[PATCH] D135132: [SourceManager] Improve getFileIDLocal.

[Haojian Wu via Phabricator via cfe-commits]

hokein created this revision.
hokein added reviewers: sammccall, nickdesaulniers.
Herald added a project: All.
hokein requested review of this revision.
Herald added a project: clang.

Prune the search space -- If we know offset(LastFileIDLookup) < SearchOffset, we
can prune the initial binary-search range from [0, end) to [LastFileIDlookup, end).

It reduces the binary search scan by ~30%.

SemaExpr.cpp:   1393437 -> 1035426
FindTarget.cpp: 1275930 -> 920087

Linux kernel:
getFileIDLocal: 2.45% -> 2.15%


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D135132

Files:
  clang/lib/Basic/SourceManager.cpp


Index: clang/lib/Basic/SourceManager.cpp
===================================================================
--- clang/lib/Basic/SourceManager.cpp
+++ clang/lib/Basic/SourceManager.cpp
@@ -790,15 +790,20 @@
 
   // See if this is near the file point - worst case we start scanning from the
   // most newly created FileID.
-  const SrcMgr::SLocEntry *I;
 
-  if (LastFileIDLookup.ID < 0 ||
-      LocalSLocEntryTable[LastFileIDLookup.ID].getOffset() < SLocOffset) {
-    // Neither loc prunes our search.
-    I = LocalSLocEntryTable.end();
-  } else {
-    // Perhaps it is near the file point.
-    I = LocalSLocEntryTable.begin()+LastFileIDLookup.ID;
+  // "I" is an iterator that points to a FileID whose offset is known to be
+  // larger than SLocOffset.
+  const SrcMgr::SLocEntry *I = LocalSLocEntryTable.end();
+  // LessIndex - This is the lower bound of the range that we're searching.
+  // We know that the offset corresponding to the FileID is is less than
+  // SLocOffset.
+  unsigned LessIndex = 0;
+  if (LastFileIDLookup.ID >= 0) {
+    // Use the LastFileIDLookup to prune the search space.
+    if (LocalSLocEntryTable[LastFileIDLookup.ID].getOffset() < SLocOffset)
+      LessIndex = LastFileIDLookup.ID;
+    else
+      I = LocalSLocEntryTable.begin() + LastFileIDLookup.ID;
   }
 
   // Find the FileID that contains this.  "I" is an iterator that points to a
@@ -820,10 +825,6 @@
   // Convert "I" back into an index.  We know that it is an entry whose index is
   // larger than the offset we are looking for.
   unsigned GreaterIndex = I - LocalSLocEntryTable.begin();
-  // LessIndex - This is the lower bound of the range that we're searching.
-  // We know that the offset corresponding to the FileID is is less than
-  // SLocOffset.
-  unsigned LessIndex = 0;
   NumProbes = 0;
   while (true) {
     unsigned MiddleIndex = (GreaterIndex-LessIndex)/2+LessIndex;


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