[lld] r353146 - [COFF] Avoid O(n^2) accesses into PartialSections

[Martin Storsjo via llvm-commits]

Author: mstorsjo
Date: Tue Feb  5 00:16:10 2019
New Revision: 353146

URL: http://llvm.org/viewvc/llvm-project?rev=353146&view=rev
Log:
[COFF] Avoid O(n^2) accesses into PartialSections

For MinGW, unique partial sections are much more common, e.g.
comdat functions get sections named e.g. text$symbol.

A moderate sized example of this contains over 200K Chunks
which create 174K unique PartialSections. Prior to SVN r352928
(D57574), linking this took around 1,5 seconds for me, while
it afterwards takes around 13 minutes. After this patch, the
linking time is back to what it was before.

The std::find_if in findPartialSection will do a linear scan of
the whole container until a match is found. To use something like
binary_search or the std::set container's own methods, we'd need
to already have a PartialSection*.

Reinstate a proper map instead of having a set with a custom sorting
comparator.

Differential Revision: https://reviews.llvm.org/D57666

Modified:
    lld/trunk/COFF/Writer.cpp

Modified: lld/trunk/COFF/Writer.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/Writer.cpp?rev=353146&r1=353145&r2=353146&view=diff
==============================================================================
--- lld/trunk/COFF/Writer.cpp (original)
+++ lld/trunk/COFF/Writer.cpp Tue Feb  5 00:16:10 2019
@@ -159,15 +159,12 @@ public:
 // PartialSection represents a group of chunks that contribute to an
 // OutputSection. Collating a collection of PartialSections of same name and
 // characteristics constitutes the OutputSection.
-class PartialSection {
+class PartialSectionKey {
 public:
-  PartialSection(StringRef N, uint32_t Chars)
-      : Name(N), Characteristics(Chars) {}
   StringRef Name;
   unsigned Characteristics;
-  std::vector<Chunk *> Chunks;
 
-  bool operator<(const PartialSection &Other) const {
+  bool operator<(const PartialSectionKey &Other) const {
     int C = Name.compare(Other.Name);
     if (C == 1)
       return false;
@@ -177,10 +174,13 @@ public:
   }
 };
 
-struct PartialLess {
-  bool operator()(PartialSection *L, PartialSection *R) const {
-    return *L < *R;
-  }
+class PartialSection {
+public:
+  PartialSection(StringRef N, uint32_t Chars)
+      : Name(N), Characteristics(Chars) {}
+  StringRef Name;
+  unsigned Characteristics;
+  std::vector<Chunk *> Chunks;
 };
 
 // The writer writes a SymbolTable result to a file.
@@ -234,7 +234,7 @@ private:
   uint32_t getSizeOfInitializedData();
 
   std::unique_ptr<FileOutputBuffer> &Buffer;
-  std::set<PartialSection *, PartialLess> PartialSections;
+  std::map<PartialSectionKey, PartialSection *> PartialSections;
   std::vector<OutputSection *> OutputSections;
   std::vector<char> Strtab;
   std::vector<llvm::object::coff_symbol16> OutputSymtab;
@@ -628,7 +628,8 @@ bool Writer::fixGnuImportChunks() {
 
   // Make sure all .idata$* section chunks are mapped as RDATA in order to
   // be sorted into the same sections as our own synthesized .idata chunks.
-  for (PartialSection *PSec : PartialSections) {
+  for (auto It : PartialSections) {
+    PartialSection *PSec = It.second;
     if (!PSec->Name.startswith(".idata"))
       continue;
     if (PSec->Characteristics == RDATA)
@@ -642,7 +643,8 @@ bool Writer::fixGnuImportChunks() {
   bool HasIdata = false;
   // Sort all .idata$* chunks, grouping chunks from the same library,
   // with alphabetical ordering of the object fils within a library.
-  for (PartialSection *PSec : PartialSections) {
+  for (auto It : PartialSections) {
+    PartialSection *PSec = It.second;
     if (!PSec->Name.startswith(".idata"))
       continue;
 
@@ -773,8 +775,8 @@ void Writer::createSections() {
 
   // Process an /order option.
   if (!Config->Order.empty())
-    for (PartialSection *PSec : PartialSections)
-      sortBySectionOrder(PSec->Chunks);
+    for (auto It : PartialSections)
+      sortBySectionOrder(It.second->Chunks);
 
   if (HasIdata)
     locateImportTables();
@@ -783,7 +785,8 @@ void Writer::createSections() {
   // '$' and all following characters in input section names are
   // discarded when determining output section. So, .text$foo
   // contributes to .text, for example. See PE/COFF spec 3.2.
-  for (PartialSection *PSec : PartialSections) {
+  for (auto It : PartialSections) {
+    PartialSection *PSec = It.second;
     StringRef Name = getOutputSectionName(PSec->Name);
     uint32_t OutChars = PSec->Characteristics;
 
@@ -1771,19 +1774,16 @@ void Writer::addBaserelBlocks(std::vecto
 
 PartialSection *Writer::createPartialSection(StringRef Name,
                                              uint32_t OutChars) {
-  PartialSection *PSec = findPartialSection(Name, OutChars);
+  PartialSection *&PSec = PartialSections[{Name, OutChars}];
   if (PSec)
     return PSec;
   PSec = make<PartialSection>(Name, OutChars);
-  PartialSections.insert(PSec);
   return PSec;
 }
 
 PartialSection *Writer::findPartialSection(StringRef Name, uint32_t OutChars) {
-  auto It = find_if(PartialSections, [&](PartialSection *P) {
-    return P->Name == Name && P->Characteristics == OutChars;
-  });
+  auto It = PartialSections.find({Name, OutChars});
   if (It != PartialSections.end())
-    return *It;
+    return It->second;
   return nullptr;
 }