Hi,
On 05/04/2020 12:47 pm, Ned Batchelder wrote:
On 4/3/20 11:13 AM, joannah nanjekye wrote:
Hey all,
From my CS theory, a control flow graph models a program flow and one
of its main characteristics is it has one entry and exit point. IIRC,
CPython’s compilation process involves generation of a control flow graph.
Contrary to peephole optimizations, optimizations on the control flow
graph are more global allowing us to have complex and global
optimizations like branch and checkpoint eliminations etc.
I have seen several implementations of control flow optimizations. The
one I am familiar with is the V8 control flow optimizer.
I tried to investigate this for one of my directed courses last fall
but I want to know if there are people who have been thinking about
this for CPython and what their thoughts are.
Please make it possible to disable any optimizations. Sometimes
programs are run to understand the program, not for speed (for example,
during debugging, or coverage measurement.)
I have to disagree. The behavior of a program should not depend on
whether a flag is set or not. It makes debugging harder, IMO, if the
behavior changes depending on some flag.
For example, `pass` statements should never be executed.
This is an "optimization", but it is easier to understand if `pass` is
never executed. It becomes confusing it "executes" some of the time,
depending on some compiler flag.
IMO, a coverage tool should be not rely on `pass`, `while True:` and
other similarly trivial statements existing in the bytecode.
By "trivial statements", I mean any unconditional control flow
statements, conditional control flow statements with a constant test,
"pass", and any expression statements that are constant and without side
effect.
E.g. the following statements are "trivial":
try:
while False:
pass
1 + 1
On the other hand, other more powerful optimization should always
preserve the observable behavior of the program.
Since the observable behavior is unchanged, there would be no need to
turn them off.
Cheers,
Mark.
--Ned.
--
//Best,
Joannah Nanjekye
/"You think you know when you learn, are more sure when you can write,
even more when you can teach, but certain when you can program."
Alan J. Perlis/
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