Teach getColumnNumber to use the line cache to get the start of the line if its on the same line as the last call to getLineNumber. Prevents needing to scan backwards for the new line. Fixes PR14106.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@166265 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Basic/SourceManager.cpp

@@ -1029,6 +1029,17 @@ unsigned SourceManager::getColumnNumber(FileID FID, unsigned FilePos,
     return 1;
   }
 
+  // See if we just calculated the line number for this FilePos and can use
+  // that to lookup the start of the line instead of searching for it.
+  if (LastLineNoFileIDQuery == FID &&
+      LastLineNoContentCache->SourceLineCache != 0) {
+    unsigned *SourceLineCache = LastLineNoContentCache->SourceLineCache;
+    unsigned LineStart = SourceLineCache[LastLineNoResult - 1];
+    unsigned LineEnd = SourceLineCache[LastLineNoResult];
+    if (FilePos >= LineStart && FilePos < LineEnd)
+      return FilePos - LineStart + 1;
+  }
+
   const char *Buf = MemBuf->getBufferStart();
   unsigned LineStart = FilePos;
   while (LineStart && Buf[LineStart-1] != '\n' && Buf[LineStart-1] != '\r')