[AsmPrinter][ELF] Support profile-guided section prefix for jump tabl…

…es' (read-only) data sections (#122215)

#122183 adds a codegen pass to
infer machine jump table entry's hotness from the MBB hotness. This is a
follow-up PR to produce `.hot` and or `.unlikely` section prefix for
jump table's (read-only) data sections in the relocatable `.o` files.

When this patch is enabled, linker will see {`.rodata`, `.rodata.hot`,
`.rodata.unlikely`} in input sections. It can map `.rodata.hot` and
`.rodata` in the input sections to `.rodata.hot` in the executable, and
map `.rodata.unlikely` into `.rodata` with a pending extension to
`--keep-text-section-prefix` like
059e7cb,
or with a linker script.

1. To partition hot and jump tables, the AsmPrinter pass slices a function's jump table indices into two groups, one for hot and the other for cold jump tables. It then emits hot jump tables into a `.hot`-prefixed data section and cold ones into a `.unlikely`-prefixed data section, retaining the relative order of `LJT<N>` labels within each group.

2. [ELF only] To have data sections with _dynamic_ names (e.g., `.rodata.hot[.func]`), we implement
`TargetLoweringObjectFile::getSectionForJumpTable` method that accepts a `MachineJumpTableEntry` parameter, and update `selectELFSectionForGlobal` to generate `.hot` or `.unlikely` based on
MJTE's hotness.
    - The dynamic JT section name doesn't depend on `-ffunction-section=true` or `-funique-section-names=true`, even though it leverages the similar underlying mechanism to have a MCSection with on-demand name as `-ffunction-section` does.

3. The new code path is off by default.
    - Typically, `TargetOptions` conveys clang or LLVM tools' options to code generation passes. To follow the pattern, add option `EnableStaticDataPartitioning` bit in `TargetOptions` and make it
readable through `TargetMachine`.
    - To enable the new code path in tools like `llc`, `partition-static-data-sections` option is introduced in
`CodeGen/CommandFlags.h/cpp`.
    -  A subsequent patch
([draft](8f36a13)) will add a clang option to enable the new code path.

---------

Co-authored-by: Ellis Hoag <[email protected]>

llvm/include/llvm/CodeGen/CommandFlags.h

@@ -136,6 +136,8 @@ bool getEmitCallSiteInfo();
 
 bool getEnableMachineFunctionSplitter();
 
+bool getEnableStaticDataPartitioning();
+
 bool getEnableDebugEntryValues();
 
 bool getValueTrackingVariableLocations();

llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h

@@ -76,6 +76,9 @@ class TargetLoweringObjectFileELF : public TargetLoweringObjectFile {
 
   MCSection *getSectionForJumpTable(const Function &F,
                                     const TargetMachine &TM) const override;
+  MCSection *
+  getSectionForJumpTable(const Function &F, const TargetMachine &TM,
+                         const MachineJumpTableEntry *JTE) const override;
   MCSection *getSectionForLSDA(const Function &F, const MCSymbol &FnSym,
                                const TargetMachine &TM) const override;
 

llvm/include/llvm/Target/TargetLoweringObjectFile.h

@@ -21,6 +21,7 @@
 namespace llvm {
 
 struct Align;
+struct MachineJumpTableEntry;
 class Constant;
 class DataLayout;
 class Function;
@@ -135,6 +136,10 @@ class TargetLoweringObjectFile : public MCObjectFileInfo {
 
   virtual MCSection *getSectionForJumpTable(const Function &F,
                                             const TargetMachine &TM) const;
+  virtual MCSection *
+  getSectionForJumpTable(const Function &F, const TargetMachine &TM,
+                         const MachineJumpTableEntry *JTE) const;
+
   virtual MCSection *getSectionForLSDA(const Function &, const MCSymbol &,
                                        const TargetMachine &) const {
     return LSDASection;

llvm/include/llvm/Target/TargetMachine.h

@@ -305,6 +305,10 @@ class TargetMachine {
     return Options.FunctionSections;
   }
 
+  bool getEnableStaticDataPartitioning() const {
+    return Options.EnableStaticDataPartitioning;
+  }
+
   /// Return true if visibility attribute should not be emitted in XCOFF,
   /// corresponding to -mignore-xcoff-visibility.
   bool getIgnoreXCOFFVisibility() const {

llvm/include/llvm/Target/TargetOptions.h

@@ -145,7 +145,8 @@ namespace llvm {
           TrapUnreachable(false), NoTrapAfterNoreturn(false), TLSSize(0),
           EmulatedTLS(false), EnableTLSDESC(false), EnableIPRA(false),
           EmitStackSizeSection(false), EnableMachineOutliner(false),
-          EnableMachineFunctionSplitter(false), SupportsDefaultOutlining(false),
+          EnableMachineFunctionSplitter(false),
+          EnableStaticDataPartitioning(false), SupportsDefaultOutlining(false),
           EmitAddrsig(false), BBAddrMap(false), EmitCallSiteInfo(false),
           SupportsDebugEntryValues(false), EnableDebugEntryValues(false),
           ValueTrackingVariableLocations(false), ForceDwarfFrameSection(false),
@@ -312,6 +313,9 @@ namespace llvm {
     /// Enables the MachineFunctionSplitter pass.
     unsigned EnableMachineFunctionSplitter : 1;
 
+    /// Enables the StaticDataSplitter pass.
+    unsigned EnableStaticDataPartitioning : 1;
+
     /// Set if the target supports default outlining behaviour.
     unsigned SupportsDefaultOutlining : 1;
 

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -2868,11 +2868,26 @@ void AsmPrinter::emitJumpTableInfo() {
           MJTI->getEntryKind() == MachineJumpTableInfo::EK_LabelDifference64,
       F);
 
-  SmallVector<unsigned> JumpTableIndices;
+  if (!TM.Options.EnableStaticDataPartitioning) {
+    emitJumpTableImpl(*MJTI, llvm::to_vector(llvm::seq<unsigned>(JT.size())),
+                      JTInDiffSection);
+    return;
+  }
+
+  SmallVector<unsigned> HotJumpTableIndices, ColdJumpTableIndices;
+  // When static data partitioning is enabled, collect jump table entries that
+  // go into the same section together to reduce the amount of section switch
+  // statements.
   for (unsigned JTI = 0, JTSize = JT.size(); JTI < JTSize; ++JTI) {
-    JumpTableIndices.push_back(JTI);
+    if (JT[JTI].Hotness == MachineFunctionDataHotness::Cold) {
+      ColdJumpTableIndices.push_back(JTI);
+    } else {
+      HotJumpTableIndices.push_back(JTI);
+    }
   }
-  emitJumpTableImpl(*MJTI, JumpTableIndices, JTInDiffSection);
+
+  emitJumpTableImpl(*MJTI, HotJumpTableIndices, JTInDiffSection);
+  emitJumpTableImpl(*MJTI, ColdJumpTableIndices, JTInDiffSection);
 }
 
 void AsmPrinter::emitJumpTableImpl(const MachineJumpTableInfo &MJTI,
@@ -2884,7 +2899,13 @@ void AsmPrinter::emitJumpTableImpl(const MachineJumpTableInfo &MJTI,
   const TargetLoweringObjectFile &TLOF = getObjFileLowering();
   const Function &F = MF->getFunction();
   const std::vector<MachineJumpTableEntry> &JT = MJTI.getJumpTables();
-  MCSection *JumpTableSection = TLOF.getSectionForJumpTable(F, TM);
+  MCSection *JumpTableSection = nullptr;
+  if (TM.Options.EnableStaticDataPartitioning) {
+    JumpTableSection =
+        TLOF.getSectionForJumpTable(F, TM, &JT[JumpTableIndices.front()]);
+  } else {
+    JumpTableSection = TLOF.getSectionForJumpTable(F, TM);
+  }
 
   const DataLayout &DL = MF->getDataLayout();
   if (JTInDiffSection) {

llvm/lib/CodeGen/CommandFlags.cpp

@@ -103,6 +103,7 @@ CGOPT(bool, EnableStackSizeSection)
 CGOPT(bool, EnableAddrsig)
 CGOPT(bool, EmitCallSiteInfo)
 CGOPT(bool, EnableMachineFunctionSplitter)
+CGOPT(bool, EnableStaticDataPartitioning)
 CGOPT(bool, EnableDebugEntryValues)
 CGOPT(bool, ForceDwarfFrameSection)
 CGOPT(bool, XRayFunctionIndex)
@@ -480,6 +481,12 @@ codegen::RegisterCodeGenFlags::RegisterCodeGenFlags() {
       cl::init(false));
   CGBINDOPT(EnableMachineFunctionSplitter);
 
+  static cl::opt<bool> EnableStaticDataPartitioning(
+      "partition-static-data-sections",
+      cl::desc("Partition data sections using profile information."),
+      cl::init(false));
+  CGBINDOPT(EnableStaticDataPartitioning);
+
   static cl::opt<bool> ForceDwarfFrameSection(
       "force-dwarf-frame-section",
       cl::desc("Always emit a debug frame section."), cl::init(false));
@@ -586,6 +593,7 @@ codegen::InitTargetOptionsFromCodeGenFlags(const Triple &TheTriple) {
   Options.ExceptionModel = getExceptionModel();
   Options.EmitStackSizeSection = getEnableStackSizeSection();
   Options.EnableMachineFunctionSplitter = getEnableMachineFunctionSplitter();
+  Options.EnableStaticDataPartitioning = getEnableStaticDataPartitioning();
   Options.EmitAddrsig = getEnableAddrsig();
   Options.EmitCallSiteInfo = getEmitCallSiteInfo();
   Options.EnableDebugEntryValues = getEnableDebugEntryValues();

llvm/lib/CodeGen/StaticDataSplitter.cpp

@@ -35,19 +35,11 @@ using namespace llvm;
 
 #define DEBUG_TYPE "static-data-splitter"
 
-STATISTIC(NumHotJumpTables, "Number of hot jump tables seen");
-STATISTIC(NumColdJumpTables, "Number of cold jump tables seen");
+STATISTIC(NumHotJumpTables, "Number of hot jump tables seen.");
+STATISTIC(NumColdJumpTables, "Number of cold jump tables seen.");
 STATISTIC(NumUnknownJumpTables,
-          "Number of jump tables with unknown hotness. Option "
-          "-static-data-default-hotness specifies the hotness.");
-
-static cl::opt<MachineFunctionDataHotness> StaticDataDefaultHotness(
-    "static-data-default-hotness", cl::Hidden,
-    cl::desc("This option specifies the hotness of static data when profile "
-             "information is unavailable"),
-    cl::init(MachineFunctionDataHotness::Hot),
-    cl::values(clEnumValN(MachineFunctionDataHotness::Hot, "hot", "Hot"),
-               clEnumValN(MachineFunctionDataHotness::Cold, "cold", "Cold")));
+          "Number of jump tables with unknown hotness. They are from functions "
+          "without profile information.");
 
 class StaticDataSplitter : public MachineFunctionPass {
   const MachineBranchProbabilityInfo *MBPI = nullptr;
@@ -156,13 +148,6 @@ bool StaticDataSplitter::splitJumpTables(MachineFunction &MF) {
   if (ProfileAvailable)
     return splitJumpTablesWithProfiles(MF, *MJTI);
 
-  // If function profile is unavailable (e.g., module not instrumented, or new
-  // code paths lacking samples), -static-data-default-hotness specifies the
-  // hotness.
-  for (size_t JTI = 0; JTI < MJTI->getJumpTables().size(); JTI++)
-    MF.getJumpTableInfo()->updateJumpTableEntryHotness(
-        JTI, StaticDataDefaultHotness);
-
   return true;
 }
 

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -24,6 +24,7 @@
 #include "llvm/CodeGen/BasicBlockSectionUtils.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineJumpTableInfo.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
 #include "llvm/CodeGen/MachineModuleInfoImpls.h"
 #include "llvm/IR/Comdat.h"
@@ -648,7 +649,8 @@ static StringRef getSectionPrefixForGlobal(SectionKind Kind, bool IsLarge) {
 static SmallString<128>
 getELFSectionNameForGlobal(const GlobalObject *GO, SectionKind Kind,
                            Mangler &Mang, const TargetMachine &TM,
-                           unsigned EntrySize, bool UniqueSectionName) {
+                           unsigned EntrySize, bool UniqueSectionName,
+                           const MachineJumpTableEntry *JTE) {
   SmallString<128> Name =
       getSectionPrefixForGlobal(Kind, TM.isLargeGlobalValue(GO));
   if (Kind.isMergeableCString()) {
@@ -669,7 +671,19 @@ getELFSectionNameForGlobal(const GlobalObject *GO, SectionKind Kind,
 
   bool HasPrefix = false;
   if (const auto *F = dyn_cast<Function>(GO)) {
-    if (std::optional<StringRef> Prefix = F->getSectionPrefix()) {
+    // Jump table hotness takes precedence over its enclosing function's hotness
+    // if it's known. The function's section prefix is used if jump table entry
+    // hotness is unknown.
+    if (JTE && JTE->Hotness != MachineFunctionDataHotness::Unknown) {
+      if (JTE->Hotness == MachineFunctionDataHotness::Hot) {
+        raw_svector_ostream(Name) << ".hot";
+      } else {
+        assert(JTE->Hotness == MachineFunctionDataHotness::Cold &&
+               "Hotness must be cold");
+        raw_svector_ostream(Name) << ".unlikely";
+      }
+      HasPrefix = true;
+    } else if (std::optional<StringRef> Prefix = F->getSectionPrefix()) {
       raw_svector_ostream(Name) << '.' << *Prefix;
       HasPrefix = true;
     }
@@ -767,8 +781,8 @@ calcUniqueIDUpdateFlagsAndSize(const GlobalObject *GO, StringRef SectionName,
   // implicitly for this symbol e.g. .rodata.str1.1, then we don't need
   // to unique the section as the entry size for this symbol will be
   // compatible with implicitly created sections.
-  SmallString<128> ImplicitSectionNameStem =
-      getELFSectionNameForGlobal(GO, Kind, Mang, TM, EntrySize, false);
+  SmallString<128> ImplicitSectionNameStem = getELFSectionNameForGlobal(
+      GO, Kind, Mang, TM, EntrySize, false, /*MJTE=*/nullptr);
   if (SymbolMergeable &&
       Ctx.isELFImplicitMergeableSectionNamePrefix(SectionName) &&
       SectionName.starts_with(ImplicitSectionNameStem))
@@ -874,7 +888,8 @@ MCSection *TargetLoweringObjectFileELF::getExplicitSectionGlobal(
 static MCSectionELF *selectELFSectionForGlobal(
     MCContext &Ctx, const GlobalObject *GO, SectionKind Kind, Mangler &Mang,
     const TargetMachine &TM, bool EmitUniqueSection, unsigned Flags,
-    unsigned *NextUniqueID, const MCSymbolELF *AssociatedSymbol) {
+    unsigned *NextUniqueID, const MCSymbolELF *AssociatedSymbol,
+    const MachineJumpTableEntry *MJTE = nullptr) {
 
   auto [Group, IsComdat, ExtraFlags] = getGlobalObjectInfo(GO, TM);
   Flags |= ExtraFlags;
@@ -893,7 +908,7 @@ static MCSectionELF *selectELFSectionForGlobal(
     }
   }
   SmallString<128> Name = getELFSectionNameForGlobal(
-      GO, Kind, Mang, TM, EntrySize, UniqueSectionName);
+      GO, Kind, Mang, TM, EntrySize, UniqueSectionName, MJTE);
 
   // Use 0 as the unique ID for execute-only text.
   if (Kind.isExecuteOnly())
@@ -967,17 +982,23 @@ MCSection *TargetLoweringObjectFileELF::getUniqueSectionForFunction(
 
 MCSection *TargetLoweringObjectFileELF::getSectionForJumpTable(
     const Function &F, const TargetMachine &TM) const {
+  return getSectionForJumpTable(F, TM, /*JTE=*/nullptr);
+}
+
+MCSection *TargetLoweringObjectFileELF::getSectionForJumpTable(
+    const Function &F, const TargetMachine &TM,
+    const MachineJumpTableEntry *JTE) const {
   // If the function can be removed, produce a unique section so that
   // the table doesn't prevent the removal.
   const Comdat *C = F.getComdat();
   bool EmitUniqueSection = TM.getFunctionSections() || C;
-  if (!EmitUniqueSection)
+  if (!EmitUniqueSection && !TM.getEnableStaticDataPartitioning())
     return ReadOnlySection;
 
   return selectELFSectionForGlobal(getContext(), &F, SectionKind::getReadOnly(),
                                    getMangler(), TM, EmitUniqueSection,
                                    ELF::SHF_ALLOC, &NextUniqueID,
-                                   /* AssociatedSymbol */ nullptr);
+                                   /* AssociatedSymbol */ nullptr, JTE);
 }
 
 MCSection *TargetLoweringObjectFileELF::getSectionForLSDA(

llvm/lib/CodeGen/TargetPassConfig.cpp

@@ -1255,7 +1255,7 @@ void TargetPassConfig::addMachinePasses() {
       }
     }
     addPass(createMachineFunctionSplitterPass());
-    if (SplitStaticData)
+    if (SplitStaticData || TM->Options.EnableStaticDataPartitioning)
       addPass(createStaticDataSplitterPass());
   }
   // We run the BasicBlockSections pass if either we need BB sections or BB

llvm/lib/Target/TargetLoweringObjectFile.cpp

@@ -348,6 +348,12 @@ TargetLoweringObjectFile::SectionForGlobal(const GlobalObject *GO,
 
 MCSection *TargetLoweringObjectFile::getSectionForJumpTable(
     const Function &F, const TargetMachine &TM) const {
+  return getSectionForJumpTable(F, TM, /*JTE=*/nullptr);
+}
+
+MCSection *TargetLoweringObjectFile::getSectionForJumpTable(
+    const Function &F, const TargetMachine &TM,
+    const MachineJumpTableEntry *JTE) const {
   Align Alignment(1);
   return getSectionForConstant(F.getDataLayout(),
                                SectionKind::getReadOnly(), /*C=*/nullptr,