From: Richard Biener <richard.guenther@gmail.com>
To: Andrew MacLeod <amacleod@redhat.com>
Cc: gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH] PR tree-optimization/103254 - Limit depth for all GORI expressions.
Date: Fri, 19 Nov 2021 19:13:54 +0100 [thread overview]
Message-ID: <95DF6FE9-478F-4084-A453-5054C40EF877@gmail.com> (raw)
In-Reply-To: <a1db9cb8-41e0-87b2-52ba-da08272e59f2@redhat.com>
On November 19, 2021 4:00:01 PM GMT+01:00, Andrew MacLeod <amacleod@redhat.com> wrote:
>On 11/19/21 04:21, Richard Biener wrote:
>> On Thu, Nov 18, 2021 at 8:30 PM Andrew MacLeod via Gcc-patches
>> <gcc-patches@gcc.gnu.org> wrote:
>>> At issue here is the dynamic approach we currently use for outgoing edge
>>> calculations. It isn't normally a problem, but once you get a very
>>> large number of possible outgoing values (ie very long unrolled blocks)
>>> with pairs of values on a statement, and individual queries for each
>>> one, it becomes exponential if they relate to each other.
>>>
>>> So. We previously introduced a param for PR 100081 which limited the
>>> depth of logical expressions GORI would pursue because we always make 2
>>> or 4 queries for each logical expression, which accelerated the
>>> exponential behaviour much quicker.
>>>
>>> This patch reuses that to apply to any statement which has 2 ssa-names,
>>> as the potential for 2 queries can happen then as well.. leading to
>>> exponential behaviour. Each statement we step back through with
>>> multiple ssa-names decreases the odds of calculating a useful outgoing
>>> range anyway. This will remove ridiculous behaviour like this PR
>>> demonstrates.
>>>
>>> Next release I plan to revamp GORI to cache outgoing values, which will
>>> eliminate all this sort of behaviour, and we can remove all such
>>> restrictions.
>>>
>>> Bootstrapped on x86_64-pc-linux-gnu with no regressions. OK for trunk?
>> I think it's reasonable. SCEV tries to limit the overall number of SSA -> def
>> visits, capturing deep vs. wide in a more uniform (and single) way.
>> (--param scev-max-expr-size and the duplicate - huh - --param
>> scev-max-expr-complexity).
>>
>> For SCEV running into the limit means fatal fail of the analysis, for ranger
>> it only means less refinement? So it might make a difference which path
>> you visit and which not (just thinking about reproducability of range analysis
>> when we say, swap operands of a stmt)?
>>
>Yes, Its means less refinement for "deeper" dependencies in the block..
>It will not be affected by operand swapping because we will either look
>at dependencies for both operands of a stmt, or neither. The path you
>visit shouldn't make any difference. Note this has nothing to do with
>generating ranges for those statements, this is only for refining
>outgoing ranges created by the condition at the bottom of the block.
>
>To be precise, we will stop looking for possible range changes once our
>dependency search, which always starts from the bottom of the block,
>encounters 6 stmts with multiple SSA operands. Statements with a single
>SSA operand will continue to build the dependency chains.
>
>so for instance, following just the chain for first operands of this
>somewhat truncated listing:
> <...>
> _137 = (short int) j_131;
> _138 = _129 & _137;
> _139 = (int) _138;
> j_140 = j_131 & _139; << depth 6 ... we don't go look at j_131
>or _139 for further dependencies
> _146 = (short int) j_140;
> _147 = _138 & _146;
> _148 = (int) _147;
> j_149 = j_140 & _148; << depth 5
> _155 = (short int) j_149;
> _156 = _147 & _155;
> _157 = (int) _156;
> j_158 = j_149 & _157; << depth 4
> _164 = (short int) j_158;
> _165 = _156 & _164;
> _166 = (int) _165;
> j_167 = j_158 & _166; <<-- depth 3
> _1 = (short int) j_167;
> _3 = _1 & _165; <<-depth 2
> _5 = (int) _3;
> j_22 = _5 & j_167; <<-- depth 1
> j_20 = j_22 + 4;
> if (j_20 == 4)
> goto <bb 3>;
>
>We'll generate dependencies, and thus able to generate outgoing ranges
>for all these ssa-names still until we hit a dependency depth of 6 (the
>current default), which for one chain comes to an end at
>
>j_140 = j_131 & _139
>
> We will be able to generate outgoing ranges for j_131 and _139 on the
>edges, but we will not try for any ssa_names used to define those.. ie,
>_138 might not be able to have outgoing ranges generated for it in this
>block.
>
>working the j_20 : [4,4] range on the true edge back thru those
>expressions, once we get that deep the odds of finding something of
>value is remote :-)
>
>We will also only generate these outgoing ranges when they are asked
>for. Many of these names are not used outside the block (especially
>back the deeper you go), and so never get asked for anyway... but you
>could :-)
I see. I suppose you could prune the list simply by asking whether a Def has a single use only (that you just followed) and follow the chain but not generate an outgoing range for the Def. Not sure if that will make a big difference in compile time.
>Anyway, this limit in no way introduces any kind of ordering variation..
>
>Andrew
>
>PS for amusement and information overload... it turns out in this hunk
>of code we end up knowing that globally the range of most of these names
>are [0, 4], and the actual ranges we COULD generate if asked:
>
>3->5 (T) _1 : short int [0, 4]
>3->5 (T) _3 : short int [0, 4]
>3->5 (T) _5 : int [0, 4]
>3->5 (T) j_20 : int [4, 4]
>3->5 (T) j_22 : int [0, 0]
>3->5 (T) _120 : short int [0, 4]
>3->5 (T) _121 : int [0, 4]
>3->5 (T) _128 : short int [0, 4]
>3->5 (T) _129 : short int [0, 4]
>3->5 (T) _130 : int [0, 4]
>3->5 (T) j_131 : int [0, 4]
>3->5 (T) _137 : short int [0, 4]
>3->5 (T) _138 : short int [0, 4]
>3->5 (T) _139 : int [0, 4]
>3->5 (T) j_140 : int [0, 4]
>3->5 (T) _146 : short int [0, 4]
>3->5 (T) _147 : short int [0, 4]
>3->5 (T) _148 : int [0, 4]
>3->5 (T) j_149 : int [0, 4]
>3->5 (T) _155 : short int [0, 4]
>3->5 (T) _156 : short int [0, 4]
>3->5 (T) _157 : int [0, 4]
>3->5 (T) j_158 : int [0, 4]
>3->5 (T) _164 : short int [0, 4]
>3->5 (T) _165 : short int [0, 4]
>3->5 (T) _166 : int [0, 4]
>3->5 (T) j_167 : int [0, 4]
>3->4 (F) _1 : short int [1, 4]
>3->4 (F) _3 : short int [1, 4]
>3->4 (F) _5 : int [1, 4]
>3->4 (F) j_20 : int [5, 8]
>3->4 (F) j_22 : int [1, 4]
>3->4 (F) _120 : short int [1, 4]
>3->4 (F) _121 : int [1, 4]
>3->4 (F) _128 : short int [1, 4]
>3->4 (F) _129 : short int [1, 4]
>3->4 (F) _130 : int [1, 4]
>3->4 (F) j_131 : int [1, 4]
>3->4 (F) _137 : short int [1, 4]
>3->4 (F) _138 : short int [1, 4]
>3->4 (F) _139 : int [1, 4]
>3->4 (F) j_140 : int [1, 4]
>3->4 (F) _146 : short int [1, 4]
>3->4 (F) _147 : short int [1, 4]
>3->4 (F) _148 : int [1, 4]
>3->4 (F) j_149 : int [1, 4]
>3->4 (F) _155 : short int [1, 4]
>3->4 (F) _156 : short int [1, 4]
>3->4 (F) _157 : int [1, 4]
>3->4 (F) j_158 : int [1, 4]
>3->4 (F) _164 : short int [1, 4]
>3->4 (F) _165 : short int [1, 4]
>3->4 (F) _166 : int [1, 4]
>3->4 (F) j_167 : int [1, 4]
>
>So the chains go slightly deeper than my example, but the end result is
>most of the names in the _30 thru _119 range on the false edge would
>come back with [0,4] instead of [1,4] with this change.
>
>The actual dependency chains from the dump:
>
>Exports: _1 _3 _5 j_20 j_22 _120 _128 _129 j_131 _137 _138
>_139 j_140 _146 _147 _148 j_149 _155 _156 _157 j_158 _164
>_165 _166 j_167
>
>The exports are the names that ranger can potentially generate a range
>on outgoing edges that may differ from the range in the block. It means
>GORI might be able to take the condition at the bottom and adjust those
>ranges. Any name not in the list will simply use the range it has in
>the block. Adjusting that param value will change the number of these.
>
>The dependency list for each ssa name is also shown: and we can see
>how the higher in the block we go, the deeper in the dependency chain
>from the bottom of the block we are, and thus there are less names in
>the list for each name
>
> _1 : _120 _128 _129 j_131 _137 _138 _139 j_140 _146
>_147 _148 j_149 _155 _156 _157 j_158 _164 _165 _166 j_167
> _3 : _1 _120 _128 _129 j_131 _137 _138 _139 j_140
>_146 _147 _148 j_149 _155 _156 _157 j_158 _164 _165 _166 j_167
> _5 : _1 _3 _120 _128 _129 j_131 _137 _138 _139 j_140
>_146 _147 _148 j_149 _155 _156 _157 j_158 _164 _165 _166 j_167
> j_20 : _1 _3 _5 j_22 _120 _128 _129 j_131 _137 _138
>_139 j_140 _146 _147 _148 j_149 _155 _156 _157 j_158 _164
>_165 _166 j_167
> j_22 : _1 _3 _5 _120 _128 _129 j_131 _137 _138 _139
>j_140 _146 _147 _148 j_149 _155 _156 _157 j_158 _164 _165
>_166 j_167
> _129 : _120 _128
> _137 : j_131
> _138 : _120 _128 _129 j_131 _137
> j_140 : j_131 _139
> _146 : j_131 _139 j_140
> _147 : _120 _128 _129 j_131 _137 _138 _139 j_140 _146
> _148 : _147
> j_149 : j_131 _139 j_140 _147 _148
> _155 : j_131 _139 j_140 _147 _148 j_149
> _156 : _120 _128 _129 j_131 _137 _138 _139 j_140 _146
>_147 _148 j_149 _155
> _157 : _120 _128 _129 j_131 _137 _138 _139 j_140 _146
>_147 _148 j_149 _155 _156
> j_158 : _120 _128 _129 j_131 _137 _138 _139 j_140 _146
>_147 _148 j_149 _155 _156 _157
> _164 : _120 _128 _129 j_131 _137 _138 _139 j_140 _146
>_147 _148 j_149 _155 _156 _157 j_158
> _165 : _120 _128 _129 j_131 _137 _138 _139 j_140 _146
>_147 _148 j_149 _155 _156 _157 j_158 _164
> _166 : _120 _128 _129 j_131 _137 _138 _139 j_140 _146
>_147 _148 j_149 _155 _156 _157 j_158 _164 _165
> j_167 : _120 _128 _129 j_131 _137 _138 _139 j_140 _146
>_147 _148 j_149 _155 _156 _157 j_158 _164 _165 _166
>
next prev parent reply other threads:[~2021-11-19 18:13 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-18 19:28 Andrew MacLeod
2021-11-19 9:21 ` Richard Biener
2021-11-19 15:00 ` Andrew MacLeod
2021-11-19 18:13 ` Richard Biener [this message]
2021-11-19 18:53 ` Andrew MacLeod
2021-11-19 16:43 ` Andrew MacLeod
2021-11-19 10:15 ` Aldy Hernandez
2021-11-19 14:38 ` Andrew MacLeod
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