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Re: [PATCH AutoFDO]Restoring indirect call value profile transformation


On Sun, Dec 16, 2018 at 9:11 AM Andi Kleen <ak@linux.intel.com> wrote:
>
> "bin.cheng" <bin.cheng@linux.alibaba.com> writes:
>
> > Hi,
> >
> > Due to ICE and mal-functional bugs, indirect call value profile transformation
> > is disabled on GCC-7/8/trunk.  This patch restores the transformation.  The
> > main issue is AutoFDO should store cgraph_node's profile_id of callee func in
> > the first histogram value's counter, rather than pointer to callee's name string
> > as it is now.
> > With the patch, some "Indirect call -> direct call" tests pass with autofdo, while
> > others are unstable.  I think the instability is caused by poor perf data collected
> > during regrets run, and can confirm these tests pass if good perf data could be
> > collected in manual experiments.
>
> Would be good to make the tests stable, otherwise we'll just have
> regressions in the future again.
>
> The problem is that the tests don't run long enough and don't get enough samples?
Yes, take g++.dg/tree-prof/morefunc.C as an example:
-  int i;
-  for (i = 0; i < 1000; i++)
+  int i, j;
+  for (i = 0; i < 1000000; i++)
+    for (j = 0; j < 50; j++)
      g += tc->foo();
    if (g<100) g++;
 }
@@ -27,8 +28,9 @@ void test1 (A *tc)
 static __attribute__((always_inline))
 void test2 (B *tc)
 {
-  int i;
+  int i, j;
   for (i = 0; i < 1000000; i++)
+    for (j = 0; j < 50; j++)

I have to increase loop count like this to get stable pass on my
machine.  The original count (1000) is too small to be sampled.

>
> Could add some loop?
> Or possibly increase the sampling frequency in perf (-F or -c)?
Maybe, I will have a try.
> Or run them multiple times and use gcov_merge to merge the files?
Without changing loop count or sampling frequency, this is not likely
to be helpful, since perf doesn't hit the small loop in most cases.

Thanks,
bin
>
>
> > FYI, an update about AutoFDO status:
> > All AutoFDO ICEs in regtest are fixed, while several tests still failing fall in below
> > three categories:
>
> Great!
>
> Of course it still ICEs with LTO?
>
> Right now there is no test case for this I think. Probably one should be added.
>
> -Andi


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