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

[Richard Sandiford <richard.sandiford@arm.com>]

Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>> Am 03.02.2023 um 15:20 schrieb Richard Sandiford via Gcc-patches <gcc-patches@gcc.gnu.org>:
>> 
>> Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>>> 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.
>> 
>> Going purely from this description and without having looked
>> at the code properly, would it be possible to link all current
>> values together, not just those with the same hash?  And would
>> that help?  It looks like the list is already doubly-linked,
>> and there's spare room to store a "start of new hash" marker.
>
> We already do have equivalent values linked, but I’m not sure that’s what you are suggesting.

I was thinking of linking every active value in the table together,
but with entries for the same hash being consecutive.  That way, things
like invalidate_memory can just walk the list and ignore the hash table.

> Those should also have the same hash value, so both lists are somewhat redundant and we might be able to save some storage here by making this a list of lists of same hash and value list?

I thought the value-equality list was to establish that (e.g.)
(reg R1) and (reg R2) are known to have the same value, despite
being different expressions with different hash values.

But I suppose if we reused an existing hash table structure (with its
own mechanism for handling collisions), it would make sense to use the
equivalent-value list to join everything together, rather than the
same-hash list.  Again, there could be a marker to establish the start
of a new equivalent-value sublist.

Thanks,
Richard
>
>> 
>> Thanks,
>> Richard
>> 
>>> 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?
>>> 
>>>    * 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;
>>> \f
>>> +/* 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