Cache pointers to histograms for recording event times.

base/metrics/histogram_macros.h

Index: base/metrics/histogram_macros.h
diff --git a/base/metrics/histogram_macros.h b/base/metrics/histogram_macros.h
index fa23bea720ce3f825a32551c14f2062891cd94bf..5a5c1bd346f4030a154937aaa0031ee59026499b 100644
--- a/base/metrics/histogram_macros.h
+++ b/base/metrics/histogram_macros.h
@@ -109,6 +109,23 @@
                           histogram_factory_get_invocation);                  \
   } while (0)
 
+// Support a collection of histograms, perhaps one for each entry in an

[Alexei Svitkine (slow), 2016/03/30 16:47:38]

This seems to be an esoteric use case - and while suffixed histograms are fairly
common, I don't think it's often the case that the suffixes always correspond to
an enum.

Unless there's more than one place that needs (which doesn't look like just from
this CL - but maybe you've found other cases?), I think it's better to define
this macro in the same file that uses it. If we find more cases, then it can be
provided more generally.

[bcwhite, 2016/03/30 17:33:08]

This macro just manages a vector.  It knows nothing about the actual histogram
names.  The factory_get_invocation deals with those details.
I think there will be.  Another culprit are certain "disk" tracking which index
1,2,3,4,5.  I haven't investigated; I just saw them being repeatedly loaded.

[Alexei Svitkine (slow), 2016/04/06 19:57:17]

Right, but it requires having an index that also corresponds to a custom name.
that you're passing to the histogram. (If you pass the same name, it doesn't
work right.)

Now, I could believe that more than one place would want this, but I'd like to
see it (i.e. two places using) before agreeing that we should have it in base
histograms code. Can you take a look at the other call site you were planning to
look at and see if it can benefit from this? If so, both can be added to the
same CL.

Also, it would be useful to have a comment with an example use, since it's not
super clear from this comment.

One other thought - if we do want to have it here, perhaps we could just make
STATIC_HISTOGRAM_POINTER_BLOCK() use this new macro with an index of 0?
 

[bcwhite, 2016/04/07 16:37:42]

Gotcha.  Right.
Okay.  I'll move it into event.cc with a comment saying it can be moved to
histogram_macros if it proves useful elsewhere.
Done.

+// enumeration. This macro manages a block of pointers, adding to a specific
+// one by its index.
+#define STATIC_HISTOGRAM_POINTER_GROUP(constant_histogram_name, index,        \
+                                       constant_maximum,                      \
+                                       histogram_add_method_invocation,       \
+                                       histogram_factory_get_invocation)      \
+  do {                                                                        \
+    static base::subtle::AtomicWord atomic_histograms[constant_maximum];      \
+    DCHECK_LE(0, index);                                                      \
+    DCHECK_LT(index, constant_maximum);                                       \
+    HISTOGRAM_POINTER_USE(&atomic_histograms[index], constant_histogram_name, \
+                          histogram_add_method_invocation,                    \
+                          histogram_factory_get_invocation);                  \
+  } while (0)
+
+
 //------------------------------------------------------------------------------
 // Provide easy general purpose histogram in a macro, just like stats counters.
 // The first four macros use 50 buckets.