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[lld][ELF] Support for zero flag section groups 

This change introduces support for zero flag ELF section groups to lld.
lld already supports COMDAT sections, which in ELF are a special type of
ELF section groups. These are generally useful to enable linker GC where
you want a group of sections to always travel together, that is to be
either retained or discarded as a whole, but without the COMDAT
semantics. Other ELF linkers already support zero flag ELF section
groups and this change helps us reach feature parity.

Differential Revision: https://reviews.llvm.org/D96636
This commit is contained in:
Petr Hosek 2021-02-12 14:26:31 -08:00
parent fbee4a0c79
commit bfa4235e6e
2 changed files with 86 additions and 42 deletions
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17
lld/ELF/InputFiles.cpp
View File

@ -609,27 +609,20 @@ void ObjFile<ELFT>::initializeSections(bool ignoreComdats) {
StringRef signature = getShtGroupSignature(objSections, sec); StringRef signature = getShtGroupSignature(objSections, sec);
this->sections[i] = &InputSection::discarded; this->sections[i] = &InputSection::discarded;
ArrayRef<Elf_Word> entries = ArrayRef<Elf_Word> entries =
CHECK(obj.template getSectionContentsAsArray<Elf_Word>(sec), this); CHECK(obj.template getSectionContentsAsArray<Elf_Word>(sec), this);
if (entries.empty()) if (entries.empty())
fatal(toString(this) + ": empty SHT_GROUP"); fatal(toString(this) + ": empty SHT_GROUP");
// The first word of a SHT_GROUP section contains flags. Currently, Elf_Word flag = entries[0];
// the standard defines only "GRP_COMDAT" flag for the COMDAT group. if (flag && flag != GRP_COMDAT)
// An group with the empty flag doesn't define anything; such sections
// are just skipped.
if (entries[0] == 0)
continue;
if (entries[0] != GRP_COMDAT)
fatal(toString(this) + ": unsupported SHT_GROUP format"); fatal(toString(this) + ": unsupported SHT_GROUP format");
bool isNew = bool keepGroup =
ignoreComdats || (flag & GRP_COMDAT) == 0 || ignoreComdats ||
symtab->comdatGroups.try_emplace(CachedHashStringRef(signature), this) symtab->comdatGroups.try_emplace(CachedHashStringRef(signature), this)
.second; .second;
if (isNew) { if (keepGroup) {
if (config->relocatable) if (config->relocatable)
this->sections[i] = createInputSection(sec); this->sections[i] = createInputSection(sec);
selectedGroups.push_back(entries); selectedGroups.push_back(entries);

111
lld/test/ELF/gc-sections-group.s
View File

@ -7,48 +7,99 @@
# RUN: ld.lld --gc-sections %t.o -o %t.dead # RUN: ld.lld --gc-sections %t.o -o %t.dead
# RUN: llvm-readobj -S %t.dead | FileCheck %s --check-prefix=CHECK-DEAD # RUN: llvm-readobj -S %t.dead | FileCheck %s --check-prefix=CHECK-DEAD
## .mynote.bar is retained because it is not in a group. ## .mynote.ccc is retained because it is not in a group.
# CHECK-DEAD-NOT: Name: .myanote.foo # CHECK-DEAD-NOT: Name: .myanote.aaa
# CHECK-DEAD-NOT: Name: .mytext.foo # CHECK-DEAD-NOT: Name: .mytext.aaa
# CHECK-DEAD-NOT: Name: .mybss.foo # CHECK-DEAD-NOT: Name: .mybss.aaa
# CHECK-DEAD-NOT: Name: .mynote.foo # CHECK-DEAD-NOT: Name: .mynote.aaa
# CHECK-DEAD: Name: .mynote.bar # CHECK-DEAD-NOT: Name: .myanote.bbb
# CHECK-DEAD-NOT: Name: .mytext.bbb
# CHECK-DEAD-NOT: Name: .mybss.bbb
# CHECK-DEAD-NOT: Name: .mynote.bbb
# CHECK-DEAD: Name: .mynote.ccc
# RUN: ld.lld --gc-sections %t.o -o %t -e anote_foo # RUN: ld.lld --gc-sections %t.o -o %t -e anote_aaa
# RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=CHECK-LIVE # RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=CHECK-LIVE-GROUP
# RUN: ld.lld --gc-sections %t.o -o %t -e foo # RUN: ld.lld --gc-sections %t.o -o %t -e aaa
# RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=CHECK-LIVE # RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=CHECK-LIVE-GROUP
# RUN: ld.lld --gc-sections %t.o -o %t -e bss_foo # RUN: ld.lld --gc-sections %t.o -o %t -e bss_aaa
# RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=CHECK-LIVE # RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=CHECK-LIVE-GROUP
## note_foo as the entry point does not make much sense because it is defined ## note_zero as the entry point does not make much sense because it is defined
## in a non-SHF_ALLOC section. This is just to demonstrate the behavior. ## in a non-SHF_ALLOC section. This is just to demonstrate the behavior.
# RUN: ld.lld --gc-sections %t.o -o %t -e note_foo # RUN: ld.lld --gc-sections %t.o -o %t -e note_aaa
# RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=CHECK-LIVE # RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=CHECK-LIVE-GROUP
# CHECK-LIVE: Name: .myanote.foo # CHECK-LIVE-GROUP: Name: .myanote.aaa
# CHECK-LIVE: Name: .mytext.foo # CHECK-LIVE-GROUP: Name: .mytext.aaa
# CHECK-LIVE: Name: .mybss.foo # CHECK-LIVE-GROUP: Name: .mybss.aaa
# CHECK-LIVE: Name: .mynote.foo # CHECK-LIVE-GROUP: Name: .mynote.aaa
# CHECK-LIVE: Name: .mynote.bar # CHECK-LIVE-GROUP-NOT: Name: .myanote.bbb
# CHECK-LIVE-GROUP-NOT: Name: .mytext.bbb
# CHECK-LIVE-GROUP-NOT: Name: .mybss.bbb
# CHECK-LIVE-GROUP-NOT: Name: .mynote.bbb
# CHECK-LIVE-GROUP: Name: .mynote.ccc
.globl anote_foo, foo, bss_foo, note_foo # RUN: ld.lld --gc-sections %t.o -o %t -e anote_bbb
# RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=CHECK-LIVE-COMDAT
# RUN: ld.lld --gc-sections %t.o -o %t -e bbb
# RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=CHECK-LIVE-COMDAT
# RUN: ld.lld --gc-sections %t.o -o %t -e bss_bbb
# RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=CHECK-LIVE-COMDAT
.section .myanote.foo,"aG",@note,foo,comdat ## note_bbb as the entry point does not make much sense because it is defined
anote_foo: ## in a non-SHF_ALLOC section. This is just to demonstrate the behavior.
# RUN: ld.lld --gc-sections %t.o -o %t -e note_bbb
# RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=CHECK-LIVE-COMDAT
# CHECK-LIVE-COMDAT-NOT: Name: .myanote.aaa
# CHECK-LIVE-COMDAT-NOT: Name: .mytext.aaa
# CHECK-LIVE-COMDAT-NOT: Name: .mybss.aaa
# CHECK-LIVE-COMDAT-NOT: Name: .mynote.aaa
# CHECK-LIVE-COMDAT: Name: .myanote.bbb
# CHECK-LIVE-COMDAT: Name: .mytext.bbb
# CHECK-LIVE-COMDAT: Name: .mybss.bbb
# CHECK-LIVE-COMDAT: Name: .mynote.bbb
# CHECK-LIVE-COMDAT: Name: .mynote.ccc
## These sections are in a zero flag group `aaa`.
.globl anote_aaa, aaa, bss_aaa, note_aaa
.section .myanote.aaa,"aG",@note,aaa
anote_aaa:
.byte 0 .byte 0
.section .mytext.foo,"axG",@progbits,foo,comdat .section .mytext.aaa,"axG",@progbits,aaa
foo: aaa:
.byte 0 .byte 0
.section .mybss.foo,"awG",@nobits,foo,comdat .section .mybss.aaa,"awG",@nobits,aaa
bss_foo: bss_aaa:
.byte 0 .byte 0
.section .mynote.foo,"G",@note,foo,comdat .section .mynote.aaa,"G",@note,aaa
note_foo: note_aaa:
.byte 0 .byte 0
.section .mynote.bar,"",@note ## These sections are in a COMDAT group `bbb`.
.globl anote_bbb, bbb, bss_bbb, note_bbb
.section .myanote.bbb,"aG",@note,bbb,comdat
anote_bbb:
.byte 0
.section .mytext.bbb,"axG",@progbits,bbb,comdat
bbb:
.byte 0
.section .mybss.bbb,"awG",@nobits,bbb,comdat
bss_bbb:
.byte 0
.section .mynote.bbb,"G",@note,bbb,comdat
note_bbb:
.byte 0
## This section isn't in any group.
.section .mynote.ccc,"",@note
.byte 0 .byte 0