[PATCH] D70653: [AutoFDO] Properly merge context-sensitive profile of inlinee back to outlined function

[Wei Mi via Phabricator via llvm-commits]

wmi added inline comments.


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Comment at: llvm/include/llvm/ProfileData/SampleProfReader.h:363
+  /// FunctionSamples if it doesn't exist.
+  FunctionSamples *getOrCreateSamplesFor(const Function &F) {
+    std::string FGUID;
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wmi wrote:
> wenlei wrote:
> > wmi wrote:
> > > I found a potential problem here. When we create a new profile for a function when merging inline instance profile back, if the function somehow has no debug information, the annotation will fail and a warning will be issued. We may want to skip those functions without debug information (The warning is issued in SampleProfileLoader::getFunctionLoc)  
> > For these cases, if the function without debug info wasn't inlined, we would still get the warning, right? I though the warning in this case is no different and as helpful comparing to other cases, or did I miss anything? 
> Although without the patch, we may also see the warning because of the source drift or compiler upgrade, in reality we   rarely see that happens. I am worried that the patch may increase the chance of such warning and break many builds.
> 
> So I will look into it and see what causes the warning and whether it is a rare case or not.      
I looked into it and found the case was not rare.

For the function without debug information, in afdo profile there was no inline instance. The function was added to localNotInlinedCallSites because findFunctionSamplesAt returned a function sample as long as the location matched (the imprecise matching is designed for indirect promotion). In this case, the FunctionSamples returned didn't belong to the function without debug information.

I applied file specific -Wno-error to disable the warning whenever I saw a case like this in a large benchmark build, and I found the same problem in at least three files (If I continue the process, I may find more).

To solve the problem, maybe check debug information in findFunctionSamplesAt before return a FunctionSamples when callee name doesn't match the name in FunctionSamples.  If debug information doesn't exists for a function, inlining it won't bring more context sensitive information because no profile annotation is possible. 


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