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SubjectRe: [patch 0/2] Immediate Values - jump patching update

* Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:

> Hi Ingo,
>
> Here is the update to the jump patching optimization taking care of
> Peter's comments about register liveliness and instruction re-use by
> gcc optimizations. A good thing : it actually simplifies the code.
> Unfortunately, it adds 3 bytes to the instructions in i-cache because
> I now have to use a 5-bytes mov instruction so I can replace it with a
> 5-bytes jump. Therefore, 9 bytes are added to rather small functions
> (5-bytes mov + 2-bytes test + 2 bytes conditional branch) and 13 bytes
> are added to larger functions which needs a 6 bytes conditional branch
> at the branch site.
>
> Instead of having to execute a sequence of nop, nop and jump, we now
> only have to execute the near jump, which jumps either at the address
> following the conditional branch or at the target address of the
> conditional branch, depending on the immediate value variable state.
>
> Thanks to Peter for the review.

thanks Mathieu, i've queued them up for more testing. Your previous
queue already looked good here so i pushed it out into sched-devel.git
as you probably noticed.

Sidenote, without trying to bikeshed paint this issue too much: are we
absolutely sure that (on modern CPU architectures) such a short jump is
better than just 2-3 NOPs in a sequence? It's a minor (sub-cycle) detail
in any case.

Ingo


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