[BOLT] Create .text.warm for 3-way splitting (#73863)

This commit explicitly adds a warm code section, .text.warm, when
-split-functions -split-strategy=cdsplit is used. This replaces the
previous approach of using .text.cold.0 as warm and .text.cold.1 as cold
in 3-way function splitting. NFC.

Author: ShatianWang

bolt/include/bolt/Core/BinaryContext.h

@@ -614,6 +614,11 @@ class BinaryContext {
   /// Indicates if the function ordering of the binary is finalized.
   bool HasFinalizedFunctionOrder{false};
 
+  /// Indicates if a separate .text.warm section is needed that contains
+  /// function fragments with
+  /// FunctionFragment::getFragmentNum() == FragmentNum::warm()
+  bool HasWarmSection{false};
+
   /// Is the binary always loaded at a fixed address. Shared objects and
   /// position-independent executables (PIEs) are examples of binaries that
   /// will have HasFixedLoadAddress set to false.
@@ -930,6 +935,8 @@ class BinaryContext {
 
   const char *getMainCodeSectionName() const { return ".text"; }
 
+  const char *getWarmCodeSectionName() const { return ".text.warm"; }
+
   const char *getColdCodeSectionName() const { return ".text.cold"; }
 
   const char *getHotTextMoverSectionName() const { return ".text.mover"; }

bolt/include/bolt/Core/BinaryFunction.h

@@ -1236,6 +1236,8 @@ class BinaryFunction {
       return SmallString<32>(CodeSectionName);
     if (Fragment == FragmentNum::cold())
       return SmallString<32>(ColdCodeSectionName);
+    if (BC.HasWarmSection && Fragment == FragmentNum::warm())
+      return SmallString<32>(BC.getWarmCodeSectionName());
     return formatv("{0}.{1}", ColdCodeSectionName, Fragment.get() - 1);
   }
 

bolt/include/bolt/Core/FunctionLayout.h

@@ -63,6 +63,7 @@ class FragmentNum {
 
   static constexpr FragmentNum main() { return FragmentNum(0); }
   static constexpr FragmentNum cold() { return FragmentNum(1); }
+  static constexpr FragmentNum warm() { return FragmentNum(2); }
 };
 
 /// A freestanding subset of contiguous blocks of a function.

bolt/lib/Core/BinaryEmitter.cpp

@@ -287,7 +287,10 @@ void BinaryEmitter::emitFunctions() {
 
   // Mark the end of hot text.
   if (opts::HotText) {
-    Streamer.switchSection(BC.getTextSection());
+    if (BC.HasWarmSection)
+      Streamer.switchSection(BC.getCodeSection(BC.getWarmCodeSectionName()));
+    else
+      Streamer.switchSection(BC.getTextSection());
     Streamer.emitLabel(BC.getHotTextEndSymbol());
   }
 }

bolt/lib/Passes/SplitFunctions.cpp

@@ -160,16 +160,15 @@ struct SplitCacheDirected final : public SplitStrategy {
     // Assign fragments based on the computed best split index.
     // All basic blocks with index up to the best split index become hot.
     // All remaining blocks are warm / cold depending on if count is
-    // greater than 0 or not.
-    FragmentNum Main(0);
-    FragmentNum Cold(1);
-    FragmentNum Warm(2);
+    // greater than zero or not.
     for (size_t Index = 0; Index < BlockOrder.size(); Index++) {
       BinaryBasicBlock *BB = BlockOrder[Index];
       if (Index <= BestSplitIndex)
-        BB->setFragmentNum(Main);
+        BB->setFragmentNum(FragmentNum::main());
       else
-        BB->setFragmentNum(BB->getKnownExecutionCount() > 0 ? Warm : Cold);
+        BB->setFragmentNum(BB->getKnownExecutionCount() > 0
+                               ? FragmentNum::warm()
+                               : FragmentNum::cold());
     }
   }
 
@@ -313,6 +312,7 @@ void SplitFunctions::runOnFunctions(BinaryContext &BC) {
     else
       Strategy = std::make_unique<SplitProfile2>();
     opts::AggressiveSplitting = true;
+    BC.HasWarmSection = true;
     break;
   case SplitFunctionsStrategy::Profile2:
     Strategy = std::make_unique<SplitProfile2>();

bolt/lib/Rewrite/RewriteInstance.cpp

@@ -3478,11 +3478,21 @@ std::vector<BinarySection *> RewriteInstance::getCodeSections() {
     if (B->getName() == BC->getHotTextMoverSectionName())
       return false;
 
-    // Depending on the option, put main text at the beginning or at the end.
-    if (opts::HotFunctionsAtEnd)
-      return B->getName() == BC->getMainCodeSectionName();
-    else
-      return A->getName() == BC->getMainCodeSectionName();
+    // Depending on opts::HotFunctionsAtEnd, place main and warm sections in
+    // order.
+    if (opts::HotFunctionsAtEnd) {
+      if (B->getName() == BC->getMainCodeSectionName())
+        return true;
+      if (A->getName() == BC->getMainCodeSectionName())
+        return false;
+      return (B->getName() == BC->getWarmCodeSectionName());
+    } else {
+      if (A->getName() == BC->getMainCodeSectionName())
+        return true;
+      if (B->getName() == BC->getMainCodeSectionName())
+        return false;
+      return (A->getName() == BC->getWarmCodeSectionName());
+    }
   };
 
   // Determine the order of sections.
@@ -3524,6 +3534,9 @@ void RewriteInstance::mapCodeSections(BOLTLinker::SectionMapper MapSection) {
 
     // Allocate sections starting at a given Address.
     auto allocateAt = [&](uint64_t Address) {
+      const char *LastNonColdSectionName = BC->HasWarmSection
+                                               ? BC->getWarmCodeSectionName()
+                                               : BC->getMainCodeSectionName();
       for (BinarySection *Section : CodeSections) {
         Address = alignTo(Address, Section->getAlignment());
         Section->setOutputAddress(Address);
@@ -3532,13 +3545,13 @@ void RewriteInstance::mapCodeSections(BOLTLinker::SectionMapper MapSection) {
         // Hugify: Additional huge page from right side due to
         // weird ASLR mapping addresses (4KB aligned)
         if (opts::Hugify && !BC->HasFixedLoadAddress &&
-            Section->getName() == BC->getMainCodeSectionName())
+            Section->getName() == LastNonColdSectionName)
           Address = alignTo(Address, Section->getAlignment());
       }
 
       // Make sure we allocate enough space for huge pages.
       ErrorOr<BinarySection &> TextSection =
-          BC->getUniqueSectionByName(BC->getMainCodeSectionName());
+          BC->getUniqueSectionByName(LastNonColdSectionName);
       if (opts::HotText && TextSection && TextSection->hasValidSectionID()) {
         uint64_t HotTextEnd =
             TextSection->getOutputAddress() + TextSection->getOutputSize();
@@ -4396,6 +4409,21 @@ void RewriteInstance::updateELFSymbolTable(
     return NewIndex;
   };
 
+  // Get the extra symbol name of a split fragment; used in addExtraSymbols.
+  auto getSplitSymbolName = [&](const FunctionFragment &FF,
+                                const ELFSymTy &FunctionSymbol) {
+    SmallString<256> SymbolName;
+    if (BC->HasWarmSection)
+      SymbolName =
+          formatv("{0}.{1}", cantFail(FunctionSymbol.getName(StringSection)),
+                  FF.getFragmentNum() == FragmentNum::warm() ? "warm" : "cold");
+    else
+      SymbolName = formatv("{0}.cold.{1}",
+                           cantFail(FunctionSymbol.getName(StringSection)),
+                           FF.getFragmentNum().get() - 1);
+    return SymbolName;
+  };
+
   // Add extra symbols for the function.
   //
   // Note that addExtraSymbols() could be called multiple times for the same
@@ -4423,9 +4451,8 @@ void RewriteInstance::updateELFSymbolTable(
            Function.getLayout().getSplitFragments()) {
         if (FF.getAddress()) {
           ELFSymTy NewColdSym = FunctionSymbol;
-          const SmallString<256> SymbolName = formatv(
-              "{0}.cold.{1}", cantFail(FunctionSymbol.getName(StringSection)),
-              FF.getFragmentNum().get() - 1);
+          const SmallString<256> SymbolName =
+              getSplitSymbolName(FF, FunctionSymbol);
           NewColdSym.st_name = AddToStrTab(SymbolName);
           NewColdSym.st_shndx =
               Function.getCodeSection(FF.getFragmentNum())->getIndex();