From ee5d1a04494390ab8dbece1e32cb00312ecce4b1 Mon Sep 17 00:00:00 2001
From: Fangrui Song <maskray@google.com>
Date: Mon, 2 Nov 2020 14:36:25 -0800
Subject: [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/lib/CodeGen/TargetLoweringObjectFileImpl.cpp | 36 +++++++++++++++++++++++
 1 file changed, 36 insertions(+)

(limited to 'llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp')

diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index de0071685799..1772e7f4e55d 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/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
-- 
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