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SubjectRe: {PATCH] Re: wasting time on page fault


On Tue, 10 Oct 2000, Jamie Lokier wrote:
>
> The assembler doesn't use nops for alignment -- it inserts longer
> instructions that are effectively nops, either 1 or two. For larger
> stretches, the assembler inserts a jmp itself for alignment.

Note that some of them are not very good no-ops. At least at some point
the "long" no-op was

lea 0x00000000(%esi),%esi

and you could cause AGI stalls and non-pairing with two of them in a row -
making the long no-op sequence potentially quite slow. The one-byte no-op
is recognized by the CPU as being a no-op (even though technically it
originally was a "xchg ax,ax" instruction decoding-wise - it doesn't
actually get executed as that in any modern CPU and thus avoids all the
dependencies implied in the traditional decode).

It may be that the special "lea" no-op is also recognized as such in newer
CPU's, but it wasn't in the early ones.

Linus

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