[SeaBIOS] [PATCHv3 0/2] Emulate "leal" instruction to workaround x86emu bugs

Paolo Bonzini pbonzini at redhat.com
Mon Apr 13 16:40:52 CEST 2015

On 11/04/2015 20:07, Kevin O'Connor wrote:
> > > With a few additional checks it's possible to emulate all the leal
> > > cases without requiring a function call.  Although this makes the
> > > vgafixup.py code a little more complex it eliminates the need for the
> > > "emulate_leal" function and it produces better code.  In my tests,
> > > almost all "leal" instructions are replaced with 4 (or fewer)
> > > instructions.
> > 
> > Maybe it is possible to tell gcc to not produce this instruction
> > in the first place?
> That would certainly be better.  Any idea how to do that?

Not really.  IIRC, GCC really likes having a way to do additions that
doesn't modify the flags.

When porting to an architectures that does not support that (e.g. some
old CISC ones), you have to use a completely different mechanism to
describe conditional branches and stores---which is legacy and much less
optimizable than the newer mechanism.  x86 uses the new one.


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