[PGO] Fix handling of cold entry count for instrumented PGO

teresajohnson · teresajohnson · commit 915897e21b12 · 2017-12-18T20:02:43.000Z
Summary: In r277849, getEntryCount was changed to return None when the entry count was 0, specifically for SamplePGO where it means no samples were recorded. However, for instrumentation PGO a 0 entry count should be returned directly, since it does mean that the function was completely cold. Otherwise we end up treating these functions conservatively in isFunctionEntryCold() and isColdBB(). Instead, for SamplePGO use -1 when there are no samples, and change getEntryCount to return None when the value is -1. Reviewers: danielcdh, davidxl Subscribers: llvm-commits Differential Revision: https://reviews.llvm.org/D41307 llvm-svn: 321018
diff --git a/llvm/lib/IR/Function.cpp b/llvm/lib/IR/Function.cpp
@@ -1333,7 +1333,9 @@ Optional<uint64_t> Function::getEntryCount() const {
       if (MDS->getString().equals("function_entry_count")) {
         ConstantInt *CI = mdconst::extract<ConstantInt>(MD->getOperand(1));
         uint64_t Count = CI->getValue().getZExtValue();
-        if (Count == 0)
+        // A value of -1 is used for SamplePGO when there were no samples.
+        // Treat this the same as unknown.
+        if (Count == (uint64_t)-1)
           return None;
         return Count;
       }
diff --git a/llvm/lib/Transforms/IPO/SampleProfile.cpp b/llvm/lib/Transforms/IPO/SampleProfile.cpp
@@ -1583,7 +1583,10 @@ bool SampleProfileLoaderLegacyPass::runOnModule(Module &M) {
 }
 
 bool SampleProfileLoader::runOnFunction(Function &F, ModuleAnalysisManager *AM) {
-  F.setEntryCount(0);
+  // Initialize the entry count to -1, which will be treated conservatively
+  // by getEntryCount as the same as unknown (None). If we have samples this
+  // will be overwritten in emitAnnotations.
+  F.setEntryCount(-1);
   std::unique_ptr<OptimizationRemarkEmitter> OwnedORE;
   if (AM) {
     auto &FAM =
diff --git a/llvm/test/Transforms/SampleProfile/entry_counts.ll b/llvm/test/Transforms/SampleProfile/entry_counts.ll
@@ -9,8 +9,8 @@ entry:
   ret void, !dbg !9
 }
 
-; This function does not have profile, check if function_entry_count is 0
-; CHECK: {{.*}} = !{!"function_entry_count", i64 0}
+; This function does not have profile, check if function_entry_count is -1
+; CHECK: {{.*}} = !{!"function_entry_count", i64 -1}
 define void @no_profile() {
 entry:
   ret void
