Re: [patch 4/7] Immediate Values - i386 Optimization

[Andi Kleen <ak@muc.de>]

> - Either I use a "r" constraint and let gcc produce the instructions,
>   that I need to assume to have correct size so I can align their
>   immediate values (therefore, taking the offset from the end of the
>   instruction will not help). Here, if gas changes its behavior
>   dramatically for a given immediate value size, it will break.

I wouldn't expect it to do that, but you could perhaps add a self
test somewhere to check for it.

> 
> - Second choice is to stick to a particular register, choosing the one
>   with the less side-effect, and encoding the instruction ourselves. I
>   start to think that this second solution might be safer, even though
>   we wouldn't let the compiler select the register which has the less
>   impact by itself.

Such effects caused occassional bugs in the alternative() implementation
which requires a maximum size for the replacement.

But in this case it should be safe enough to trust gas stability.

-Andi
-
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/