Re: [Qemu-devel] [PATCH v2 0/6] ARM: AREG0 conversion

[Richard Henderson <rth@twiddle.net>]

On 03/29/2012 11:42 AM, Laurent Desnogues wrote:
> That will indeed probably make the real problem, which is that
> this patch increases the size of generated code, less obvious
> on small benchmarks that don't put pressure on instruction
> cache.  But the fact is that generated code is larger and will
> have to execute more instructions, so no matter what you do,
> this will have an impact on speed.

While this is true, the benefit of using a more standard calling
convention on reliability and debug-ability is enormous.

Consider the i686 host, where we currently obscond with EBP.
While I'm not aware of any current problems with -O0 or spill
failures under optimization, it's not inconceivable.

Consider sparc-linux host, where we have *no* call-saved global
register at all, and (currently) try very hard to use a call-
clobbered global register, with occasionally disastrous results.
See the patch set I posted recently where I give up on this entirely
and make sparc use a TLS variable instead of a hard register at all.
This regresses the Sparc host on speed for a progression in reliability.
The conversion to explicit env arguments fixes essentially all of
the speed regression since we then receive ENV in %o0 instead of
having to read from TLS.


r~