IR: Introduce !elf_section_properties for setting section properties.

[pcc]

This new metadata type may be used to set sh_type and sh_entsize on a
global's section. The intent is that it will be used to mark up
CFI jump table sections.

[llvmbot]

@llvm/pr-subscribers-backend-x86

@llvm/pr-subscribers-llvm-ir

Author: Peter Collingbourne (pcc)

Changes

This new metadata type may be used to set sh_type and sh_entsize on a
global's section.

TODO:

• Add tests.

---

Full diff: https://github.com/llvm/llvm-project/pull/149260.diff

2 Files Affected:

• (modified) llvm/include/llvm/IR/FixedMetadataKinds.def (+1)
• (modified) llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp (+37-23)

diff --git a/llvm/include/llvm/IR/FixedMetadataKinds.def b/llvm/include/llvm/IR/FixedMetadataKinds.def
index df572e8791e13..16fd9f4996176 100644
--- a/llvm/include/llvm/IR/FixedMetadataKinds.def
+++ b/llvm/include/llvm/IR/FixedMetadataKinds.def
@@ -53,3 +53,4 @@ LLVM_FIXED_MD_KIND(MD_DIAssignID, "DIAssignID", 38)
 LLVM_FIXED_MD_KIND(MD_coro_outside_frame, "coro.outside.frame", 39)
 LLVM_FIXED_MD_KIND(MD_mmra, "mmra", 40)
 LLVM_FIXED_MD_KIND(MD_noalias_addrspace, "noalias.addrspace", 41)
+LLVM_FIXED_MD_KIND(MD_elf_section_properties, "elf_section_properties", 42)
diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index 5454cd475f5ed..2c523b58d0de4 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -653,10 +653,11 @@ 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,
+                           bool UniqueSectionName,
                            const MachineJumpTableEntry *JTE) {
   SmallString<128> Name =
       getSectionPrefixForGlobal(Kind, TM.isLargeGlobalValue(GO));
+  unsigned EntrySize = getEntrySizeForKind(Kind);
   if (Kind.isMergeableCString()) {
     // We also need alignment here.
     // FIXME: this is getting the alignment of the character, not the
@@ -790,8 +791,8 @@ 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, /*MJTE=*/nullptr);
+  SmallString<128> ImplicitSectionNameStem =
+      getELFSectionNameForGlobal(GO, Kind, Mang, TM, false, /*MJTE=*/nullptr);
   if (SymbolMergeable &&
       Ctx.isELFImplicitMergeableSectionNamePrefix(SectionName) &&
       SectionName.starts_with(ImplicitSectionNameStem))
@@ -802,8 +803,9 @@ calcUniqueIDUpdateFlagsAndSize(const GlobalObject *GO, StringRef SectionName,
   return NextUniqueID++;
 }
 
-static std::tuple<StringRef, bool, unsigned>
-getGlobalObjectInfo(const GlobalObject *GO, const TargetMachine &TM) {
+static std::tuple<StringRef, bool, unsigned, unsigned, unsigned>
+getGlobalObjectInfo(const GlobalObject *GO, const TargetMachine &TM,
+                    StringRef SectionName, SectionKind Kind) {
   StringRef Group = "";
   bool IsComdat = false;
   unsigned Flags = 0;
@@ -814,7 +816,23 @@ getGlobalObjectInfo(const GlobalObject *GO, const TargetMachine &TM) {
   }
   if (TM.isLargeGlobalValue(GO))
     Flags |= ELF::SHF_X86_64_LARGE;
-  return {Group, IsComdat, Flags};
+
+  unsigned Type, EntrySize;
+  if (MDNode *MD = GO->getMetadata(LLVMContext::MD_elf_section_properties)) {
+    Type = cast<ConstantAsMetadata>(MD->getOperand(0))
+        ->getValue()
+        ->getUniqueInteger()
+        .getZExtValue();
+    EntrySize = cast<ConstantAsMetadata>(MD->getOperand(1))
+        ->getValue()
+        ->getUniqueInteger()
+        .getZExtValue();
+  } else {
+    Type = getELFSectionType(SectionName, Kind);
+    EntrySize = getEntrySizeForKind(Kind);
+  }
+
+  return {Group, IsComdat, Flags, Type, EntrySize};
 }
 
 static StringRef handlePragmaClangSection(const GlobalObject *GO,
@@ -850,18 +868,18 @@ static MCSection *selectExplicitSectionGlobal(const GlobalObject *GO,
   Kind = getELFKindForNamedSection(SectionName, Kind);
 
   unsigned Flags = getELFSectionFlags(Kind, TM.getTargetTriple());
-  auto [Group, IsComdat, ExtraFlags] = getGlobalObjectInfo(GO, TM);
+  auto [Group, IsComdat, ExtraFlags, Type, EntrySize] =
+      getGlobalObjectInfo(GO, TM, SectionName, Kind);
   Flags |= ExtraFlags;
 
-  unsigned EntrySize = getEntrySizeForKind(Kind);
   const unsigned UniqueID = calcUniqueIDUpdateFlagsAndSize(
       GO, SectionName, Kind, TM, Ctx, Mang, Flags, EntrySize, NextUniqueID,
       Retain, ForceUnique);
 
   const MCSymbolELF *LinkedToSym = getLinkedToSymbol(GO, TM);
-  MCSectionELF *Section = Ctx.getELFSection(
-      SectionName, getELFSectionType(SectionName, Kind), Flags, EntrySize,
-      Group, IsComdat, UniqueID, LinkedToSym);
+  MCSectionELF *Section =
+      Ctx.getELFSection(SectionName, Type, Flags, EntrySize, Group, IsComdat,
+                        UniqueID, LinkedToSym);
   // Make sure that we did not get some other section with incompatible sh_link.
   // This should not be possible due to UniqueID code above.
   assert(Section->getLinkedToSymbol() == LinkedToSym &&
@@ -899,13 +917,6 @@ static MCSectionELF *selectELFSectionForGlobal(
     const TargetMachine &TM, bool EmitUniqueSection, unsigned Flags,
     unsigned *NextUniqueID, const MCSymbolELF *AssociatedSymbol,
     const MachineJumpTableEntry *MJTE = nullptr) {
-
-  auto [Group, IsComdat, ExtraFlags] = getGlobalObjectInfo(GO, TM);
-  Flags |= ExtraFlags;
-
-  // Get the section entry size based on the kind.
-  unsigned EntrySize = getEntrySizeForKind(Kind);
-
   bool UniqueSectionName = false;
   unsigned UniqueID = MCSection::NonUniqueID;
   if (EmitUniqueSection) {
@@ -916,15 +927,18 @@ static MCSectionELF *selectELFSectionForGlobal(
       (*NextUniqueID)++;
     }
   }
-  SmallString<128> Name = getELFSectionNameForGlobal(
-      GO, Kind, Mang, TM, EntrySize, UniqueSectionName, MJTE);
+  SmallString<128> Name =
+      getELFSectionNameForGlobal(GO, Kind, Mang, TM, UniqueSectionName, MJTE);
+
+  auto [Group, IsComdat, ExtraFlags, Type, EntrySize] =
+      getGlobalObjectInfo(GO, TM, Name, Kind);
+  Flags |= ExtraFlags;
 
   // Use 0 as the unique ID for execute-only text.
   if (Kind.isExecuteOnly())
     UniqueID = 0;
-  return Ctx.getELFSection(Name, getELFSectionType(Name, Kind), Flags,
-                           EntrySize, Group, IsComdat, UniqueID,
-                           AssociatedSymbol);
+  return Ctx.getELFSection(Name, Type, Flags, EntrySize, Group, IsComdat,
+                           UniqueID, AssociatedSymbol);
 }
 
 static MCSection *selectELFSectionForGlobal(

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff HEAD~1 HEAD --extensions cpp -- llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

View the diff from clang-format here.

diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index 2eb1e9de3..d65da59ea 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -799,13 +799,13 @@ getGlobalObjectInfo(const GlobalObject *GO, const TargetMachine &TM,
   unsigned Type, EntrySize;
   if (MDNode *MD = GO->getMetadata(LLVMContext::MD_elf_section_properties)) {
     Type = cast<ConstantAsMetadata>(MD->getOperand(0))
-        ->getValue()
-        ->getUniqueInteger()
-        .getZExtValue();
+               ->getValue()
+               ->getUniqueInteger()
+               .getZExtValue();
     EntrySize = cast<ConstantAsMetadata>(MD->getOperand(1))
-        ->getValue()
-        ->getUniqueInteger()
-        .getZExtValue();
+                    ->getValue()
+                    ->getUniqueInteger()
+                    .getZExtValue();
   } else {
     Type = getELFSectionType(SectionName, Kind);
     EntrySize = getEntrySizeForKind(Kind);

[MaskRay]

Add document to llvm/docs/LangRef.rst named metadata?

[pcc]

Done