Skip to content

Commit

Permalink
[AsmPrinter] Split up .gcc_except_table
Browse files Browse the repository at this point in the history
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
  • Loading branch information
MaskRay committed Nov 2, 2020
1 parent 1fcd5d5 commit ee5d1a0
Show file tree
Hide file tree
Showing 6 changed files with 102 additions and 2 deletions.
2 changes: 2 additions & 0 deletions llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
Original file line number Diff line number Diff line change
Expand Up @@ -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,
Expand Down
1 change: 0 additions & 1 deletion llvm/include/llvm/MC/MCObjectFileInfo.h
Original file line number Diff line number Diff line change
Expand Up @@ -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; }
Expand Down
4 changes: 4 additions & 0 deletions llvm/include/llvm/Target/TargetLoweringObjectFile.h
Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand Down
3 changes: 2 additions & 1 deletion llvm/lib/CodeGen/AsmPrinter/EHStreamer.cpp
Original file line number Diff line number Diff line change
Expand Up @@ -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) {
Expand Down
36 changes: 36 additions & 0 deletions llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand Down
58 changes: 58 additions & 0 deletions llvm/test/CodeGen/X86/gcc_except_table-multi.ll
Original file line number Diff line number Diff line change
@@ -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(...)

0 comments on commit ee5d1a0

Please sign in to comment.