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    SubjectRe: [PATCH] Make shr to divide by power of 2
    On Fri, Aug 07, 2009 at 09:09:36PM -0600, Robert Hancock wrote:
    > On 08/07/2009 12:50 AM, Andi Kleen wrote:
    >> Sergey Senozhatsky<sergey.senozhatsky@gmail.com> writes:
    >>> Of course, compiler supposed to do something like
    >>> add $0x7,%eax
    >>> shr $0x3,%eax
    >>> instead of div. Should we rely on this?
    >>
    >> Yes. We should rely on this.
    >
    > It may depend on the selected CPU type that gcc is optimizing for - I
    > believe that on some P4s the shift may actually be slower than the divide..

    DIV should be always slower than a SHIFT.

    But it has nothing really to do with the CPU. The point is that the compiler
    always selects a suitable one by itself. Rewriting x / 2 to x >> 1 is
    one of the easiest exercises in compiler optimizations.

    The only case when the compiler cannot do this easily by itself is
    when the dividend is not a constant.

    That said -Os sometimes screws us up on this, but it's still not worth
    doing this change manually.

    -Andi

    --
    ak@linux.intel.com -- Speaking for myself only.


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