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Re: disabling branch probability guessing (patch) [another patch]

>>>>> "Andi" == Andi Kleen <> writes:

 > On Tue, Jan 16, 2001 at 08:55:36AM -0800, Joe Buck wrote:
 >> Andi Kleen writes:
 >> > Cool. This is useful for the linux kernel too. I found out that the 
 >> > prediction guessing undoes some of the carefully crafted goto woods
 >> > to keep fast paths jumpless.
 >> I'm not sure that this is really the right solution.  In all examples
 >> in which I've seen gotos used by good programmers, the goto is an
 >> exceptional condition and should be predicted not-taken.  Can't we
 >> just flag branches originally entered as gotos?

 > I agree that that would be a good heuristic. Assume goto as not taken. 

Alright.  I did some playing around to predict gotos as not taken
(patch below).  It seems to work pretty good.

I have a few questions though...

I tested this on a couple of x86 bootstraps.  My "benchmark" was
running a freshly built gcc (with and without the new prediction)
against "gcc/stmt.i" and timing the compilations.

The patch below speeds things up by a few nanoseconds-- if that.  The
unix "time" doesn't seem to be very good for benchmarks.

What program can I use to time other programs?

What's a good way of doing benchmarks?

Is there some kind free benchmarking program I can use?

What's a good architecture to test this out-- preferable one with a
high penalty for missed branch predictions?

And finally, is the patch below correct?  I deduct that gotos are
jumps with text labels.


Index: predict.c
RCS file: /cvs/gcc/egcs/gcc/predict.c,v
retrieving revision 1.13
diff -c -r1.13 predict.c
*** predict.c	2000/05/19 22:27:26	1.13
--- predict.c	2001/01/20 06:47:42
*** 111,117 ****
    for (i = 0; i < n_basic_blocks - 1; i++)
        rtx last_insn = BLOCK_END (i);
!       rtx cond, earliest;
        int prob;
        edge e;
--- 111,117 ----
    for (i = 0; i < n_basic_blocks - 1; i++)
        rtx last_insn = BLOCK_END (i);
!       rtx cond, earliest, label;
        int prob;
        edge e;
*** 121,126 ****
--- 121,135 ----
        if (find_reg_note (last_insn, REG_BR_PROB, 0))
+       /* Predict gotos as false.  */
+       label = JUMP_LABEL (last_insn);
+       if (GET_CODE (label) == CODE_LABEL
+ 	  && XSTR (label, 6))
+ 	{
+ 	  prob = PROB_UNLIKELY;
+ 	  goto emitnote;
+ 	}
        cond = get_condition (last_insn, &earliest);
        if (! cond)

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