[issue30501] Produce optimized code for boolean conditions

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue30501] Produce optimized code for boolean conditions

[Serhiy Storchaka]

Serhiy Storchaka added the comment:


New changeset 36ff451ebae41f09560bff582c95946474d898f8 by Serhiy Storchaka in 
branch 'master':
bpo-30501: Make the compiler producing optimized code for condition 
expressions. (#1851)
https://github.com/python/cpython/commit/36ff451ebae41f09560bff582c95946474d898f8


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[issue30501] Produce optimized code for boolean conditions

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

> Any high-level benchmarks (i.e. other than pybench and similar programs) 
> impacted by this?

Sorry, I don't have any data. Currently I'm not able to run large benchmarks. I 
would be surprised if this increases the performance of a macrobenchmark even 
by 1%. But this change is a step in the direction of getting rid from the 
peephole optimizer.

> Will this negatively impact code coverage reporting or code tracing (by 
> optimizing across basic blocks)?

I can't imagine the case. The compiler doesn't generate separate line numbers 
neither for JUMP instructions, nor for auxilary stack manipulating instructions 
in chained comparison, nor for UNARY_NOT. And these are the only things that 
are changed. Line number marks are left the same in optimized code, and they 
points to the same instructions. I think that coverage tools and debugger are 
not affected.

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[issue30501] Produce optimized code for boolean conditions

[Emily Morehouse]

Changes by Emily Morehouse :


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nosy: +emilyemorehouse

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[issue30501] Produce optimized code for boolean conditions

[Raymond Hettinger]

Raymond Hettinger added the comment:

The improved code does look better.  I'm +1 on this proposal as long as we're 
sure it doesn't introduce any new problems.

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[issue30501] Produce optimized code for boolean conditions

[Raymond Hettinger]

Raymond Hettinger added the comment:

Will this negatively impact code coverage reporting or code tracing (by 
optimizing across basic blocks)?

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[issue30501] Produce optimized code for boolean conditions

[Antoine Pitrou]

Antoine Pitrou added the comment:

Any high-level benchmarks (i.e. other than pybench and similar programs) 
impacted by this?

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nosy: +pitrou

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[issue30501] Produce optimized code for boolean conditions

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

It would be hard to get the same optimization in the peepholer.

For the first example, "if not a and b:", it is easy, just replace 
UNARY_NOT+POP_JUMP_IF_FALSE with POP_JUMP_IF_TRUE. The more complex third 
example, "if not (a and b) and c:", would need multiple passes (and more 
complex examples can need more passes, having the quadratic complexity in 
corner cases). The second example, with chained comparisons, is the most 
complex. It uses the fact that after the conditional jump and some stack 
operations we got a false value on the stack. This is too complex for the 
peepholer.

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[issue30501] Produce optimized code for boolean conditions

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Some examples.

if not a and b: x

Unpatched:
  1   0 LOAD_NAME0 (a)
  2 UNARY_NOT
  4 POP_JUMP_IF_FALSE   14
  6 LOAD_NAME1 (b)
  8 POP_JUMP_IF_FALSE   14
 10 LOAD_NAME2 (x)
 12 POP_TOP
>>   14 LOAD_CONST   0 (None)
 16 RETURN_VALUE

Patched:
  1   0 LOAD_NAME0 (a)
  2 POP_JUMP_IF_TRUE12
  4 LOAD_NAME1 (b)
  6 POP_JUMP_IF_FALSE   12
  8 LOAD_NAME2 (x)
 10 POP_TOP
>>   12 LOAD_CONST   0 (None)
 14 RETURN_VALUE

if a <= b < c: x

Unpatched:
  1   0 LOAD_NAME0 (a)
  2 LOAD_NAME1 (b)
  4 DUP_TOP
  6 ROT_THREE
  8 COMPARE_OP   1 (<=)
 10 JUMP_IF_FALSE_OR_POP18
 12 LOAD_NAME2 (c)
 14 COMPARE_OP   0 (<)
 16 JUMP_FORWARD 4 (to 22)
>>   18 ROT_TWO
 20 POP_TOP
>>   22 POP_JUMP_IF_FALSE   28
 24 LOAD_NAME3 (x)
 26 POP_TOP
>>   28 LOAD_CONST   0 (None)
 30 RETURN_VALUE

Patched:
  1   0 LOAD_NAME0 (a)
  2 LOAD_NAME1 (b)
  4 DUP_TOP
  6 ROT_THREE
  8 COMPARE_OP   1 (<=)
 10 POP_JUMP_IF_FALSE   20
 12 LOAD_NAME2 (c)
 14 COMPARE_OP   0 (<)
 16 POP_JUMP_IF_FALSE   28
 18 JUMP_FORWARD 4 (to 24)
>>   20 POP_TOP
 22 JUMP_FORWARD 4 (to 28)
>>   24 LOAD_NAME3 (x)
 26 POP_TOP
>>   28 LOAD_CONST   0 (None)
 30 RETURN_VALUE

if not (a and b) and c: x

Unpatched:
  1   0 LOAD_NAME0 (a)
  2 JUMP_IF_FALSE_OR_POP 6
  4 LOAD_NAME1 (b)
>>6 UNARY_NOT
  8 POP_JUMP_IF_FALSE   18
 10 LOAD_NAME2 (c)
 12 POP_JUMP_IF_FALSE   18
 14 LOAD_NAME3 (x)
 16 POP_TOP
>>   18 LOAD_CONST   0 (None)
 20 RETURN_VALUE

Patched:
  1   0 LOAD_NAME0 (a)
  2 POP_JUMP_IF_FALSE8
  4 LOAD_NAME1 (b)
  6 POP_JUMP_IF_TRUE16
>>8 LOAD_NAME2 (c)
 10 POP_JUMP_IF_FALSE   16
 12 LOAD_NAME3 (x)
 14 POP_TOP
>>   16 LOAD_CONST   0 (None)
 18 RETURN_VALUE

Note that the __bool__() method of every value is evaluated only once.

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[issue30501] Produce optimized code for boolean conditions

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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pull_requests: +1934

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[issue30501] Produce optimized code for boolean conditions

[Serhiy Storchaka]

New submission from Serhiy Storchaka:

The peephole optimizer optimizes some boolean conditions. For example in "if 
not a:" it replaces UNARY_NOT+POP_JUMP_IF_FALSE with POP_JUMP_IF_TRUE, and in 
"if a and b:" it makes checking the boolean value of a only once. But it is 
unable to optimize more complex conditions, like "if not a and b:".

Proposed patch makes the compiler producing optimized code for conditions. It 
supports expressions with arbitrary complexity containing the "not" operator, 
the "and" and "or" binary operators, the "if" ternary operator, and chained 
comparisons, used as conditions in the ternary operator, in the "if", "while" 
and "assert" statements, and in generator expressions and comprehensions.

It would be possible to remove the part of the peepholer optimizer, but it is 
needed also for optimizing the "and" and "or" operators in non-boolean context. 
Doing this optimization in the compiler is possible but too cumbersome, it 
requires 3 times more code that in the proposed patch. May be I'll find the 
more general solution in future.

--
components: Interpreter Core
messages: 294674
nosy: benjamin.peterson, brett.cannon, ncoghlan, rhettinger, serhiy.storchaka, 
yselivanov
priority: normal
severity: normal
stage: patch review
status: open
title: Produce optimized code for boolean conditions
type: performance
versions: Python 3.7

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