[PATCH] D18622: Replace the use of MaxFunctionCount module flag

[Easwaran Raman via llvm-commits]

eraman added inline comments.

================
Comment at: include/llvm/ProfileData/ProfileCommon.h:194
@@ +193,3 @@
+    return nullptr;
+  // Computing profile summary for a module involves parsing a fairly large
+  // metadata and could be expensive. We use a simple cache of the last seen
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davidxl wrote:
> I suggest doing some compile time measurement before using the caching mechanism. Besides, this won't affect O2 compile time at all.
Here are some numbers:
* It takes ~ 1.05 ms on a machine with Intel Xeon E5-2690 with 2.9GHz clock frequency per call to computeProfileSummary.  Or 3000 cycles. The time was obtained by using a NamedRegionTimer to measure 1M calls to computeProfileSummary and I took the user + sys time.

* To put this in perspective, while compiling a large real application, the time taken by computeProfileSummary is  25-30% of the time taken by CallAnalyzer's analyzeCall. So not caching will increase in a 25-30% increase in analyzeCall (which is now the only client of computeProfileSummary). 

Of course, as you point out this is only in the PGO mode and doesn't affect O2 compiles, but still it seems like worthwhile caching this,


http://reviews.llvm.org/D18622