[ELF] Deduplicate names of local symbols only with -O2

The deduplication requires a DenseMap of the same size of the local part of
.strtab . I optimized it in e20544543478b259eb09fa0a253d4fb1a5525d9e but it is
still quite slow.

For Release build of clang, deduplication makes .strtab 1.1% smaller and makes the link 3% slower.
For chrome, deduplication makes .strtab 0.1% smaller and makes the link 6% slower.

I suggest that we only perform the optimization with -O2 (default is -O1).
Not deduplicating local symbol names will simplify parallel symbol table write.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D118577

1 files changed, 5 insertions, 2 deletions

diff --git a/lld/ELF/SyntheticSections.cpp b/lld/ELF/SyntheticSections.cpp
index 36a226c..986c130 100644
--- a/lld/ELF/SyntheticSections.cpp
+++ b/lld/ELF/SyntheticSections.cpp
@@ -1230,7 +1230,8 @@ StringTableSection::StringTableSection(StringRef name, bool dynamic)
     : SyntheticSection(dynamic ? (uint64_t)SHF_ALLOC : 0, SHT_STRTAB, 1, name),
       dynamic(dynamic) {
   // ELF string tables start with a NUL byte.
-  addString("");
+  strings.push_back("");
+  size = 1;
 }
 
 // Adds a string to the string table. If `hashIt` is true we hash and check for
@@ -1243,6 +1244,8 @@ unsigned StringTableSection::addString(StringRef s, bool hashIt) {
     if (!r.second)
       return r.first->second;
   }
+  if (s.empty())
+    return 0;
   unsigned ret = this->size;
   this->size = this->size + s.size() + 1;
   strings.push_back(s);
@@ -2155,7 +2158,7 @@ void SymbolTableBaseSection::addSymbol(Symbol *b) {
   // Adding a local symbol to a .dynsym is a bug.
   assert(this->type != SHT_DYNSYM || !b->isLocal());
 
-  bool hashIt = b->isLocal();
+  bool hashIt = b->isLocal() && config->optimize >= 2;
   symbols.push_back({b, strTabSec.addString(b->getName(), hashIt)});
 }