Re: [PATCH] Replace hash function from bcache with fast_hash

["Christian Biesinger via gdb-patches" <gdb-patches at sourceware dot org>]

On Tue, Dec 3, 2019 at 2:36 PM Simon Marchi <simark@simark.ca> wrote:
>
> On 2019-12-02 8:02 p.m., Christian Biesinger via gdb-patches wrote:
> > This function is not just slower than xxhash, it is slower than
> > even libiberty's iterative_hash, so there does not seem to be
> > a reason for it to exist.
> >
> > ------------------------------------------------------------
> > Benchmark                     Time           CPU Iterations
> > ------------------------------------------------------------
> > BM_xxh3                      11 ns         11 ns   66127192
> > BM_xxh32                     19 ns         19 ns   36792609
> > BM_xxh64                     16 ns         16 ns   42941328
> > BM_city32                    26 ns         26 ns   27028370
> > BM_city64                    17 ns         17 ns   40472793
> > BM_iterative_hash            77 ns         77 ns    9088854
> > BM_bcache_hash              125 ns        125 ns    5599232
> >
> > gdb/ChangeLog:
> >
> > 2019-12-02  Christian Biesinger  <cbiesinger@google.com>
> >
> >       * bcache.c (hash): Remove.
> >       (hash_continue): Remove.
> >       * bcache.h (hash): Remove.
> >       (hash_continue): Remove.
> >       (struct bcache) <ctor>: Update.
> >       * psymtab.c (psymbol_hash): Update.
> >       * stabsread.c (hashname): Update.
> >       * utils.h (fast_hash): Add an argument for a start value,
> >       defaulting to zero.
>
> LGTM, with the nits below fixed.
>
> > diff --git a/gdb/bcache.h b/gdb/bcache.h
> > index 15dcc63440..96f6d6813f 100644
> > --- a/gdb/bcache.h
> > +++ b/gdb/bcache.h
> > @@ -138,13 +138,12 @@
> >
> >  struct bstring;
> >
> > -/* The hash functions */
> > -extern unsigned long hash (const void *addr, int length);
> > -extern unsigned long hash_continue (const void *addr, int length,
> > -                                    unsigned long h);
> > -
> >  struct bcache
> >  {
> > +  static unsigned long default_hash (const void *ptr, int length) {
>
> Brace on the next line.

Done.

> > +    return fast_hash (ptr, length, 0);
> > +  }
>
> Can this method be private, just like `compare` is?

Done.

> > +
> >    /* Allocate a bcache.  HASH_FN and COMPARE_FN can be used to pass in
> >       custom hash, and compare functions to be used by this bcache.  If
> >       HASH_FUNCTION is NULL hash() is used and if COMPARE_FUNCTION is
>
> This line of documentation should be updated, probably hash -> fast_hash.

Done.

> > diff --git a/gdb/utils.h b/gdb/utils.h
> > index 79c8a6fc8d..68376dac83 100644
> > --- a/gdb/utils.h
> > +++ b/gdb/utils.h
> > @@ -571,17 +571,18 @@ extern void copy_bitwise (gdb_byte *dest, ULONGEST dest_offset,
> >                         const gdb_byte *source, ULONGEST source_offset,
> >                         ULONGEST nbits, int bits_big_endian);
> >
> > -/* A fast hashing function.  This can be used to hash strings in a fast way
> > +/* A fast hashing function.  This can be used to hash data in a fast way
> >     when the length is known.  If no fast hashing library is available, falls
> > -   back to iterative_hash from libiberty.  */
> > +   back to iterative_hash from libiberty.  START_VALUE can be set to
> > +   continue hashing from a previous value.  */
> >
> >  static inline unsigned int
> > -fast_hash (const char* str, size_t len)
> > +fast_hash (const void* ptr, size_t len, unsigned int start_value = 0)
>
> - const void* ptr
> + const void *ptr

Done, thanks.

Will push now with those fixes.

Christian