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Re: [PATCH] PR/7344, O(n^2) branch prediction


On Sat, 7 Sep 2002, Brad Lucier wrote:

> Did you test this new patch to see if it reduced the long runtimes
> you reported for branch prediction?  I ran it on the test-case in
> 
> http://gcc.gnu.org/ml/gcc/2002-08/msg01559.html
> 
> and discovered no difference in run times.
> 
> Brad
> 
> > I think you're right. Here is the revised patch.
> > 
> > 2002-09-06  Toshiyasu Morita  <toshiyasu.morita@hsa.hitachi.com>
> > 
> > 	* predict.c (process_note_prediction): Ignore multiway
> > 	branches to avoid excessively long compile times (PR/7344)
> 

This patch is to fix PR/7344, so it doesn't fix any other problems.

Yep, I know there are many other problems. The testcase I "fixed" was
reduced from infinite compile time down to about 2.x minutes.

If you look at the PR/7344 testcase there really isn't any code in it
except for a switch statement. Yet GCSE winds up taking about 30% of the
2.x minutes of compile time for some reason, and instruction scheduling
winds up another 30% or so.

That testcase really shouldn't take more than a few seconds to compile
IMHO.

Toshi



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