[PATCH] D46145: Use a buffer when allocating relocations

[Rafael Avila de Espindola via Phabricator via llvm-commits]

espindola updated this revision to Diff 144365.
espindola added a comment.

Delete call to reserve.

The results are

master                765.5   total, 563.7 peak
with reserve       747.76 total, 545.3 peak
without reserve 747.63 total, 545.3 peak


https://reviews.llvm.org/D46145

Files:
  ELF/Relocations.cpp
  ELF/Writer.cpp


Index: ELF/Writer.cpp
===================================================================
--- ELF/Writer.cpp
+++ ELF/Writer.cpp
@@ -859,16 +859,29 @@
 
 template <class ELFT>
 void Writer<ELFT>::forEachRelSec(std::function<void(InputSectionBase &)> Fn) {
+  std::vector<Relocation> Buffer;
+
+  // We allocate a lot of relocations. By using a single buffer for
+  // all sections we ensure that the final vector in each section has
+  // the exact size.
+  auto WithBuffer = [&](InputSectionBase &Sec) {
+    swap(Sec.Relocations, Buffer);
+    Fn(Sec);
+    swap(Sec.Relocations, Buffer);
+    Sec.Relocations.insert(Sec.Relocations.end(), Buffer.begin(), Buffer.end());
+    Buffer.clear();
+  };
+
   // Scan all relocations. Each relocation goes through a series
   // of tests to determine if it needs special treatment, such as
   // creating GOT, PLT, copy relocations, etc.
   // Note that relocations for non-alloc sections are directly
   // processed by InputSection::relocateNonAlloc.
   for (InputSectionBase *IS : InputSections)
     if (IS->Live && isa<InputSection>(IS) && (IS->Flags & SHF_ALLOC))
-      Fn(*IS);
+      WithBuffer(*IS);
   for (EhInputSection *ES : InX::EhFrame->Sections)
-    Fn(*ES);
+    WithBuffer(*ES);
 }
 
 // This function generates assignments for predefined symbols (e.g. _end or
Index: ELF/Relocations.cpp
===================================================================
--- ELF/Relocations.cpp
+++ ELF/Relocations.cpp
@@ -1015,10 +1015,6 @@
 template <class ELFT, class RelTy>
 static void scanRelocs(InputSectionBase &Sec, ArrayRef<RelTy> Rels) {
   OffsetGetter GetOffset(Sec);
-
-  // Not all relocations end up in Sec.Relocations, but a lot do.
-  Sec.Relocations.reserve(Rels.size());
-
   for (auto I = Rels.begin(), End = Rels.end(); I != End;)
     scanReloc<ELFT>(Sec, GetOffset, I, End);
 }


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