[Clang][SourceManager] optimize getFileIDLocal()

[nickdesaulniers]

Summary

A recent Linux kernel commit exposed a performance cliff in Clang. Calls
to SourceManager::getFileIDLocal() when there's a cache miss against
LastFileIDLookup can be relatively expensive, as getFileIDLocal() tries
a few linear probes, then falls back to binary search. The use of
SourceManager::isOffsetInFileID() is also relatively expensive (both
isOffsetInFileID and getFileIDLocal dominated a trace of the performance
cliff case).

As a FIXME notes (and as @kadircet helpfully noted in review of D80681),
there's a few optimizations we can do here since we've already
identified that an offset is local (as opposed to "loaded").

This patch was forked off of D80681, which additionally did this and
modified some caching behavior, as we expect this change to be less
controversial.

In terms of optimizations, we've already determined that the SLocOffset
parameter to SourceManager::getFileIDLocal() is local in the caller
SourceManager::getFileIDSlow(). Also, there's an early continue in the
binary search loop in getFileIDLocal() that are duplicated in
isOffsetInFileID() as pointed out by @kadircet.

Take advantage of these to optimize the binary search patch, and remove
the FIXME.

[kadircet]

LGTM, thanks!

Btw, I've recently learned about http://llvm-compile-time-tracker.com/ so you might want to check the effect afterwards.

[kadircet]

For the record: http://llvm-compile-time-tracker.com/compare.php?from=ed8184b7814df4310dbad065a9a1c3bb8f3bfa86&to=408efffbe4a52bae05f1677a47eb3ccfd5cdc1d3&stat=instructions

https://photos.app.goo.gl/fJUS5jrMaJjvvmo1A

clang/lib/Basic/SourceManager.cpp

while (true) {
     // 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;
     }
 
     // If the middle index contains the value, succeed and return.
-    // FIXME: This could be made faster by using a function that's aware of
-    // being in the local area.
-    if (isOffsetInFileID(FileID::get(MiddleIndex), SLocOffset)) {
+    if (MiddleIndex + 1 == LocalSLocEntryTable.size() ||
+        SLocOffset < getLocalSLocEntry(MiddleIndex + 1).getOffset()) {
       FileID Res = FileID::get(MiddleIndex);
 
       // If this isn't a macro expansion, remember it.  We have good locality
       // across FileID lookups.
       if (!LocalSLocEntryTable[MiddleIndex].isExpansion())
         LastFileIDLookup = Res;
       NumBinaryProbes += NumProbes;
       return Res;