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

H. Peter Anvin
2009-09-28T21:44:18Z

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.

-hpa

[1] Common subcase:

movl global, %reg ; 6 bytes (unless reg is eax on 32 bits)
movl $immed, %reg ; 5 bytes