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

Petr Hosek via llvm-commits llvm-commits at lists.llvm.org
Fri May 3 16:09:24 PDT 2024


https://github.com/petrhosek created https://github.com/llvm/llvm-project/pull/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.

>From 78193f68a149e378fbb180a1a5fa1f4551f2af66 Mon Sep 17 00:00:00 2001
From: Petr Hosek <phosek at google.com>
Date: Fri, 3 May 2024 15:48:31 -0700
Subject: [PATCH] [Clang] -fseparate-named-sections option

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.
---
 clang/include/clang/Basic/CodeGenOptions.def  |  1 +
 clang/include/clang/Driver/Options.td         |  5 +++
 clang/lib/CodeGen/BackendUtil.cpp             |  1 +
 llvm/include/llvm/CodeGen/CommandFlags.h      |  2 ++
 llvm/include/llvm/Target/TargetMachine.h      |  4 +++
 llvm/include/llvm/Target/TargetOptions.h      | 20 ++++++-----
 llvm/lib/CodeGen/CommandFlags.cpp             |  8 +++++
 .../CodeGen/TargetLoweringObjectFileImpl.cpp  | 16 +++++++--
 .../X86/elf-separate-named-sections.ll        | 36 +++++++++++++++++++
 9 files changed, 81 insertions(+), 12 deletions(-)
 create mode 100644 llvm/test/CodeGen/X86/elf-separate-named-sections.ll

diff --git a/clang/include/clang/Basic/CodeGenOptions.def b/clang/include/clang/Basic/CodeGenOptions.def
index 340b08dd7e2a33..b964e45574782c 100644
--- a/clang/include/clang/Basic/CodeGenOptions.def
+++ b/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
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index 9f86808145d9ab..deb1c2253c1ed2 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -4136,6 +4136,11 @@ 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,
diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index 22c3f8642ad8eb..119ec4704002cf 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/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;
diff --git a/llvm/include/llvm/CodeGen/CommandFlags.h b/llvm/include/llvm/CodeGen/CommandFlags.h
index 244dabd38cf65b..d5448d781363d4 100644
--- a/llvm/include/llvm/CodeGen/CommandFlags.h
+++ b/llvm/include/llvm/CodeGen/CommandFlags.h
@@ -122,6 +122,8 @@ bool getUniqueSectionNames();
 
 bool getUniqueBasicBlockSectionNames();
 
+bool getSeparateNamedSections();
+
 llvm::EABI getEABIVersion();
 
 llvm::DebuggerKind getDebuggerTuningOpt();
diff --git a/llvm/include/llvm/Target/TargetMachine.h b/llvm/include/llvm/Target/TargetMachine.h
index 48ea3cfe02775b..1ba99730ca7024 100644
--- a/llvm/include/llvm/Target/TargetMachine.h
+++ b/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 {
diff --git a/llvm/include/llvm/Target/TargetOptions.h b/llvm/include/llvm/Target/TargetOptions.h
index d37e9d9576ba75..25d1f3f81fc142 100644
--- a/llvm/include/llvm/Target/TargetOptions.h
+++ b/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 +277,8 @@ namespace llvm {
     /// Use unique names for basic block sections.
     unsigned UniqueBasicBlockSectionNames : 1;
 
+    unsigned SeparateNamedSections : 1;
+
     /// Emit target-specific trap instruction for 'unreachable' IR instructions.
     unsigned TrapUnreachable : 1;
 
diff --git a/llvm/lib/CodeGen/CommandFlags.cpp b/llvm/lib/CodeGen/CommandFlags.cpp
index 14ac4b2102c2fa..677460a2d8e401 100644
--- a/llvm/lib/CodeGen/CommandFlags.cpp
+++ b/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());
diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index 2a77a683a901f1..1217559e1e2276 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -75,6 +75,10 @@ static cl::opt<bool> JumpTableInFunctionSection(
     "jumptable-in-function-section", cl::Hidden, cl::init(false),
     cl::desc("Putting Jump Table in function section"));
 
+static cl::opt<bool> UniqueExplicitSections(
+    "unique-explicit-sections", cl::Hidden, cl::init(true),
+    cl::desc("Putting symbols with explicit section into unique sections"));
+
 static void GetObjCImageInfo(Module &M, unsigned &Version, unsigned &Flags,
                              StringRef &Section) {
   SmallVector<Module::ModuleFlagEntry, 8> ModuleFlags;
@@ -733,16 +737,22 @@ 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
   // implicitly for this symbol e.g. .rodata.str1.1, then we don't need
diff --git a/llvm/test/CodeGen/X86/elf-separate-named-sections.ll b/llvm/test/CodeGen/X86/elf-separate-named-sections.ll
new file mode 100644
index 00000000000000..0631b604afab8e
--- /dev/null
+++ b/llvm/test/CodeGen/X86/elf-separate-named-sections.ll
@@ -0,0 +1,36 @@
+; Test that global values with explicit sections are placed into unique sections.
+
+; RUN: llc < %s 2>&1 | FileCheck %s
+; RUN: llc -separate-named-sections < %s 2>&1 | FileCheck %s --check-prefix=SEPARATE
+target triple="x86_64-unknown-unknown-elf"
+
+define i32 @f() section "custom_text" {
+    entry:
+    ret i32 0
+}
+
+define i32 @g() section "custom_text" {
+    entry:
+    ret i32 0
+}
+
+; CHECK: .section custom_text,"ax", at progbits{{$}}
+; CHECK: f:
+; CHECK: g:
+
+; SEPARATE: .section custom_text,"ax", at progbits,unique,1{{$}}
+; SEPARATE: f:
+; SEPARATE: .section custom_text,"ax", at progbits,unique,2{{$}}
+; SEPARATE: g:
+
+ at i = global i32 0, section "custom_data", align 8
+ at j = global i32 0, section "custom_data", align 8
+
+; CHECK: .section custom_data,"aw", at progbits{{$}}
+; CHECK: i:
+; CHECK: j:
+
+; SEPARATE: .section custom_data,"aw", at progbits,unique,3{{$}}
+; SEPARATE: i:
+; SEPARATE: .section custom_data,"aw", at progbits,unique,4{{$}}
+; SEPARATE: j:



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