[Pyrex] [Cython] switch statement

[Stefan Behnel]

Hi,

Robert Bradshaw top-posted:
> On Mar 12, 2008, at 11:32 AM, Simon Burton wrote:
>> Does anyone know if gcc can optimise a switch done with nested if's  
>> as well as it optimises a real switch ? (eg. using computed-gotos)
>
> I did some experiments with this, and the results seemed to indicate
> it does not. Also, nested ifs performed poorly compared to a flat
> list of ifs (for a medium-sized (5-50) set of conditions).

Interesting. Does that also hold for boolean "and/or" expressions? Cython
translates those to nested ifs. (I guess that's difficult to compare, though,
as it short-circuits.)


> It would be nice to have some syntax for switch statements. Several
> alternatives have been rejected for Python: http://www.python.org/dev/
> peps/pep-3103/

That's because they have Python semantics, not C semantics. Python's language
semantics make it a bit harder to come up with a meaningful "switch" design
than a plain C language "check this integer value against a list of other
integers" switch statement.

Regarding syntax: my opinion on this is that we should just do without any new
syntax, take our cool plugin optimiser infrastructure and just replace the
most obvious

    if c_int_val == 1:
        code1()
    elif c_int_val == 2:
        code2()
    elif c_int_val == some_other_constant_c_int:
        code3()
    else:
        code4()

by a straight C switch statement. No special casing if it involves variable
values, Python values, or otherwise non compile-time known values (i.e. DEF is
allowed), or any other comparison operators than "==". Just an optimisation
for the most simple case that a C switch statement can handle:

- if-elif-else chain with exactly one operator at each step: '=='
  -> decidable right after parsing (or even in the parser)
- lhs always the same plain C integer variable
  -> "same name" decidable right after parsing, "type" after type annalysis
- rhs a compile-time constant C integer value
  -> "type" decidable after type annalysis

This should be easy and fast enough to decide.

That way, people can decide what trade-off between Python semantics and
optimisation they want. And if we find more cases that we can support, we can
improve the plugin instead of the syntax.

Stefan