Re: VM jump table address kept in a register

[Ludovic Courtès]

Hi,

Andy Wingo <address@hidden> skribis:

> I looked at specifying registers for the IP, SP, and FP as well.  I got
> positive improvements with IP and FP but not (oddly) with SP.  Should we
> try this?
>
>     #define IP_REG asm ("r13")
>     #define FP_REG asm ("r14")

I tried, and it’s noticeably worse than letting it choose %rbp and
%rbx.

I don’t have any explanation, though.  It could be that loads to/from
r13/r14 are longer to decode, or that it is stressing GCC’s register
allocator in a bad way.

Thanks,
Ludo’.