Messages in this thread | | | Date | Mon, 25 Oct 2010 15:31:05 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] tracing: Cleanup the convoluted softirq tracepoints |
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On 10/21/2010 12:56 PM, Thomas Gleixner wrote: > On Thu, 21 Oct 2010, Steven Rostedt wrote: > >> On Thu, 2010-10-21 at 18:18 +0200, Thomas Gleixner wrote: >>> >>> On Tue, 19 Oct 2010, Steven Rostedt wrote: >>> >>>> On Wed, 2010-10-20 at 00:04 +0200, Thomas Gleixner wrote: >>>> >>>>> hpa just posted code which does the _RIGHT_ _THING_ independent of any >>>>> compiler madness and you tracer folks just missed it. >>>> >>>> Thomas, >>>> >>>> Can you try this patch and see if it makes the object code better? >>> >>> Nope, same result. >> >> Yeah, I figured. Do you have CC_OPTIMIZE_FOR_SIZE set? And if you do, >> what happens if you disable it? > > Hmm. Indeed. That gets rid of the double jump. >
-Os unfortunately drops a bunch of optimizations.
With gcc 4.5.1 there is actually a way to guarantee to get rid of double jumps, which is that you tell gcc that it is branching to one of two targets:
asm goto("1: .byte 0xe9 ; .long %l[t_no]-2f\n" "2:\n" /* patching infrastructure goes here */ : : "i" (bit) : : t_no, t_yes); __builtin_unreachable(); t_no: return false; t_yes: return true;
[The open-coding of the jump is necessary to force the 5-byte form instead of the 2-byte form.]
The patching machinery can recognize the case where the jump offset is zero and patch in a NOP instead.
There does, however, seem to be a couple of problems:
a) gcc 4.5.1 is required due to a bug in previous versions of gcc when an asm goto doesn't have a fallthrough case.
b) it seems to encourage gcc to actively jump around as it reorders blocks, since gcc no longer sees a fallthrough case at all.
Not sure I have a good solution for this, at least not with current gcc.
-hpa
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