Richard Sandiford <[email protected]> writes:
> Richard Biener <[email protected]> writes:
>> On Fri, 4 Apr 2025, Richard Sandiford wrote:
>>
>>> Another problem in PR101523 was that, after each successful 2->2
>>> combination attempt, distribute_links would search further and further
>>> for the next combinable use of the i2 destination. Each search would
>>> start at i2 itself, making the search quadratic in the worst case.
>>>
>>> In a 2->2 combination, if i2 is unchanged, the search can start at i3
>>> instead of i2. The same thing applies to i2 when distributing i2's
>>> links, since the only changes to earlier instructions are the deletion
>>> of i0 and i1.
>>>
>>> This change, combined with the previous split_i2i3 patch, gives a
>>> 34.6% speedup in combine for the testcase in PR101523. Combine
>>> goes from being 41% to 34% of compile time.
>>
>> From my analysis this patch is OK (I do wonder why not always
>> starting from iN when distributing links for iN, but I guess
>> that combining into parallel & splitting can actually move
>> link sources/destinations in odd ways
>
> Right. In particular, if we split part of i3 into i2, some log
> links can move up from i3 to i2. I think it would be safe to
> pass iN unconditionally and then use LUIDs to check whether the
...I meant pass i2 unconditionally for i3links, on the basis that
we certainly don't need to search before i2.
> passed-in instruction is before or after the definition, picking
> the later of the two. But that felt like too much of a rabiit hole.
>
> Richard
>
>
>> - I don't claim I fully understand this).
>>
>> Richard.
>>
>>> gcc/
>>> PR testsuite/116398
>>> * combine.cc (distribute_links): Take an optional start point.
>>> (try_combine): If only i3 has changed, only distribute i3's links,
>>> not i2's. Start the search for the new use from i3 rather than
>>> from the definition instruction. Likewise start the search for
>>> the new use from i2 when distributing i2's links.
>>> ---
>>> gcc/combine.cc | 27 +++++++++++++++++++--------
>>> 1 file changed, 19 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/gcc/combine.cc b/gcc/combine.cc
>>> index 0664f8dd433..9eae1a0111e 100644
>>> --- a/gcc/combine.cc
>>> +++ b/gcc/combine.cc
>>> @@ -472,7 +472,7 @@ static void move_deaths (rtx, rtx, int, rtx_insn *, rtx
>>> *);
>>> static bool reg_bitfield_target_p (rtx, rtx);
>>> static void distribute_notes (rtx, rtx_insn *, rtx_insn *, rtx_insn *,
>>> rtx, rtx, rtx);
>>> -static void distribute_links (struct insn_link *);
>>> +static void distribute_links (struct insn_link *, rtx_insn * = nullptr);
>>> static void mark_used_regs_combine (rtx);
>>> static void record_promoted_value (rtx_insn *, rtx);
>>> static bool unmentioned_reg_p (rtx, rtx);
>>> @@ -4590,10 +4590,15 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn
>>> *i1, rtx_insn *i0,
>>> NULL_RTX, NULL_RTX, NULL_RTX);
>>> }
>>>
>>> - distribute_links (i3links);
>>> - distribute_links (i2links);
>>> - distribute_links (i1links);
>>> - distribute_links (i0links);
>>> + if (only_i3_changed)
>>> + distribute_links (i3links, i3);
>>> + else
>>> + {
>>> + distribute_links (i3links);
>>> + distribute_links (i2links, i2);
>>> + distribute_links (i1links);
>>> + distribute_links (i0links);
>>> + }
>>>
>>> if (REG_P (i2dest))
>>> {
>>> @@ -14984,10 +14989,13 @@ distribute_notes (rtx notes, rtx_insn *from_insn,
>>> rtx_insn *i3, rtx_insn *i2,
>>>
>>> /* Similarly to above, distribute the LOG_LINKS that used to be present on
>>> I3, I2, and I1 to new locations. This is also called to add a link
>>> - pointing at I3 when I3's destination is changed. */
>>> + pointing at I3 when I3's destination is changed.
>>> +
>>> + If START is nonnull and an insn, we know that the next location for each
>>> + link is no earlier than START. */
>>>
>>> static void
>>> -distribute_links (struct insn_link *links)
>>> +distribute_links (struct insn_link *links, rtx_insn *start)
>>> {
>>> struct insn_link *link, *next_link;
>>>
>>> @@ -15053,7 +15061,10 @@ distribute_links (struct insn_link *links)
>>> I3 to I2. Also note that not much searching is typically done here
>>> since most links don't point very far away. */
>>>
>>> - for (insn = NEXT_INSN (link->insn);
>>> + insn = start;
>>> + if (!insn || NOTE_P (insn))
>>> + insn = NEXT_INSN (link->insn);
>>> + for (;
>>> (insn && (this_basic_block->next_bb == EXIT_BLOCK_PTR_FOR_FN (cfun)
>>> || BB_HEAD (this_basic_block->next_bb) != insn));
>>> insn = NEXT_INSN (insn))
>>>
