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    SubjectRe: [patch 5/8] Immediate Values - x86 Optimization (update)
    * H. Peter Anvin (hpa@zytor.com) wrote:
    > Mathieu Desnoyers wrote:
    >> Andi seemed to trust gas stability and you answered:
    >> "The comment was referring to x86-64, but I incorrectly remembered that
    >> applying to "movq $imm,%reg" as opposed to loading from an absolute
    >> address. gas actually has a special opcode (movabs) for the 64-bit
    >> version of the latter variant, which is only available with %rax and its
    >> subregisters.
    >> Nevermind, in other words. It's still true, though, that the immediate
    >> will always be the last thing in the instruction -- that's a fixture of
    >> the instruction format."
    >> So, in the end, is there a way to make x86_64 use a fixed-size opcode
    >> for the 1, 2, 4 and 8 bytes load immediates or we will have to force the
    >> use of a specific register ?
    >> (and we can't take a pointer from the end of the instruction, because we
    >> need to align the immediate value correctly)
    >
    > For a 64-bit load, you'll always have a REX prefix. For 8-, 16- and 32-bit
    > load, the length of the instruction will depend on the register chosen,
    > unless you constrain to either all legacy or all upper registers, or you
    > force gas to generate a prefix, but I don't think there is a way to do that
    > that will work with assemblers all the way back to 2.12, which is at least
    > what we officially support (I have no idea if assemblers that far back
    > actually *work*, mind you.)
    >
    > -hpa

    Ok, so the most flexible solution that I see, that should fit for both
    i386 and x86_64 would be :


    1 byte : "=Q" : Any register accessible as rh: a, b, c, and d.
    2, 4 bytes : "=R" : Legacy register—the eight integer registers available
    on all i386 processors (a, b, c, d, si, di, bp, sp).
    8 bytes : (only for x86_64)
    "=r" : A register operand is allowed provided that it is in a
    general register.

    That should make sure x86_64 won't try to use REX prefixed opcodes for
    1, 2 and 4 bytes values.

    Does it make sense ?

    Mathieu


    --
    Mathieu Desnoyers
    Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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