Re: [PATCH] PR tree-optimization/103254 - Limit depth for all GORI expressions.

Richard Biener richard.guenther@gmail.com
Fri Nov 19 18:13:54 GMT 2021


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
>



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