[issue27236] Add CHAINED_COMPARE_OP opcode

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Thank you for your tests Josh.

I have collected additional statistics. According to it CHAINED_COMPARE_OP 
would be used 100 times less than COMPARE_OP. This is not worth adding new 
opcode.

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

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[issue27236] Add CHAINED_COMPARE_OP opcode

[Josh Rosenberg]

Josh Rosenberg added the comment:

It is a little funny though. I expect a more common test like:

if a < b < c: pass

to be unconditionally faster than the logically equivalent:

if a < b and b < c: pass

The only difference between the two should be that b is loaded only once, which 
should make the chained comparison either equivalent or faster. But thanks to 
the additional stack manipulations needed, in microbenchmarks, this isn't the 
case. I was just testing a couple simple cases in ipython (3.5, Linux, x64), 
and the chained comparison is actually slower when short-circuiting is involved:

%%timeit -r5 a, b, c = range(3, 0, -1)
if a < b < c: pass
1000 loops, best of 5: 33.7 ns per loop

%%timeit -r5 a, b, c = range(3, 0, -1)
if a < b and b < c: pass
1000 loops, best of 5: 25 ns per loop

or even without short-circuiting, but with the final test failing, chained 
loses (by a smaller margin):

%%timeit -r5 a, b, c = 0, 1, 0
if a < b < c: pass
1000 loops, best of 5: 42.5 ns per loop

%%timeit -r5 a, b, c = 0, 1, 0
if a < b and b < c: pass
1000 loops, best of 5: 39.3 ns per loop

The same pattern holds if a and c are replaced with constants, e.g. if 0 < b < 
2: (so only b is loaded by name). Yes, it's a small difference; when a, b, c 
are initialized from range(3), so all tests succeed, chained comparisons win, 
44.9 ns to 54.5 ns. But it seems kind of odd that the simplest case of a 
chained comparison is actually a pessimization in two of the three cases.

Not saying it's worth changing (I doubt chained comparisons occur in code 
enough to justify the tiny savings a dedicated op code could provide), but it's 
not something I'd reject out of hand.

--
nosy: +josh.r

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[issue27236] Add CHAINED_COMPARE_OP opcode

[Demur Rumed]

Demur Rumed added the comment:

@rhettinger can you clarify your opinion in relation to #27140 with #27095 & 
#27213 in mind?

I agree that CHAINED_COMPARE_OP is unnecessary

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[issue27236] Add CHAINED_COMPARE_OP opcode

[Raymond Hettinger]

Raymond Hettinger added the comment:

Please stop adding new opcodes for rare use cases.  That represents a sharp 
departure from our entire history of adding opcodes.

Code like "x = a < b > c < d" almost never comes up.

--
nosy: +rhettinger

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[issue27236] Add CHAINED_COMPARE_OP opcode

[Serhiy Storchaka]

New submission from Serhiy Storchaka:

For now complex code is generated for chained comparing.

$ echo "x = a < b > c < d" | ./python -m dis
  1   0 LOAD_NAME0 (a)
  2 LOAD_NAME1 (b)
  4 DUP_TOP
  6 ROT_THREE
  8 COMPARE_OP   0 (<)
 10 JUMP_IF_FALSE_OR_POP28
 12 LOAD_NAME2 (c)
 14 DUP_TOP
 16 ROT_THREE
 18 COMPARE_OP   4 (>)
 20 JUMP_IF_FALSE_OR_POP28
 22 LOAD_NAME3 (d)
 24 COMPARE_OP   0 (<)
 26 JUMP_FORWARD 4 (to 32)
>>   28 ROT_TWO
 30 POP_TOP
>>   32 STORE_NAME   4 (x)
 34 LOAD_CONST   0 (None)
 36 RETURN_VALUE

Proposed patch adds CHAINED_COMPARE_OP opcode that does all necessary stack 
manipulatios. Using it the generated code is simpler:

$ echo "x = a < b > c < d" | ./python -m dis
  1   0 LOAD_NAME0 (a)
  2 LOAD_NAME1 (b)
  4 CHAINED_COMPARE_OP   0 (<)
  6 JUMP_IF_FALSE_OR_POP18
  8 LOAD_NAME2 (c)
 10 CHAINED_COMPARE_OP   4 (>)
 12 JUMP_IF_FALSE_OR_POP18
 14 LOAD_NAME3 (d)
 16 COMPARE_OP   0 (<)
>>   18 STORE_NAME   4 (x)
 20 LOAD_CONST   0 (None)
 22 RETURN_VALUE

--
components: Interpreter Core
files: chained_compare_op.patch
keywords: patch
messages: 267466
nosy: Demur Rumed, Mark.Shannon, serhiy.storchaka
priority: normal
severity: normal
stage: patch review
status: open
title: Add CHAINED_COMPARE_OP opcode
type: enhancement
versions: Python 3.6
Added file: http://bugs.python.org/file43247/chained_compare_op.patch

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