[clang] [clang] Speedup getFileIDLocal with a separate offset table. (PR #146604)

[Erich Keane via cfe-commits]

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@@ -719,6 +719,8 @@ class SourceManager : public RefCountedBase<SourceManager> {
   /// Positive FileIDs are indexes into this table. Entry 0 indicates an invalid
   /// expansion.
   SmallVector<SrcMgr::SLocEntry, 0> LocalSLocEntryTable;
+  /// An in-parallel offset table, merely used for speeding up FileID lookup.
+  SmallVector<SourceLocation::UIntTy> LocalLocOffsetTable;
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erichkeane wrote:

Because of the size/cache sensitivity of this, might it be better to not use the SmallVector and either use a normal std::vector (no small-vector opt, see the table above does this with the `, 0`), or a very large small-vector-opt here?  Do we have a good idea what size these usually do?


https://github.com/llvm/llvm-project/pull/146604