[RuntimeDyld] Fix performance problem in resolveRelocations with many sections

Summary:
Rather than just iterating over all sections and checking whether we have relocations for them, iterate over the relocation map instead. This showed up heavily in an artificial julia benchmark that does lots of compilation. On that particular benchmark, this patch gives
~15% performance improvements. As far as I can tell the primary reason why the original
loop was so expensive is that Relocations[i] actually constructs a relocationList (allocating memory & doing lots of other unnecessary computing) if none is found.

Reviewers: lhames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13545

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249942 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp b/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp
index a2557cbb05534c6be5573d4e9ea2fe4ebf3b7bd6..b2f017c94cb8f7876b1b4014de202c4d114bfab0 100644 (file)
--- a/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp
+++ b/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp
@@ -91,18 +91,18 @@ void RuntimeDyldImpl::resolveRelocations() {
   // First, resolve relocations associated with external symbols.
   resolveExternalSymbols();
 
-  // Just iterate over the sections we have and resolve all the relocations
-  // in them. Gross overkill, but it gets the job done.
-  for (int i = 0, e = Sections.size(); i != e; ++i) {
+  // Iterate over all outstanding relocations
+  for (auto it = Relocations.begin(), e = Relocations.end(); it != e; ++it) {
     // The Section here (Sections[i]) refers to the section in which the
     // symbol for the relocation is located.  The SectionID in the relocation
     // entry provides the section to which the relocation will be applied.
-    uint64_t Addr = Sections[i].LoadAddress;
-    DEBUG(dbgs() << "Resolving relocations Section #" << i << "\t"
+    int Idx = it->getFirst();
+    uint64_t Addr = Sections[Idx].LoadAddress;
+    DEBUG(dbgs() << "Resolving relocations Section #" << Idx << "\t"
                  << format("%p", (uintptr_t)Addr) << "\n");
-    resolveRelocationList(Relocations[i], Addr);
-    Relocations.erase(i);
+    resolveRelocationList(it->getSecond(), Addr);
   }
+  Relocations.clear();
 
   // Print out sections after relocation.
   DEBUG(