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

[Richard Biener <rguenther@suse.de>]

> Am 03.02.2023 um 16:55 schrieb Richard Sandiford via Gcc-patches <gcc-patches@gcc.gnu.org>:
> 
> 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.

Ah, yeah.  Even better might be a generation count for memory like there’s one (but only for a subset of cases?!) for pseudos.  That would avoid the walking altogether.

>> 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.

I’d have to double check, I was just cursory sweeping over the code after being pointed to it from a profile of a testcase.  Most of CSE.cc dates back to the initial source revision …

Richard 

> 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