (with warp factor set to 3 rather than the
default 10, to get the round time up over 10s) than without computed
gotos.  With my patch applied, the computed goto version is ~5.5% faster
than without computed gotos by the same measure.  On this platform,
Pystone rates at ~86k (no computed gotos), ~85k (computed gotos) and
~82k (computed gotos + my patch).

For comparison, this machine running Windows XP (32 bit) with the
python.org builds rates ~92k pystones for 2.6.1 and ~81k for 3.1a2. 
Pybench isn't distributed in the MSI installers :-(