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

[Mathieu Desnoyers]

* H. Peter Anvin (hpa@xxxxxxxxx) 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
> 

-- 
Mathieu Desnoyers
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