[clang] Improve getFileIDLocal binary search. (#146510)

Avoid reading the `LocalSLocEntryTable` twice per loop iteration. NFC.

https://llvm-compile-time-tracker.com/compare.php?from=0b6ddb02efdcbdac9426e8d857499ea0580303cd&to=1aa335ccfb07ba96177b89b1933aa6b980fa14f6&stat=instructions:u

Author: hokein

clang/lib/Basic/SourceManager.cpp

@@ -811,6 +811,8 @@ FileID SourceManager::getFileIDSlow(SourceLocation::UIntTy SLocOffset) const {
 /// loaded one.
 FileID SourceManager::getFileIDLocal(SourceLocation::UIntTy SLocOffset) const {
   assert(SLocOffset < NextLocalOffset && "Bad function choice");
+  assert(SLocOffset >= LocalSLocEntryTable[0].getOffset() &&
+         "Invalid SLocOffset");
 
   // After the first and second level caches, I see two common sorts of
   // behavior: 1) a lot of searched FileID's are "near" the cached file
@@ -854,35 +856,24 @@ FileID SourceManager::getFileIDLocal(SourceLocation::UIntTy SLocOffset) const {
       break;
   }
 
-  NumProbes = 0;
-  while (true) {
-    unsigned MiddleIndex = (GreaterIndex-LessIndex)/2+LessIndex;
-    SourceLocation::UIntTy MidOffset =
-        getLocalSLocEntry(MiddleIndex).getOffset();
-
-    ++NumProbes;
-
-    // If the offset of the midpoint is too large, chop the high side of the
-    // range to the midpoint.
-    if (MidOffset > SLocOffset) {
-      GreaterIndex = MiddleIndex;
-      continue;
-    }
+  while (LessIndex < GreaterIndex) {
+    ++NumBinaryProbes;
 
-    // If the middle index contains the value, succeed and return.
-    if (MiddleIndex + 1 == LocalSLocEntryTable.size() ||
-        SLocOffset < getLocalSLocEntry(MiddleIndex + 1).getOffset()) {
-      FileID Res = FileID::get(MiddleIndex);
+    unsigned MiddleIndex = LessIndex + (GreaterIndex - LessIndex) / 2;
 
-      // Remember it.  We have good locality across FileID lookups.
-      LastFileIDLookup = Res;
-      NumBinaryProbes += NumProbes;
-      return Res;
-    }
+    SourceLocation::UIntTy MidOffset =
+        LocalSLocEntryTable[MiddleIndex].getOffset();
 
-    // Otherwise, move the low-side up to the middle index.
-    LessIndex = MiddleIndex;
+    if (MidOffset <= SLocOffset)
+      LessIndex = MiddleIndex + 1;
+    else
+      GreaterIndex = MiddleIndex;
   }
+
+  // At this point, LessIndex is the index of the *first element greater than*
+  // SLocOffset. The element we are actually looking for is the one immediately
+  // before it.
+  return LastFileIDLookup = FileID::get(LessIndex - 1);
 }
 
 /// Return the FileID for a SourceLocation with a high offset.