Write relocation sections contiguously.

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

1 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/MC/ELFObjectWriter.cpp b/llvm/lib/MC/ELFObjectWriter.cpp
index 8cb01c43edd..b293afc56a0 100644
--- a/llvm/lib/MC/ELFObjectWriter.cpp
+++ b/llvm/lib/MC/ELFObjectWriter.cpp
@@ -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(