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Write relocation sections contiguously.

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Linkers normally read all the relocations upfront to compute the references
between sections. Putting them together is a bit more cache friendly.

I benchmarked linking a Release+Asserts clang with gold on a vm. I tried all
4 combinations of --gc-sections/no --gc-section hot and cold cache.

I cleared the cache with

echo 3 > /proc/sys/vm/drop_caches

and warmed it up by running the link once before timing the subsequent ones.

With cold cache and --gc-sections the time goes from

1.86130781665 +- 0.01713126697463843 seconds
to
1.82370735105 +- 0.014127522318814516 seconds

With cold cache and no --gc-sections the time goes from

1.6087245435500002 +- 0.012999066825178644 seconds
to
1.5687122041500001 +- 0.013145850126026619 seconds

With hot cache and no --gc-sections the time goes from

0.926200939 ( +-  0.33% ) seconds
to
0.907200079 ( +-  0.31% ) seconds

With hot cache and gc sections the time goes from

1.183038049 ( +-  0.34% ) seconds
to
1.147355862 ( +-  0.39% ) seconds

llvm-svn: 235165
This commit is contained in:
Rafael Espindola 2015-04-17 08:11:38 +00:00
parent 4fc2911327
commit 3b98c374cd
5 changed files with 18 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

9
lib/MC/ELFObjectWriter.cpp
View File

@ -943,6 +943,8 @@ void ELFObjectWriter::computeIndexMap(MCAssembler &Asm,
SectionIndexMap[&Section] = Index++;
}
std::vector<const MCSectionELF *> RelSections;
for (MCAssembler::iterator it = Asm.begin(),
ie = Asm.end(); it != ie; ++it) {
const MCSectionData &SD = *it;
@ -956,9 +958,14 @@ void ELFObjectWriter::computeIndexMap(MCAssembler &Asm,
if (MCSectionData *RelSD = createRelocationSection(Asm, SD)) {
const MCSectionELF *RelSection =
static_cast<const MCSectionELF *>(&RelSD->getSection());
SectionIndexMap[RelSection] = Index++;
RelSections.push_back(RelSection);
}
}
// Put relocation sections close together. The linker reads them
// first, so this improves cache locality.
for (const MCSectionELF * Sec: RelSections)
SectionIndexMap[Sec] = Index++;
}
void ELFObjectWriter::computeSymbolTable(

4
test/MC/ARM/eh-directive-section-comdat.s
View File

@ -52,7 +52,7 @@ func1:
@ The second, third, and fourth word should correspond to the section index
@ of .TEST1, .ARM.extab.TEST1, and .ARM.exidx.TEST1.
@-------------------------------------------------------------------------------
@ CHECK: 0000: 01000000 05000000 06000000 08000000 |................|
@ CHECK: 0000: 01000000 05000000 06000000 07000000 |................|
@ CHECK: )
@ CHECK: }
@ -96,7 +96,7 @@ func1:
@ Check the .ARM.exidx.TEST1 section
@-------------------------------------------------------------------------------
@ CHECK: Section {
@ CHECK: Index: 8
@ CHECK: Index: 7
@ CHECK: Name: .ARM.exidx.TEST1
@ CHECK: Type: SHT_ARM_EXIDX (0x70000001)
@-------------------------------------------------------------------------------

6
test/MC/ARM/eh-directive-section.s
View File

@ -109,7 +109,7 @@ func2:
@ Check the TEST2 section (without the dot in the beginning)
@-------------------------------------------------------------------------------
@ CHECK: Section {
@ CHECK: Index: 9
@ CHECK: Index: 7
@ CHECK: Name: TEST2
@ CHECK: SectionData (
@ CHECK: 0000: 1EFF2FE1 |../.|
@ -143,7 +143,7 @@ func2:
@-------------------------------------------------------------------------------
@ This section should linked with TEST2 section.
@-------------------------------------------------------------------------------
@ CHECK: Link: 9
@ CHECK: Link: 7
@-------------------------------------------------------------------------------
@ The first word should be relocated to the code address in TEST2 section.
@ -175,6 +175,6 @@ func2:
@ CHECK: }
@ CHECK: Symbol {
@ CHECK: Name: func2
@ CHECK: Section: TEST2 (0x9)
@ CHECK: Section: TEST2 (0x7)
@ CHECK: }
@ CHECK: ]

6
test/MC/ARM/eh-link.s
View File

@ -43,7 +43,7 @@
@ CHECK-NEXT: }
@ CHECK: Section {
@ CHECK: Index: 9
@ CHECK: Index: 8
@ CHECK-NEXT: Name: .text
@ CHECK-NEXT: Type: SHT_PROGBITS
@ CHECK-NEXT: Flags [
@ -60,7 +60,7 @@
@ CHECK-NEXT: EntrySize: 0
@ CHECK-NEXT: }
@ CHECK-NEXT: Section {
@ CHECK-NEXT: Index: 10
@ CHECK-NEXT: Index: 9
@ CHECK-NEXT: Name: .ARM.exidx
@ CHECK-NEXT: Type: SHT_ARM_EXIDX
@ CHECK-NEXT: Flags [
@ -71,7 +71,7 @@
@ CHECK-NEXT: Address: 0x0
@ CHECK-NEXT: Offset:
@ CHECK-NEXT: Size: 8
@ CHECK-NEXT: Link: 9
@ CHECK-NEXT: Link: 8
@ CHECK-NEXT: Info: 0
@ CHECK-NEXT: AddressAlignment: 4
@ CHECK-NEXT: EntrySize: 0

4
test/MC/ELF/comdat-reloc.s
View File

@ -16,9 +16,9 @@ world:
// CHECK: Name: .group
// CHECK-NOT: SectionData
// CHECK: SectionData
// CHECK-NEXT: 0000: 01000000 06000000 07000000
// CHECK-NEXT: 0000: 01000000 05000000 07000000
// CHECK: Index: 6
// CHECK: Index: 5
// CHECK-NEXT: Name: .text.world
// CHECK-NOT: Section {
// CHECK: SHF_GROUP