[BOLT][NFC] Make YamlProfileToFunction a DenseMap

[aaupov]

YAML function profiles have sparse function IDs, assigned from
sequential function IDs from profiled binary. For example, for one large
binary, YAML profile has 15K functions, but the highest ID is ~600K,
close to number of functions in the profiled binary.

In matchProfileToFunction, YamlProfileToFunction vector was resized
to match function ID, which entails a 40X overcommit. Change the type of
YamlProfileToFunction to DenseMap to reduce memory utilization.

#99891 makes use of it for profile lookup associated with a given binary
function.

[llvmbot]

@llvm/pr-subscribers-bolt

Author: Amir Ayupov (aaupov)

Changes

YAML function profiles have sparse function IDs, assigned from
sequential function IDs from profiled binary. For example, for one large
binary, YAML profile has 15K functions, but the highest ID is ~600K,
close to number of functions in the profiled binary.

In matchProfileToFunction, YamlProfileToFunction vector was resized
to match function ID, which entails a 40X overcommit.

Change the type of YamlProfileToFunction to DenseMap to curb memory
utilization.

---

Full diff: https://github.com/llvm/llvm-project/pull/108712.diff

2 Files Affected:

• (modified) bolt/include/bolt/Profile/YAMLProfileReader.h (+1-3)
• (modified) bolt/lib/Profile/YAMLProfileReader.cpp (+3-10)

diff --git a/bolt/include/bolt/Profile/YAMLProfileReader.h b/bolt/include/bolt/Profile/YAMLProfileReader.h
index bd5a86fd676a59..a6f0fd6f3251f0 100644
--- a/bolt/include/bolt/Profile/YAMLProfileReader.h
+++ b/bolt/include/bolt/Profile/YAMLProfileReader.h
@@ -105,7 +105,7 @@ class YAMLProfileReader : public ProfileReaderBase {
   yaml::bolt::BinaryProfile YamlBP;
 
   /// Map a function ID from a YAML profile to a BinaryFunction object.
-  std::vector<BinaryFunction *> YamlProfileToFunction;
+  DenseMap<uint32_t, BinaryFunction *> YamlProfileToFunction;
 
   using FunctionSet = std::unordered_set<const BinaryFunction *>;
   /// To keep track of functions that have a matched profile before the profile
@@ -162,8 +162,6 @@ class YAMLProfileReader : public ProfileReaderBase {
   /// Update matched YAML -> BinaryFunction pair.
   void matchProfileToFunction(yaml::bolt::BinaryFunctionProfile &YamlBF,
                               BinaryFunction &BF) {
-    if (YamlBF.Id >= YamlProfileToFunction.size())
-      YamlProfileToFunction.resize(YamlBF.Id + 1);
     YamlProfileToFunction[YamlBF.Id] = &BF;
     YamlBF.Used = true;
 
diff --git a/bolt/lib/Profile/YAMLProfileReader.cpp b/bolt/lib/Profile/YAMLProfileReader.cpp
index 67ed32017667d6..3bd09508fb76e1 100644
--- a/bolt/lib/Profile/YAMLProfileReader.cpp
+++ b/bolt/lib/Profile/YAMLProfileReader.cpp
@@ -238,9 +238,7 @@ bool YAMLProfileReader::parseFunctionProfile(
     BB.setExecutionCount(YamlBB.ExecCount);
 
     for (const yaml::bolt::CallSiteInfo &YamlCSI : YamlBB.CallSites) {
-      BinaryFunction *Callee = YamlCSI.DestId < YamlProfileToFunction.size()
-                                   ? YamlProfileToFunction[YamlCSI.DestId]
-                                   : nullptr;
+      BinaryFunction *Callee = YamlProfileToFunction.lookup(YamlCSI.DestId);
       bool IsFunction = Callee ? true : false;
       MCSymbol *CalleeSymbol = nullptr;
       if (IsFunction)
@@ -707,7 +705,7 @@ Error YAMLProfileReader::readProfile(BinaryContext &BC) {
       break;
     }
   }
-  YamlProfileToFunction.resize(YamlBP.Functions.size() + 1);
+  YamlProfileToFunction.reserve(YamlBP.Functions.size());
 
   // Computes hash for binary functions.
   if (opts::MatchProfileWithFunctionHash) {
@@ -760,12 +758,7 @@ Error YAMLProfileReader::readProfile(BinaryContext &BC) {
   NormalizeByCalls = usesEvent("branches");
   uint64_t NumUnused = 0;
   for (yaml::bolt::BinaryFunctionProfile &YamlBF : YamlBP.Functions) {
-    if (YamlBF.Id >= YamlProfileToFunction.size()) {
-      // Such profile was ignored.
-      ++NumUnused;
-      continue;
-    }
-    if (BinaryFunction *BF = YamlProfileToFunction[YamlBF.Id])
+    if (BinaryFunction *BF = YamlProfileToFunction.lookup(YamlBF.Id))
       parseFunctionProfile(*BF, YamlBF);
     else
       ++NumUnused;

[maksfb]

I'm curious if using uint32_t leads to more compact allocation compared to uint64_t?

[aaupov]

I don't think it does: DenseMap stores objects in buckets where bucket is a KV pair where the second element (pointer) would force the alignment to be 8b.

[aaupov]

But the YamlBF.Id is uint32, so I decided to match it:

llvm-project/bolt/include/bolt/Profile/ProfileYAMLMapping.h

Lines 200 to 203 in 6b21cf8

	struct BinaryFunctionProfile {
	std::string Name;
	uint32_t NumBasicBlocks{0};
	uint32_t Id{0};