[Clang] -fseparate-named-sections option (#91028)

When set, the compiler will use separate unique sections for global
symbols in named special sections (e.g. symbols that are annotated with
__attribute__((section(...)))). Doing so enables linker GC to collect
unused symbols without having to use a different section per-symbol.

Author: petrhosek

clang/docs/ReleaseNotes.rst

@@ -303,6 +303,11 @@ New Compiler Flags
   allow late parsing certain attributes in specific contexts where they would
   not normally be late parsed.
 
+- ``-fseparate-named-sections`` uses separate unique sections for global
+  symbols in named special sections (i.e. symbols annotated with
+  ``__attribute__((section(...)))``. This enables linker GC to collect unused
+  symbols without having to use a per-symbol section.
+
 Deprecated Compiler Flags
 -------------------------
 

clang/include/clang/Basic/CodeGenOptions.def

@@ -57,6 +57,7 @@ CODEGENOPT(UniqueSectionNames, 1, 1) ///< Set for -funique-section-names.
 CODEGENOPT(UniqueBasicBlockSectionNames, 1, 1) ///< Set for -funique-basic-block-section-names,
                                                ///< Produce unique section names with
                                                ///< basic block sections.
+CODEGENOPT(SeparateNamedSections, 1, 0) ///< Set for -fseparate-named-sections.
 CODEGENOPT(EnableAIXExtendedAltivecABI, 1, 0) ///< Set for -mabi=vec-extabi. Enables the extended Altivec ABI on AIX.
 CODEGENOPT(XCOFFReadOnlyPointers, 1, 0) ///< Set for -mxcoff-roptr.
 CODEGENOPT(AllTocData, 1, 0) ///< AIX -mtocdata

clang/include/clang/Driver/Options.td

@@ -4159,6 +4159,11 @@ defm unique_section_names : BoolFOption<"unique-section-names",
   NegFlag<SetFalse, [], [ClangOption, CC1Option],
           "Don't use unique names for text and data sections">,
   PosFlag<SetTrue>>;
+defm separate_named_sections : BoolFOption<"separate-named-sections",
+  CodeGenOpts<"SeparateNamedSections">, DefaultFalse,
+  PosFlag<SetTrue, [], [ClangOption, CC1Option],
+          "Use separate unique sections for named sections (ELF Only)">,
+  NegFlag<SetFalse>>;
 
 defm split_machine_functions: BoolFOption<"split-machine-functions",
   CodeGenOpts<"SplitMachineFunctions">, DefaultFalse,

clang/lib/CodeGen/BackendUtil.cpp

@@ -423,6 +423,7 @@ static bool initTargetOptions(DiagnosticsEngine &Diags,
   Options.UniqueSectionNames = CodeGenOpts.UniqueSectionNames;
   Options.UniqueBasicBlockSectionNames =
       CodeGenOpts.UniqueBasicBlockSectionNames;
+  Options.SeparateNamedSections = CodeGenOpts.SeparateNamedSections;
   Options.TLSSize = CodeGenOpts.TLSSize;
   Options.EnableTLSDESC = CodeGenOpts.EnableTLSDESC;
   Options.EmulatedTLS = CodeGenOpts.EmulatedTLS;

clang/test/CodeGen/fseparate-named-sections.c

@@ -0,0 +1,28 @@
+// REQUIRES: x86-registered-target
+
+// RUN: %clang_cc1 -triple x86_64-pc-linux -S -o - < %s | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-pc-linux -S -fseparate-named-sections -o - < %s | FileCheck %s --check-prefix=SEPARATE
+
+__attribute__((section("custom_text"))) void f(void) {}
+__attribute__((section("custom_text"))) void g(void) {}
+
+// CHECK: .section custom_text,"ax",@progbits{{$}}
+// CHECK: f:
+// CHECK: g:
+
+// SEPARATE: .section custom_text,"ax",@progbits,unique,1{{$}}
+// SEPARATE: f:
+// SEPARATE: .section custom_text,"ax",@progbits,unique,2{{$}}
+// SEPARATE: g:
+
+__attribute__((section("custom_data"))) int i = 0;
+__attribute__((section("custom_data"))) int j = 0;
+
+// CHECK: .section custom_data,"aw",@progbits{{$}}
+// CHECK: i:
+// CHECK: j:
+
+// SEPARATE: .section custom_data,"aw",@progbits,unique,3{{$}}
+// SEPARATE: i:
+// SEPARATE: .section custom_data,"aw",@progbits,unique,4{{$}}
+// SEPARATE: j:

llvm/include/llvm/CodeGen/CommandFlags.h

@@ -122,6 +122,8 @@ bool getUniqueSectionNames();
 
 bool getUniqueBasicBlockSectionNames();
 
+bool getSeparateNamedSections();
+
 llvm::EABI getEABIVersion();
 
 llvm::DebuggerKind getDebuggerTuningOpt();

llvm/include/llvm/Target/TargetMachine.h

@@ -288,6 +288,10 @@ class TargetMachine {
     return Options.UniqueBasicBlockSectionNames;
   }
 
+  bool getSeparateNamedSections() const {
+    return Options.SeparateNamedSections;
+  }
+
   /// Return true if data objects should be emitted into their own section,
   /// corresponds to -fdata-sections.
   bool getDataSections() const {

llvm/include/llvm/Target/TargetOptions.h

@@ -144,15 +144,15 @@ namespace llvm {
           DisableIntegratedAS(false), FunctionSections(false),
           DataSections(false), IgnoreXCOFFVisibility(false),
           XCOFFTracebackTable(true), UniqueSectionNames(true),
-          UniqueBasicBlockSectionNames(false), TrapUnreachable(false),
-          NoTrapAfterNoreturn(false), TLSSize(0), EmulatedTLS(false),
-          EnableTLSDESC(false), EnableIPRA(false), EmitStackSizeSection(false),
-          EnableMachineOutliner(false), EnableMachineFunctionSplitter(false),
-          SupportsDefaultOutlining(false), EmitAddrsig(false), BBAddrMap(false),
-          EmitCallSiteInfo(false), SupportsDebugEntryValues(false),
-          EnableDebugEntryValues(false), ValueTrackingVariableLocations(false),
-          ForceDwarfFrameSection(false), XRayFunctionIndex(true),
-          DebugStrictDwarf(false), Hotpatch(false),
+          UniqueBasicBlockSectionNames(false), SeparateNamedSections(false),
+          TrapUnreachable(false), NoTrapAfterNoreturn(false), TLSSize(0),
+          EmulatedTLS(false), EnableTLSDESC(false), EnableIPRA(false),
+          EmitStackSizeSection(false), EnableMachineOutliner(false),
+          EnableMachineFunctionSplitter(false), SupportsDefaultOutlining(false),
+          EmitAddrsig(false), BBAddrMap(false), EmitCallSiteInfo(false),
+          SupportsDebugEntryValues(false), EnableDebugEntryValues(false),
+          ValueTrackingVariableLocations(false), ForceDwarfFrameSection(false),
+          XRayFunctionIndex(true), DebugStrictDwarf(false), Hotpatch(false),
           PPCGenScalarMASSEntries(false), JMCInstrument(false),
           EnableCFIFixup(false), MisExpect(false), XCOFFReadOnlyPointers(false),
           FPDenormalMode(DenormalMode::IEEE, DenormalMode::IEEE) {}
@@ -277,6 +277,9 @@ namespace llvm {
     /// Use unique names for basic block sections.
     unsigned UniqueBasicBlockSectionNames : 1;
 
+    /// Emit named sections with the same name into different sections.
+    unsigned SeparateNamedSections : 1;
+
     /// Emit target-specific trap instruction for 'unreachable' IR instructions.
     unsigned TrapUnreachable : 1;
 

llvm/lib/CodeGen/CommandFlags.cpp

@@ -96,6 +96,7 @@ CGOPT_EXP(bool, EmulatedTLS)
 CGOPT_EXP(bool, EnableTLSDESC)
 CGOPT(bool, UniqueSectionNames)
 CGOPT(bool, UniqueBasicBlockSectionNames)
+CGOPT(bool, SeparateNamedSections)
 CGOPT(EABI, EABIVersion)
 CGOPT(DebuggerKind, DebuggerTuningOpt)
 CGOPT(bool, EnableStackSizeSection)
@@ -419,6 +420,12 @@ codegen::RegisterCodeGenFlags::RegisterCodeGenFlags() {
       cl::init(false));
   CGBINDOPT(UniqueBasicBlockSectionNames);
 
+  static cl::opt<bool> SeparateNamedSections(
+      "separate-named-sections",
+      cl::desc("Use separate unique sections for named sections"),
+      cl::init(false));
+  CGBINDOPT(SeparateNamedSections);
+
   static cl::opt<EABI> EABIVersion(
       "meabi", cl::desc("Set EABI type (default depends on triple):"),
       cl::init(EABI::Default),
@@ -569,6 +576,7 @@ codegen::InitTargetOptionsFromCodeGenFlags(const Triple &TheTriple) {
   Options.BBSections = getBBSectionsMode(Options);
   Options.UniqueSectionNames = getUniqueSectionNames();
   Options.UniqueBasicBlockSectionNames = getUniqueBasicBlockSectionNames();
+  Options.SeparateNamedSections = getSeparateNamedSections();
   Options.TLSSize = getTLSSize();
   Options.EmulatedTLS =
       getExplicitEmulatedTLS().value_or(TheTriple.hasDefaultEmulatedTLS());

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -733,15 +733,20 @@ calcUniqueIDUpdateFlagsAndSize(const GlobalObject *GO, StringRef SectionName,
       Ctx.isELFGenericMergeableSection(SectionName);
   // If this is the first ocurrence of this section name, treat it as the
   // generic section
-  if (!SymbolMergeable && !SeenSectionNameBefore)
-    return MCContext::GenericSectionID;
+  if (!SymbolMergeable && !SeenSectionNameBefore) {
+    if (TM.getSeparateNamedSections())
+      return NextUniqueID++;
+    else
+      return MCContext::GenericSectionID;
+  }
 
   // Symbols must be placed into sections with compatible entry sizes. Generate
   // unique sections for symbols that have not been assigned to compatible
   // sections.
   const auto PreviousID =
       Ctx.getELFUniqueIDForEntsize(SectionName, Flags, EntrySize);
-  if (PreviousID)
+  if (PreviousID && (!TM.getSeparateNamedSections() ||
+                     *PreviousID == MCContext::GenericSectionID))
     return *PreviousID;
 
   // If the user has specified the same section name as would be created