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    SubjectRe: Code optimization <LEA Instruction>
    On Thu, Jan 27, 2000 at 05:01:48PM -0500, Richard B. Johnson wrote:
    > The Intel reference manual tells about optimizing code.
    > One of the things it states is that the LEA instruction can
    > be used to change the value of an index register faster than
    > using the ADD instruction (Page G-10, Intel '486 Rag).

    yes...

    > I note that recent 'C' compilers do this. There are a lot of
    > LEA instructions that have replaced simple adds for adding
    > a constant displacement to an index register.

    ok...

    > As usual, the Intel reference manual is wrong for all types
    > of CPUs that I have tested (486-686).

    with you...

    > I wish somebody would have tested this information before wrong
    > information went into the code-generation of the 'C' compilers.

    uh huh...

    > The following program clearly shows that many more 'addl' instructions
    > may be executed than 'leal' instructions within a given time.

    [snip example]

    > The main point is, when attempting to optimize code, it would be
    > a good idea to test the many possible instruction-sequences before
    > you commit yourself, especially when providing inline 'asm' or
    > generating compiler output.

    umm...

    I still don't see the relevance to linux-kernel (which is not x86
    specific and is not particularly tied to the intimate details of the gcc
    x86 backend). Perhaps you meant to send this to gcc-bug or take it up
    with the gcc x86 backend maintainer in personal mail instead?

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