Re: PATCH: Remove SLOW_BYTE_ACCESS from get_best_mode

[Paolo Bonzini <paolo dot bonzini at lu dot unisi dot ch>]

> I don't understand why we use byte instruction to access 32bit field.
> There may be some hardwares where 4 byte accesses are faster than one
> 32bit access. But I am not aware any x86 one. I tried this patch on
> Prescott, Nocona, Core and Core 2. There are no regressions in gcc
> testsuites nor negative performance impact on SPEC CPU 2K.

First, the patch should have subject "Rename ..." rather than "Remove". 
 As it is, it conveys the idea that there is no differentiation anymore 
in get_best_mode.

Second, I don't like the name FAST_BYTE_ACCESS_BITFIELD.  It could be 
named SLOW_BYTE_ACCESS_BITFIELD (switching the default definition and 
use to remove the !), or WORD_ACCESS_BITFIELD.

Third, documentation is needed for the new macro.

Fourth, the default value for the new target macro should be in 
defaults.h rather than in stor-layout.c.

Fifth, it seems plausible to me that SLOW_BYTE_ACCESS would still win if 
there was only one bitfield accessed.  dojump.c is using byte 
instructions when jumping, i.e. when obviously you access only one 
field, and it is a win there.  I think the changes in twolf should be 
analyzed more carefully before proposing a patch, to identify the 
pessimization there and whether it could happen with the proposed patch 
on similar code.

Paolo