[AsmPrinter] Split up .gcc_except_table

MC currently produces monolithic .gcc_except_table section. GCC can split up .gcc_except_table:

* if comdat: `.section .gcc_except_table._Z6comdatv,"aG",@progbits,_Z6comdatv,comdat`
* otherwise, if -ffunction-sections: `.section .gcc_except_table._Z3fooi,"a",@progbits`

This ensures that (a) non-prevailing copies are discarded and (b)
.gcc_except_table associated to discarded text sections can be discarded by a
.gcc_except_table-aware linker (GNU ld, but not gold or LLD)

This patches matches the GCC behavior. If -fno-unique-section-names is
specified, we don't append the suffix. If -ffunction-sections is additionally specified,
use `.section ...,unique`.

Note, if clang driver communicates that the linker is LLD and we know it
is new (11.0.0 or later) we can use SHF_LINK_ORDER to avoid string table
costs, at least in the -fno-unique-section-names case. We cannot use it on GNU
ld because as of binutils 2.35 it does not support mixed SHF_LINK_ORDER &
non-SHF_LINK_ORDER components in an output section
https://sourceware.org/bugzilla/show_bug.cgi?id=26256

For RISC-V -mrelax, this patch additionally fixes an assembler-linker
interaction problem: because a section is shrinkable, the length of a call-site
code range is not a constant. Relocations referencing the associated text
section (STT_SECTION) are needed. However, a STB_LOCAL relocation referencing a
discarded section group member from outside the group is disallowed by the ELF
specification (PR46675):

```
// a.cc
inline int comdat() { try { throw 1; } catch (int) { return 1; } return 0; }
int main() { return comdat(); }

// b.cc
inline int comdat() { try { throw 1; } catch (int) { return 1; } return 0; }
int foo() { return comdat(); }

clang++ -target riscv64-linux -c a.cc b.cc -fPIC -mno-relax
ld.lld -shared a.o b.o => ld.lld: error: relocation refers to a symbol in a discarded section:
```

-fbasic-block-sections= is similar to RISC-V -mrelax: there are outstanding relocations.

Reviewed By: jrtc27, rahmanl

Differential Revision: https://reviews.llvm.org/D83655

llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h

@@ -63,6 +63,8 @@ class TargetLoweringObjectFileELF : public TargetLoweringObjectFile {
 
   MCSection *getSectionForJumpTable(const Function &F,
                                     const TargetMachine &TM) const override;
+  MCSection *getSectionForLSDA(const Function &F,
+                               const TargetMachine &TM) const override;
 
   MCSection *
   getSectionForMachineBasicBlock(const Function &F,

llvm/include/llvm/MC/MCObjectFileInfo.h

@@ -250,7 +250,6 @@ class MCObjectFileInfo {
   MCSection *getDataSection() const { return DataSection; }
   MCSection *getBSSSection() const { return BSSSection; }
   MCSection *getReadOnlySection() const { return ReadOnlySection; }
-  MCSection *getLSDASection() const { return LSDASection; }
   MCSection *getCompactUnwindSection() const { return CompactUnwindSection; }
   MCSection *getDwarfAbbrevSection() const { return DwarfAbbrevSection; }
   MCSection *getDwarfInfoSection() const { return DwarfInfoSection; }

llvm/include/llvm/Target/TargetLoweringObjectFile.h

@@ -118,6 +118,10 @@ class TargetLoweringObjectFile : public MCObjectFileInfo {
 
   virtual MCSection *getSectionForJumpTable(const Function &F,
                                             const TargetMachine &TM) const;
+  virtual MCSection *getSectionForLSDA(const Function &F,
+                                       const TargetMachine &TM) const {
+    return LSDASection;
+  }
 
   virtual bool shouldPutJumpTableInFunctionSection(bool UsesLabelDifference,
                                                    const Function &F) const;

llvm/lib/CodeGen/AsmPrinter/EHStreamer.cpp

@@ -417,7 +417,8 @@ MCSymbol *EHStreamer::emitExceptionTable() {
   bool HaveTTData = !TypeInfos.empty() || !FilterIds.empty();
 
   // Type infos.
-  MCSection *LSDASection = Asm->getObjFileLowering().getLSDASection();
+  MCSection *LSDASection =
+      Asm->getObjFileLowering().getSectionForLSDA(MF->getFunction(), Asm->TM);
   unsigned TTypeEncoding;
 
   if (!HaveTTData) {

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -833,6 +833,42 @@ MCSection *TargetLoweringObjectFileELF::getSectionForJumpTable(
                                    /* AssociatedSymbol */ nullptr);
 }
 
+MCSection *
+TargetLoweringObjectFileELF::getSectionForLSDA(const Function &F,
+                                               const TargetMachine &TM) const {
+  // If neither COMDAT nor function sections, use the monolithic LSDA section.
+  if (!F.hasComdat() && !TM.getFunctionSections())
+    return LSDASection;
+
+  const auto *LSDA = cast<MCSectionELF>(LSDASection);
+  unsigned Flags = LSDA->getFlags();
+  StringRef Group;
+  if (F.hasComdat()) {
+    Group = F.getComdat()->getName();
+    Flags |= ELF::SHF_GROUP;
+  }
+
+  // Append the function name as the suffix like GCC, assuming
+  // -funique-section-names applies to .gcc_except_table sections.
+  if (TM.getUniqueSectionNames())
+    return getContext().getELFSection(LSDA->getName() + "." + F.getName(),
+                                      LSDA->getType(), Flags, 0, Group,
+                                      MCSection::NonUniqueID, nullptr);
+
+  // Allocate a unique ID if function sections && (integrated assembler or GNU
+  // as>=2.35). Note we could use SHF_LINK_ORDER to facilitate --gc-sections but
+  // that would require that we know the linker is a modern LLD (12.0 or later).
+  // GNU ld as of 2.35 does not support mixed SHF_LINK_ORDER &
+  // non-SHF_LINK_ORDER components in an output section
+  // https://sourceware.org/bugzilla/show_bug.cgi?id=26256
+  unsigned ID = TM.getFunctionSections() &&
+                        getContext().getAsmInfo()->useIntegratedAssembler()
+                    ? NextUniqueID++
+                    : MCSection::NonUniqueID;
+  return getContext().getELFSection(LSDA->getName(), LSDA->getType(), Flags, 0,
+                                    Group, ID, nullptr);
+}
+
 bool TargetLoweringObjectFileELF::shouldPutJumpTableInFunctionSection(
     bool UsesLabelDifference, const Function &F) const {
   // We can always create relative relocations, so use another section

llvm/test/CodeGen/X86/gcc_except_table-multi.ll

@@ -0,0 +1,58 @@
+; RUN: llc < %s -mtriple=x86_64 | FileCheck %s --check-prefixes=CHECK,NORMAL
+; RUN: llc < %s -mtriple=x86_64 -unique-section-names=false | FileCheck %s --check-prefixes=CHECK,NOUNIQUE
+; RUN: llc < %s -mtriple=x86_64 -function-sections | FileCheck %s --check-prefixes=CHECK,SEP
+; RUN: llc < %s -mtriple=x86_64 -function-sections -unique-section-names=false | FileCheck %s --check-prefixes=CHECK,SEP_NOUNIQUE
+
+;; Don't use `,unique` if GNU as<2.35.
+; RUN: llc < %s -mtriple=x86_64 -function-sections -unique-section-names=false -no-integrated-as | FileCheck %s --check-prefixes=CHECK,SEP_NOUNIQUE_GAS
+
+@_ZTIi = external constant i8*
+
+;; If the function is in a comdat group, the generated .gcc_except_table should
+;; be placed in the same group, so that .gcc_except_table can be discarded if
+;; the comdat is not prevailing. If -funique-section-names, append the function name.
+$group = comdat any
+define i32 @group() uwtable comdat personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) {
+; CHECK-LABEL:       group:
+; CHECK:             .cfi_endproc
+; NORMAL-NEXT:       .section .gcc_except_table.group,"aG",@progbits,group,comdat{{$}}
+; NOUNIQUE-NEXT:     .section .gcc_except_table,"aG",@progbits,group,comdat{{$}}
+; SEP-NEXT:          .section .gcc_except_table.group,"aG",@progbits,group,comdat{{$}}
+; SEP_NOUNIQUE-NEXT: .section .gcc_except_table,"aG",@progbits,group,comdat,unique,2
+; SEP_NOUNIQUE_GAS-NEXT: .section .gcc_except_table,"aG",@progbits,group,comdat{{$}}
+entry:
+  invoke void @ext() to label %try.cont unwind label %lpad
+lpad:
+  %0 = landingpad { i8*, i32 } catch i8* bitcast (i8** @_ZTIi to i8*)
+  br label %eh.resume
+try.cont:
+  ret i32 0
+eh.resume:
+  resume { i8*, i32 } %0
+}
+
+;; If the function is not in a comdat group, but function sections is enabled,
+;; use a separate section by either using a unique ID (integrated assembler) or
+;; a suffix (GNU as<2.35).
+define i32 @foo() uwtable personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) {
+; CHECK-LABEL:       foo:
+; CHECK:             .cfi_endproc
+; NORMAL-NEXT:       .section .gcc_except_table,"a",@progbits{{$}}
+; NOUNIQUE-NEXT:     .section .gcc_except_table,"a",@progbits{{$}}
+; SEP-NEXT:          .section .gcc_except_table.foo,"a",@progbits{{$}}
+; SEP_NOUNIQUE-NEXT: .section .gcc_except_table,"a",@progbits,unique,4
+; SEP_NOUNIQUE_GAS-NEXT: .section .gcc_except_table,"a",@progbits{{$}}
+entry:
+  invoke void @ext() to label %try.cont unwind label %lpad
+lpad:
+  %0 = landingpad { i8*, i32 } catch i8* bitcast (i8** @_ZTIi to i8*)
+  br label %eh.resume
+try.cont:
+  ret i32 0
+eh.resume:
+  resume { i8*, i32 } %0
+}
+
+declare void @ext()
+
+declare i32 @__gxx_personality_v0(...)