[NFC][StaticDataProfileInfo] Refactor StaticDataProfileInfo::getConstantSectionPrefix and extract analysis based on PGO-counter to be a helper function (#162388)

`StaticDataProfileInfo::getConstantSectionPrefix` is used twice in
codegen ([1] and [2]) to emit section prefix for constants.

Before this patch, its implementation does analysis using PGO-counters,
and PGO-counters are only available on module-internal constants.

After this patch, the PGO-counter analysis are extracted to a helper
function, and returns enum rather than StringPrefix. This way, the
follow up patch https://github.com/llvm/llvm-project/pull/163325 can
extend this function to use global variable section prefix and compute a
max (the hotter one).

[1]
975fba1b49/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp (L3014-L3019)
[2]
975fba1b49/llvm/lib/CodeGen/StaticDataAnnotator.cpp (L77-L84)

llvm/include/llvm/Analysis/StaticDataProfileInfo.h

@@ -32,8 +32,11 @@ bool IsAnnotationOK(const GlobalVariable &GV);
 /// profile information and provides methods to operate on them.
 class StaticDataProfileInfo {
 public:
-  /// Accummulate the profile count of a constant that will be lowered to static
-  /// data sections.
+  /// A constant is tracked only if the following conditions are met.
+  ///   1) It has local (i.e., private or internal) linkage.
+  //    2) Its data kind is one of {.rodata, .data, .bss, .data.rel.ro}.
+  //    3) It's eligible for section prefix annotation. See `AnnotationKind`
+  //       above for ineligible reasons.
   DenseMap<const Constant *, uint64_t> ConstantProfileCounts;
 
   /// Keeps track of the constants that are seen at least once without profile
@@ -44,6 +47,22 @@ public:
   LLVM_ABI std::optional<uint64_t>
   getConstantProfileCount(const Constant *C) const;
 
+  /// Use signed enums for enum value comparison, and make 'LukewarmOrUnknown'
+  /// as 0 so any accidentally uninitialized value will default to unknown.
+  enum class StaticDataHotness : int8_t {
+    Cold = -1,
+    LukewarmOrUnknown = 0,
+    Hot = 1,
+  };
+
+  /// Return the hotness of the constant \p C based on its profile count \p
+  /// Count.
+  LLVM_ABI StaticDataHotness getConstantHotnessUsingProfileCount(
+      const Constant *C, const ProfileSummaryInfo *PSI, uint64_t Count) const;
+
+  /// Return the string representation of the hotness enum \p Hotness.
+  LLVM_ABI StringRef hotnessToStr(StaticDataHotness Hotness) const;
+
 public:
   StaticDataProfileInfo() = default;
 

llvm/lib/Analysis/StaticDataProfileInfo.cpp

@@ -60,6 +60,36 @@ void StaticDataProfileInfo::addConstantProfileCount(
     OriginalCount = getInstrMaxCountValue();
 }
 
+StaticDataProfileInfo::StaticDataHotness
+StaticDataProfileInfo::getConstantHotnessUsingProfileCount(
+    const Constant *C, const ProfileSummaryInfo *PSI, uint64_t Count) const {
+  // The accummulated counter shows the constant is hot. Return enum 'hot'
+  // whether this variable is seen by unprofiled functions or not.
+  if (PSI->isHotCount(Count))
+    return StaticDataHotness::Hot;
+  // The constant is not hot, and seen by unprofiled functions. We don't want to
+  // assign it to unlikely sections, even if the counter says 'cold'. So return
+  // enum 'LukewarmOrUnknown'.
+  if (ConstantWithoutCounts.count(C))
+    return StaticDataHotness::LukewarmOrUnknown;
+  // The accummulated counter shows the constant is cold so return enum 'cold'.
+  if (PSI->isColdCount(Count))
+    return StaticDataHotness::Cold;
+
+  return StaticDataHotness::LukewarmOrUnknown;
+}
+
+StringRef StaticDataProfileInfo::hotnessToStr(StaticDataHotness Hotness) const {
+  switch (Hotness) {
+  case StaticDataHotness::Cold:
+    return "unlikely";
+  case StaticDataHotness::Hot:
+    return "hot";
+  default:
+    return "";
+  }
+}
+
 std::optional<uint64_t>
 StaticDataProfileInfo::getConstantProfileCount(const Constant *C) const {
   auto I = ConstantProfileCounts.find(C);
@@ -70,23 +100,10 @@ StaticDataProfileInfo::getConstantProfileCount(const Constant *C) const {
 
 StringRef StaticDataProfileInfo::getConstantSectionPrefix(
     const Constant *C, const ProfileSummaryInfo *PSI) const {
-  auto Count = getConstantProfileCount(C);
+  std::optional<uint64_t> Count = getConstantProfileCount(C);
   if (!Count)
     return "";
-  // The accummulated counter shows the constant is hot. Return 'hot' whether
-  // this variable is seen by unprofiled functions or not.
-  if (PSI->isHotCount(*Count))
-    return "hot";
-  // The constant is not hot, and seen by unprofiled functions. We don't want to
-  // assign it to unlikely sections, even if the counter says 'cold'. So return
-  // an empty prefix before checking whether the counter is cold.
-  if (ConstantWithoutCounts.count(C))
-    return "";
-  // The accummulated counter shows the constant is cold. Return 'unlikely'.
-  if (PSI->isColdCount(*Count))
-    return "unlikely";
-  // The counter says lukewarm. Return an empty prefix.
-  return "";
+  return hotnessToStr(getConstantHotnessUsingProfileCount(C, PSI, *Count));
 }
 
 bool StaticDataProfileInfoWrapperPass::doInitialization(Module &M) {