Re: [patch 5/8] Immediate Values - x86 Optimization (update)

["H. Peter Anvin" <hpa@zytor.com>]

Mathieu Desnoyers wrote:
> * H. Peter Anvin (hpa@zytor.com) wrote:
> > Mathieu Desnoyers wrote:
> > > * H. Peter Anvin (hpa@zytor.com) wrote:
> > > > Mathieu Desnoyers wrote:
> > > > > > - Use "=g" constraint for char immediate value inline assembly.
> > > > > >
> > > > > > "=g" is the same as "=rmi" which is inherently bogus.  In your actual 
> > > > > > code you use "=r", the correct constraint is "=q".
> > > > > q
> > > > >     Any register accessible as rl. In 32-bit mode, a, b, c, and d; in 
> > > > > 64-bit mode, any integer register. I am worried that "=q" might exclude 
> > > > > the si and di registers in 32-bit mode.
> > > > > What exactly is wrong with "=r" ?
> > > > For "char" (8-bit) values, sp/bp/si/di are illegal in 32-bit mode.
> > > >
> > > > Hence "=q".
> > > Ah! yep, I see, so we say:
> > > 1 byte : "=q"
> > > 2 bytes : "=r"
> > > 4 bytes : "=r"
> > > 8 bytes : "=r"
> > > ? (si and di appear to be legal for 2 and 4 bytes in 32-bit mode)
> > That's right.
> >
> > 	-hpa

Something else to watch out for... in 64-bit mode the lengths most of 
these will depend on which register is used, since whether or not a REX 
prefix is needed will vary.

As far as I can tell, you're assuming fixed length instructions, which 
is wrong unless you manually constrain yourself to only legacy registers.

	-hpa
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/