Re: [PATCH] Improve RTL CSE hash table hash usage

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Feb 3, 2023 at 3:07 PM Richard Biener via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> The RTL CSE hash table has a fixed number of buckets (32) each
> with a linked list of entries with the same hash value.  The
> actual hash values are computed using hash_rtx which uses adds
> for mixing and adds the rtx CODE as CODE << 7 (apart from some
> exceptions such as MEM).  The unsigned int typed hash value
> is then simply truncated for the actual lookup into the fixed
> size table which means that usually CODE is simply lost.
>
> The following improves this truncation by first mixing in more
> bits using xor.  It does not change the actual hash function
> since that's used outside of CSE as well.
>
> An alternative would be to bump the fixed number of buckets,
> say to 256 which would retain the LSB of CODE or to 8192 which
> can capture all 6 bits required for the last CODE.
>
> As the comment in CSE says, there's invalidate_memory and
> flush_hash_table done possibly frequently and those at least
> need to walk all slots, so when the hash table is mostly empty
> enlarging it will be a loss.  Still there should be more
> regular lookups by hash, so less collisions should pay off
> as well.
>
> Without enlarging the table a better hash function is unlikely
> going to make a big difference, simple statistics on the
> number of collisions at insertion time shows a reduction of
> around 10%.  Bumping HASH_SHIFT by 1 improves that to 30%
> at the expense of reducing the average table fill by 10%
> (all of this stats from looking just at fold-const.i at -O2).
> Increasing HASH_SHIFT more leaves the table even more sparse
> likely showing that hash_rtx uses add for mixing which is
> quite bad.  Bumping HASH_SHIFT by 2 removes 90% of all
> collisions.
>
> Experimenting with using inchash instead of adds for the
> mixing does not improve things when looking at the HASH_SHIFT
> bumped by 2 numbers.
>
> Bootstrapped and tested on x86_64-unknown-linux-gnu.
>
> Any opinions?

I have pushed this change now after re-bootstrapping and testing
on x86_64-unknown-linux-gnu.

Richard.

>         * cse.cc (HASH): Turn into inline function and mix
>         in another HASH_SHIFT bits.
>         (SAFE_HASH): Likewise.
> ---
>  gcc/cse.cc | 37 +++++++++++++++++++++++--------------
>  1 file changed, 23 insertions(+), 14 deletions(-)
>
> diff --git a/gcc/cse.cc b/gcc/cse.cc
> index 37afc88b439..4777e559b86 100644
> --- a/gcc/cse.cc
> +++ b/gcc/cse.cc
> @@ -420,20 +420,6 @@ struct table_elt
>  #define HASH_SIZE      (1 << HASH_SHIFT)
>  #define HASH_MASK      (HASH_SIZE - 1)
>
> -/* Compute hash code of X in mode M.  Special-case case where X is a pseudo
> -   register (hard registers may require `do_not_record' to be set).  */
> -
> -#define HASH(X, M)     \
> - ((REG_P (X) && REGNO (X) >= FIRST_PSEUDO_REGISTER     \
> -  ? (((unsigned) REG << 7) + (unsigned) REG_QTY (REGNO (X)))   \
> -  : canon_hash (X, M)) & HASH_MASK)
> -
> -/* Like HASH, but without side-effects.  */
> -#define SAFE_HASH(X, M)        \
> - ((REG_P (X) && REGNO (X) >= FIRST_PSEUDO_REGISTER     \
> -  ? (((unsigned) REG << 7) + (unsigned) REG_QTY (REGNO (X)))   \
> -  : safe_hash (X, M)) & HASH_MASK)
> -
>  /* Determine whether register number N is considered a fixed register for the
>     purpose of approximating register costs.
>     It is desirable to replace other regs with fixed regs, to reduce need for
> @@ -586,6 +572,29 @@ static machine_mode cse_cc_succs (basic_block, basic_block, rtx, rtx,
>
>  static const struct rtl_hooks cse_rtl_hooks = RTL_HOOKS_INITIALIZER;
>
> +/* Compute hash code of X in mode M.  Special-case case where X is a pseudo
> +   register (hard registers may require `do_not_record' to be set).  */
> +
> +static inline unsigned
> +HASH (rtx x, machine_mode mode)
> +{
> +  unsigned h = (REG_P (x) && REGNO (x) >= FIRST_PSEUDO_REGISTER
> +               ? (((unsigned) REG << 7) + (unsigned) REG_QTY (REGNO (x)))
> +               : canon_hash (x, mode));
> +  return (h ^ (h >> HASH_SHIFT)) & HASH_MASK;
> +}
> +
> +/* Like HASH, but without side-effects.  */
> +
> +static inline unsigned
> +SAFE_HASH (rtx x, machine_mode mode)
> +{
> +  unsigned h = (REG_P (x) && REGNO (x) >= FIRST_PSEUDO_REGISTER
> +               ? (((unsigned) REG << 7) + (unsigned) REG_QTY (REGNO (x)))
> +               : safe_hash (x, mode));
> +  return (h ^ (h >> HASH_SHIFT)) & HASH_MASK;
> +}
> +
>  /* Nonzero if X has the form (PLUS frame-pointer integer).  */
>
>  static bool
> --
> 2.35.3