[PATCH] D135440: [SourceManager] Speedup getFileIDLocal with a separate Offset Table.

[Nick Desaulniers via Phabricator via cfe-commits]

nickdesaulniers added inline comments.


================
Comment at: clang/include/clang/Basic/SourceManager.h:697
+  /// FileID lookup.
+  SmallVector<SourceLocation::UIntTy> LocalLocOffsetTable;
 
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nickdesaulniers wrote:
> `LocalSLocEntryTable` is a vec of `SrcMgr::SLocEntry`. `SrcMgr::SLocEntry`'s `Offset` is:
> ```
> static constexpr int OffsetBits = 8 * sizeof(SourceLocation::UIntTy) - 1;
>   SourceLocation::UIntTy Offset : OffsetBits;
> ```
> So this is a vector of 32b values vs the 24b values we need.  Does packing this by declaring a struct improve locality further?
> 
> ```
> diff --git a/clang/include/clang/Basic/SourceManager.h b/clang/include/clang/Basic/SourceManager.h
> index 70ba85bbe948..518c6ed59120 100644
> --- a/clang/include/clang/Basic/SourceManager.h
> +++ b/clang/include/clang/Basic/SourceManager.h
> @@ -692,9 +692,15 @@ class SourceManager : public RefCountedBase<SourceManager> {
>    /// Positive FileIDs are indexes into this table. Entry 0 indicates an invalid
>    /// expansion.
>    SmallVector<SrcMgr::SLocEntry, 0> LocalSLocEntryTable;
> +
> +  struct SMOffset {
> +    SourceLocation::UIntTy Offset : 24;
> +    SMOffset (SourceLocation::UIntTy O): Offset(O){}
> +    operator SourceLocation::UIntTy() const { return Offset; }
> +  };
>    /// An in-parallel SlocEntry offset table, merely used for speeding up the
>    /// FileID lookup.
> -  SmallVector<SourceLocation::UIntTy> LocalLocOffsetTable;
> +  SmallVector<SMOffset> LocalLocOffsetTable;
>  
>    /// The table of SLocEntries that are loaded from other modules.
>    ///
> ```
`struct SMOffset { ... } __attribute__((packed));`


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https://reviews.llvm.org/D135440