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[ELF] - Optimize .eh_frame section: remove CIE if all FDEs referencing it were removed. 

This patch performs little optimization for eh_frame section.
If all FDE`s that referenced CIE are removed then CIE is also removed from output. 
That can happen for example when dropping FDEs that point to dropped sections. Testcase showing that is included.
The same optimization was added to ld about 14 years ago: https://sourceware.org/ml/binutils/2001-12/msg00144.html, gold does not do that it seems.

Differential revision: http://reviews.llvm.org/D15564

llvm-svn: 256638
This commit is contained in:
George Rimar 2015-12-30 11:40:44 +00:00
parent 1589071288
commit 57286644f7
2 changed files with 51 additions and 8 deletions
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20
lld/ELF/OutputSections.cpp
View File

@ -947,7 +947,8 @@ void EHOutputSection<ELFT>::addSectionAux(
auto RelI = Rels.begin();
auto RelE = Rels.end();
DenseMap<unsigned, unsigned> OffsetToIndex;
// Maps offset to Index/Length pair.
DenseMap<unsigned, std::pair<unsigned, uint32_t>> OffsetToData;
while (!D.empty()) {
unsigned Index = S->Offsets.size();
S->Offsets.push_back(std::make_pair(Offset, -1));
@ -974,21 +975,24 @@ void EHOutputSection<ELFT>::addSectionAux(
std::pair<StringRef, StringRef> CieInfo(Entry, Personality);
auto P = CieMap.insert(std::make_pair(CieInfo, Cies.size()));
if (P.second) {
if (P.second)
Cies.push_back(C);
this->Header.sh_size += RoundUpToAlignment(Length, sizeof(uintX_t));
}
OffsetToIndex[Offset] = P.first->second;
OffsetToData[Offset] = std::make_pair(P.first->second, Length);
} else {
if (!HasReloc)
error("FDE doesn't reference another section");
InputSectionBase<ELFT> *Target = S->getRelocTarget(*RelI);
if (Target != &InputSection<ELFT>::Discarded && Target->isLive()) {
uint32_t CieOffset = Offset + 4 - ID;
auto I = OffsetToIndex.find(CieOffset);
if (I == OffsetToIndex.end())
auto I = OffsetToData.find(CieOffset);
if (I == OffsetToData.end())
error("Invalid CIE reference");
Cies[I->second].Fdes.push_back(EHRegion<ELFT>(S, Index));
std::pair<unsigned, uint32_t> &IndLen = I->second;
Cie<ELFT> &Cie = Cies[IndLen.first];
if (Cie.Fdes.empty())
this->Header.sh_size +=
RoundUpToAlignment(IndLen.second, sizeof(uintX_t));
Cie.Fdes.push_back(EHRegion<ELFT>(S, Index));
this->Header.sh_size += RoundUpToAlignment(Length, sizeof(uintX_t));
}
}

39
lld/test/ELF/eh-frame-opt.s Normal file
View File

@ -0,0 +1,39 @@
// REQUIRES: x86
// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
// RUN: ld.lld --gc-sections %t.o -o %t
// RUN: llvm-readobj -s -section-data %t | FileCheck %s
// Here we check that if all FDEs referencing a CIE
// were removed, CIE is also removed.
// CHECK: Section {
// CHECK: Index:
// CHECK: Name: .eh_frame
// CHECK-NEXT: Type: SHT_X86_64_UNWIND
// CHECK-NEXT: Flags [
// CHECK-NEXT: SHF_ALLOC
// CHECK-NEXT: ]
// CHECK-NEXT: Address: 0x10120
// CHECK-NEXT: Offset: 0x120
// CHECK-NEXT: Size: 0
// CHECK-NEXT: Link: 0
// CHECK-NEXT: Info: 0
// CHECK-NEXT: AddressAlignment: 8
// CHECK-NEXT: EntrySize: 0
// CHECK-NEXT: SectionData (
// CHECK-NEXT: )
// CHECK-NEXT: }
.section foo,"ax",@progbits
.cfi_startproc
nop
.cfi_endproc
.section bar,"ax",@progbits
.cfi_startproc
nop
nop
.cfi_endproc
.text
.globl _start;
_start: