Re^2: Why is the execution order of subexpressions undefined? (victim)

[tye]

I think you've fallen victim to the temptation that I posted about. I don't believe that Perl defines the order of evaluation of expressions based on the precedence table. That determines what the results will be but need not restrict Perl to arriving at the proscribed value in the exact order that you appear to have defined based on how Perl has so far been implemented.

It would be sad if Perl 6 were not free to optimize

  my $x = $z * ( $y + 1 ) + $w / ( $y + 1 );

By noting that $y + 1 is used twice and therefore could be computed beforehand and the result just used twice rather than computing it twice. But if you tie Perl's hand by saying that execution order is defined for such things, then Perl 6 will have no choice but to not optimize expressions, thwarting part of what was advertised as a big motivation for Parrot: the ability to optimize bytecode.

[Zaxo]

Under an "as-if" rule, a bytecode optimizer need not obey the precedence rules, but the initial interpretation of your source must. Precedence and associativity are there to ensure that the expression you write is the one that gets calculated.

[tye]

We are talking order. The order that the calculations get performed need not be as you specified (based solely on precedence and associativity). Even if + might be redefined by the programmer such that x+y != y+x, computing x+y does not require that x be computed before y.

[BrowserUk]

It would be sad if Perl 6 were not free to optimize

 my $x = $z * ( $y + 1 ) + $w / ( $y + 1 );

Why would defining execution order affect that optimisation?

That just common subexpression elimination--clearly defined by the parens. The only sane defined ordering would be to execute the contents of parens first?

[tye]

I'd be insane to calculate $z first?

So which is the defined order that you find so easy to come up with? You can only have one.

[BrowserUk]

Sub expressions are evaluated before the expressions they are a part of. Order of evaluation within a subexpression is in precedence order with ties broken left to right.

In the expression you gave, both ( $y + 1 ) are subexpressions and would be evaluated before the overall expression, left to right. The fact that they are the same subexpression gives rise to the possible optimisation.