[PATCH] D118577: [ELF] Deduplicate names of local symbols only with -O2

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
MaskRay added reviewers: bd1976llvm, ikudrin, peter.smith.
Herald added subscribers: arichardson, emaste.
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The deduplication requires a DenseMap of the same size of the local part of
.strtab . I optimized it in e20544543478b259eb09fa0a253d4fb1a5525d9e <https://reviews.llvm.org/rGe20544543478b259eb09fa0a253d4fb1a5525d9e> 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).

---

I will fix other tests if this looks good.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D118577

Files:
  lld/ELF/SyntheticSections.cpp
  lld/test/ELF/strtab-dedup.s


Index: lld/test/ELF/strtab-dedup.s
===================================================================
--- /dev/null
+++ lld/test/ELF/strtab-dedup.s
@@ -0,0 +1,20 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+
+## By default local symbol names are not deduplicated.
+# RUN: ld.lld %t.o %t.o -o %t
+# RUN: llvm-readelf -p .strtab %t | FileCheck %s --check-prefix=NODEDUP
+
+# NODEDUP:        [     1]  local
+# NODEDUP-NEXT:   [     7]  local
+# NODEDUP-EMPTY:
+
+## -O2 deduplicates local symbol names.
+# RUN: ld.lld -O2 %t.o %t.o -o %t
+# RUN: llvm-readelf -p .strtab %t | FileCheck %s --check-prefix=DEDUP
+
+# DEDUP:        [     1]  local
+# DEDUP-EMPTY:
+
+local:
+  ret
Index: lld/ELF/SyntheticSections.cpp
===================================================================
--- lld/ELF/SyntheticSections.cpp
+++ lld/ELF/SyntheticSections.cpp
@@ -2155,7 +2155,7 @@
   // 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)});
 }
 


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