Messages in this thread | | | Date | Mon, 28 Apr 2008 14:48:33 +0200 | From | Ingo Molnar <> | Subject | Re: [patch 0/2] Immediate Values - jump patching update |
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* 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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