[ThinLTO] Remove BlockCount for non partial sample profile builds

As pointed out in
https://discourse.llvm.org/t/undeterministic-thin-index-file/69985, the
block count added to distributed ThinLTO index files breaks incremental
builds on ThinLTO - if any linked file has a different number of BBs,
then the accumulated sum placed in the index files will change, causing
all ThinLTO backend compiles to be redone.

The block count is only used for scaling of partial sample profiles, and
was added in D80403 for D79831.

This patch simply removes this field from the index files of non partial
sample profile compiles, which is NFC on the output of the compiler.

We subsequently need to see if this can be removed for partial sample
profiles without signficant performance loss, or redesigned in a way
that does not destroy caching.

Differential Revision: https://reviews.llvm.org/D148746

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp b/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp
index 26ff84f..d84cfbf 100644
--- a/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp
+++ b/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp
@@ -80,6 +80,8 @@ static cl::opt<std::string> ModuleSummaryDotFile(
     "module-summary-dot-file", cl::Hidden, cl::value_desc("filename"),
     cl::desc("File to emit dot graph of new summary into"));
 
+extern cl::opt<bool> ScalePartialSampleProfileWorkingSetSize;
+
 // Walk through the operands of a given User via worklist iteration and populate
 // the set of GlobalValue references encountered. Invoked either on an
 // Instruction or a GlobalVariable (which walks its initializer).
@@ -477,7 +479,9 @@ static void computeFunctionSummary(
       }
     }
   }
-  Index.addBlockCount(F.size());
+
+  if (PSI->hasPartialSampleProfile() && ScalePartialSampleProfileWorkingSetSize)
+    Index.addBlockCount(F.size());
 
   std::vector<ValueInfo> Refs;
   if (IsThinLTO) {