Re: [patch 02/12] Immediate Values - Architecture Independent Code

Mathieu Desnoyers
2009-09-28T22:10:06Z

* H. Peter Anvin (hpa@zytor.com) wrote:
> On 09/28/2009 01:37 PM, Arjan van de Ven wrote:
> >
> > this makes me wonder what happens when a variable is used in multiple
> > places... that makes the icache overhead multiply right?
> >
>
> On x86, the icache overhead can often be zero or close to zero -- or
> even negative in a fairly common subcase[1] -- simply because you are
> dropping a displacement used to fetch a global variable with an
> immediate in the code itself.
>
> For 8- or 16-bit data items this is even more of a win in terms of
> icache space; for 64-bit data it is always a lose.
>
> It is also worth noting that the way this is implemented as a graft-on
> rather than with compiler support means that the full instruction set
> cannot exploited -- x86 can often use a memory operand or immediate as
> part of an operation. This adds icache pressure.

Indeed, these cases could make good use of compiler support to let
immediate values be added to a wider range of operations. Currently,
being limited to "mov" is somewhat limiting on x86. We could definitely
do better.

Mathieu

>
> -hpa
>
> [1] Common subcase:
>
> movl global, %reg ; 6 bytes (unless reg is eax on 32 bits)
> movl $immed, %reg ; 5 bytes