Re: Immediate values

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

Jason Baron wrote:
> 
> right we've proposed an alternative to the immediate values, which I've
> been calling 'jump label', here:
> 
> http://marc.info/?l=linux-kernel&m=125200966226921&w=2
> 
> The basic idea is that gcc, 4.5 will have support for an 'asm goto'
> construct which can refer to c code labels. Thus, we can replace a nop
> in the code stream with a 'jmp' instruction to various branch targets.
> 
> In terms of a comparison between the two, IMO, I think that the syntax
> for the immediate variables can be more readable, since it just looks
> like a conditional expression.
> 
> The immediate values do a 'mov', 'test' and then a jump, whereas jump
> label can just do a jump. So in this respect, I believe jump label can
> be more optimal. Additinally, if we want to mark sections 'cold' so they
> don't impact the istruction cache, the jump label already has the labels
> for doing so. Obviously, a performance comparison would be interesting
> as well.
> 

Direct jumps should at least theoretically be able to have better 
performance, but it would still be nice to have measurements of both.

	-hpa