[dsymutil] Be smarter in caching calls to realpath

Calling realpath is expensive but necessary to perform the uniqueing in
dsymutil. Although we already cached the results for every individual
file in the line table, we had reports of it taking 40 seconds of a 3.5
minute link.

This patch adds a second level of caching. When we do have to call
realpath, we cache its result for its parents path. We didn't replace
the existing caching, because it's fast (indexed) and saves us from
reading the line table for entries we've already seen.

For WebkitCore this results in a decrease of 11% in linking time: from
85.79 to 76.11 seconds (average over 3 runs).

Differential revision: https://reviews.llvm.org/D43511

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325757 91177308-0d34-0410-b5e6-96231b3b80d8

tools/dsymutil/DwarfLinker.cpp

@@ -100,6 +100,37 @@ namespace dsymutil {
 
 namespace {
 
+/// Small helper that resolves and caches file paths. This helps reduce the
+/// number of calls to realpath which is expensive. We assume the input are
+/// files, and cache the realpath of their parent. This way we can quickly
+/// resolve different files under the same path.
+class CachedPathResolver {
+public:
+  /// Resolve a path by calling realpath and cache its result. The returned
+  /// StringRef is interned in the given \p StringPool.
+  StringRef resolve(std::string Path, NonRelocatableStringpool &StringPool) {
+    StringRef FileName = sys::path::filename(Path);
+    SmallString<PATH_MAX> ParentPath = sys::path::parent_path(Path);
+
+    // If the ParentPath has not yet been resolved, resolve and cache it for
+    // future look-ups.
+    if (!ResolvedPaths.count(ParentPath)) {
+      SmallString<PATH_MAX> RealPath;
+      sys::fs::real_path(ParentPath, RealPath);
+      ResolvedPaths.insert({ParentPath, StringRef(RealPath).str()});
+    }
+
+    // Join the file name again with the resolved path.
+    SmallString<PATH_MAX> ResolvedPath(ResolvedPaths[ParentPath]);
+    sys::path::append(ResolvedPath, FileName);
+    return StringPool.internString(ResolvedPath);
+  }
+
+private:
+  StringMap<std::string> ResolvedPaths;
+};
+
+
 /// Retrieve the section named \a SecName in \a Obj.
 ///
 /// To accommodate for platform discrepancies, the name passed should be
@@ -234,6 +265,8 @@ class DeclContextTree {
   DeclContext Root;
   DeclContext::Map Contexts;
 
+  /// Cache resolved paths from the line table.
+  CachedPathResolver PathResolver;
 public:
   /// Get the child of \a Context described by \a DIE in \a Unit. The
   /// required strings will be interned in \a StringPool.
@@ -1945,30 +1978,24 @@ PointerIntPair<DeclContext *, 1> DeclContextTree::getChildDeclContext(
           if (!Name && Tag == dwarf::DW_TAG_namespace)
             FileNum = 1;
 
-          // FIXME: Passing U.getOrigUnit().getCompilationDir()
-          // instead of "" would allow more uniquing, but for now, do
-          // it this way to match dsymutil-classic.
           if (LT->hasFileAtIndex(FileNum)) {
             Line = dwarf::toUnsigned(DIE.find(dwarf::DW_AT_decl_line), 0);
-            // Cache the resolved paths, because calling realpath is expansive.
+            // Cache the resolved paths based on the index in the line table,
+            // because calling realpath is expansive.
             StringRef ResolvedPath = U.getResolvedPath(FileNum);
             if (!ResolvedPath.empty()) {
               FileRef = ResolvedPath;
             } else {
               std::string File;
-              bool gotFileName =
-                LT->getFileNameByIndex(FileNum, "",
-                        DILineInfoSpecifier::FileLineInfoKind::AbsoluteFilePath,
-                        File);
-              (void)gotFileName;
-              assert(gotFileName && "Must get file name from line table");
-#ifdef HAVE_REALPATH
-              char RealPath[PATH_MAX + 1];
-              RealPath[PATH_MAX] = 0;
-              if (::realpath(File.c_str(), RealPath))
-                File = RealPath;
-#endif
-              FileRef = StringPool.internString(File);
+              bool FoundFileName = LT->getFileNameByIndex(
+                  FileNum, U.getOrigUnit().getCompilationDir(),
+                  DILineInfoSpecifier::FileLineInfoKind::AbsoluteFilePath,
+                  File);
+              (void)FoundFileName;
+              assert(FoundFileName && "Must get file name from line table");
+              // Second level of caching, this time based on the file's parent
+              // path.
+              FileRef = PathResolver.resolve(File, StringPool);
               U.setResolvedPath(FileNum, FileRef);
             }
           }