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


    [1] Common subcase:

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

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