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[ICF] Ignore SHF_GROUP flag when comparing two sections. 

SHF_GROUP bit doesn't make sense in executables or DSOs, so linkers are
expected to remove that bit from section flags. We did that when we create
output sections.

This patch is to do that earlier than before. Now the flag is dropped when
we instantiate input section objects.

This change improves ICF. Previously, two sections that differ only in
SHF_GROUP flag were not merged, because when the control reached ICF,
the flag was still there. Now the flag is dropped before reaching to ICF,
so the difference is ignored naturally.

This issue was found by pcc.

Differential Revision: https://reviews.llvm.org/D34074

llvm-svn: 305134
This commit is contained in:
Rui Ueyama 2017-06-10 00:38:55 +00:00
parent c7672d66ba
commit 1c837b5fb1
4 changed files with 39 additions and 8 deletions
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12
lld/ELF/InputSection.cpp
View File

@ -73,6 +73,16 @@ InputSectionBase::InputSectionBase(InputFile *File, uint64_t Flags,
this->Alignment = V;
}
// Drop SHF_GROUP bit unless we are producing a re-linkable object file.
// SHF_GROUP is a marker that a section belongs to some comdat group.
// That flag doesn't make sense in an executable.
static uint64_t getFlags(uint64_t Flags) {
Flags &= ~(uint64_t)SHF_INFO_LINK;
if (!Config->Relocatable)
Flags &= ~(uint64_t)SHF_GROUP;
return Flags;
}
// GNU assembler 2.24 and LLVM 4.0.0's MC (the newest release as of
// March 2017) fail to infer section types for sections starting with
// ".init_array." or ".fini_array.". They set SHT_PROGBITS instead of
@ -95,7 +105,7 @@ template <class ELFT>
InputSectionBase::InputSectionBase(elf::ObjectFile<ELFT> *File,
const typename ELFT::Shdr *Hdr,
StringRef Name, Kind SectionKind)
: InputSectionBase(File, Hdr->sh_flags & ~SHF_INFO_LINK,
: InputSectionBase(File, getFlags(Hdr->sh_flags),
getType(Hdr->sh_type, Name), Hdr->sh_entsize,
Hdr->sh_link, Hdr->sh_info, Hdr->sh_addralign,
getSectionContents(File, Hdr), Name, SectionKind) {

8
lld/ELF/OutputSections.cpp
View File

@ -313,10 +313,6 @@ void OutputSectionFactory::addInputSec(InputSectionBase *IS,
return;
}
uint64_t Flags = IS->Flags;
if (!Config->Relocatable)
Flags &= ~(uint64_t)SHF_GROUP;
if (Sec) {
if (getIncompatibleFlags(Sec->Flags) != getIncompatibleFlags(IS->Flags))
error("incompatible section flags for " + Sec->Name +
@ -333,9 +329,9 @@ void OutputSectionFactory::addInputSec(InputSectionBase *IS,
"\n>>> output section " + Sec->Name + ": " +
getELFSectionTypeName(Config->EMachine, Sec->Type));
}
Sec->Flags |= Flags;
Sec->Flags |= IS->Flags;
} else {
Sec = make<OutputSection>(OutsecName, IS->Type, Flags);
Sec = make<OutputSection>(OutsecName, IS->Type, IS->Flags);
OutputSections.push_back(Sec);
}

4
lld/test/ELF/arm-icf-exidx.s
View File

@ -19,13 +19,15 @@ g:
.section .text.h
.global __aeabi_unwind_cpp_pr0
__aeabi_unwind_cpp_pr0:
nop
bx lr
// CHECK: Disassembly of section .text:
// CHECK-NEXT: f:
// CHECK-NEXT: 11000: 1e ff 2f e1 bx lr
// CHECK: __aeabi_unwind_cpp_pr0:
// CHECK-NEXT: 11004: 1e ff 2f e1 bx lr
// CHECK-NEXT: 11004: 00 f0 20 e3 nop
// CHECK-NEXT: 11008: 1e ff 2f e1 bx lr
// CHECK: Contents of section .ARM.exidx:
// CHECK-NEXT: 100d4 2c0f0000 b0b0b080 280f0000 01000000

23
lld/test/ELF/icf-comdat.s Normal file
View File

@ -0,0 +1,23 @@
# REQUIRES: x86
# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
# RUN: ld.lld %t -o %t2 --icf=all --verbose | FileCheck %s
# CHECK: selected .text.f1
# CHECK: removed .text.f2
.globl _start, f1, f2
_start:
ret
.section .text.f1,"ax"
f1:
mov $60, %rax
mov $42, %rdi
syscall
.section .text.f2,"axG",@progbits,foo,comdat
f2:
mov $60, %rax
mov $42, %rdi
syscall