[llvm] [CodeGen][StaticDataPartitioning]Place local-linkage global variables in hot or unlikely prefixed sections based on profile information (PR #125756)

[Mingming Liu via llvm-commits]

================
@@ -117,18 +142,89 @@ bool StaticDataSplitter::partitionStaticDataWithProfiles(MachineFunction &MF) {
           // Hotness is based on source basic block hotness.
           // TODO: PSI APIs are about instruction hotness. Introduce API for
           // data access hotness.
-          if (PSI->isColdBlock(&MBB, MBFI))
+          if (Count && PSI->isColdCount(*Count))
             Hotness = MachineFunctionDataHotness::Cold;
 
           if (MJTI->updateJumpTableEntryHotness(JTI, Hotness))
             ++NumChangedJumpTables;
+        } else if (Op.isGlobal()) {
+          // Find global variables with local linkage
+          const GlobalVariable *GV =
+              getLocalLinkageGlobalVariable(Op.getGlobal());
+          if (!GV || !inStaticDataSection(GV, TM))
+            continue;
+
+          // Acccumulate data profile count across machine function
+          // instructions.
+          // TODO: Analyze global variable's initializers.
+          if (Count) {
+            auto [It, Inserted] =
+                DataProfileCounts.try_emplace(GV, APInt(128, 0));
+            It->second += *Count;
+          }
         }
       }
     }
   }
   return NumChangedJumpTables > 0;
 }
 
+const GlobalVariable *
+StaticDataSplitter::getLocalLinkageGlobalVariable(const GlobalValue *GV) {
+  if (!GV || GV->isDeclarationForLinker())
+    return nullptr;
+
+  return GV->hasLocalLinkage() ? dyn_cast<GlobalVariable>(GV) : nullptr;
+}
+
+bool StaticDataSplitter::inStaticDataSection(const GlobalVariable *GV,
+                                             const TargetMachine &TM) {
+  assert(GV && "Caller guaranteed");
+
+  // Skip LLVM reserved symbols.
+  if (GV->getName().starts_with("llvm."))
----------------
mingmingl-llvm wrote:

Good question.  The globals with `llvm.` as a name prefix are usually handled specially, by many (middle-end and back-end) passes (e.g., AsmPrinter emits [special LLVM values](https://github.com/llvm/llvm-project/blob/7464dc8c7618aeb5a01998576bbcc4c88f0dde1d/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp#L730-L731) specially), and they are skipped in this pass mostly out of conservativeness. 

I moved this check before calling `inStaticDataSection` helper though for readability, and added a comment around the check. What do you think about this?

https://github.com/llvm/llvm-project/pull/125756