[llvm] r264486 - Don't force OnDiskHashTables to have a minimum of 64 buckets. That's

[David Blaikie via llvm-commits]

On Fri, Mar 25, 2016 at 6:49 PM, Richard Smith via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> Author: rsmith
> Date: Fri Mar 25 20:49:50 2016
> New Revision: 264486
>
> URL: http://llvm.org/viewvc/llvm-project?rev=264486&view=rev
> Log:
> Don't force OnDiskHashTables to have a minimum of 64 buckets. That's
> preposterously large for some lookup tables -- in C++ classes generated by
> template instantiation, it's very common for the number of lookup results
> to be
> either 1 or 2.
>
> This reduces size of a libstdc++ module by 7-15%.
>

Awesome!


>
> Modified:
>     llvm/trunk/include/llvm/Support/OnDiskHashTable.h
>
> Modified: llvm/trunk/include/llvm/Support/OnDiskHashTable.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/OnDiskHashTable.h?rev=264486&r1=264485&r2=264486&view=diff
>
> ==============================================================================
> --- llvm/trunk/include/llvm/Support/OnDiskHashTable.h (original)
> +++ llvm/trunk/include/llvm/Support/OnDiskHashTable.h Fri Mar 25 20:49:50
> 2016
> @@ -152,6 +152,22 @@ public:
>      using namespace llvm::support;
>      endian::Writer<little> LE(Out);
>
> +    // Now we're done adding entries, resize the bucket list if it's
> +    // significantly too large. (This only happens if the number of
> +    // entries is small and we're within our initial allocation of
> +    // 64 buckets.) We aim for an occupancy ratio in [3/8, 3/4).
> +    //
> +    // As a special case, if there are two or fewer entries, just
> +    // form a single bucket. A linear scan is fine in that case, and
> +    // this is very common in C++ class lookup tables. This also
> +    // guarantees we produce at least one bucket for an empty table.
> +    //
> +    // FIXME: Try computing a perfect hash function at this point.
> +    unsigned TargetNumBuckets =
> +        NumEntries <= 2 ? 1 : NextPowerOf2(NumEntries * 4 / 3);
> +    if (TargetNumBuckets != NumBuckets)
> +      resize(TargetNumBuckets);
> +
>      // Emit the payload of the table.
>      for (offset_type I = 0; I < NumBuckets; ++I) {
>        Bucket &B = Buckets[I];
>
>
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