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Re: [perl #76438] peephole optimiser could prune more dead code

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James Mastros
July 10, 2010 04:36
Re: [perl #76438] peephole optimiser could prune more dead code
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On 9 July 2010 16:56, Nicholas Clark <> wrote:
> $ ./perl -Ilib -MO=Deparse -e 'if ("Pie" eq "Good") {print}'
> '???';
> -e syntax OK
> but
> $ ./perl -Ilib -MO=Deparse -e 'if ($a && "Pie" eq "Good") {print}'
> if ($a and !1) {
>    print $_;
> }
> -e syntax OK
> which demonstrates that "Pie" eq "Good" is constant folded, but that the
> optree for the block still exists.
> The peephole optimiser is correct not to optimise this to nothing, as it
> can't know that $a is neither tied nor overloaded, so cannot assume that
> the lookup of $a has no side effects.
> However, it can know that the conditional to the if block is always false,
> and so could optimise away the ops for the block, freeing up their memory.
> Hence the code should become
>    $a and !1;
> or even the perl equivalent of
>    (void) (bool) $a;
> Wishlist, because I've no idea how much real world perl code ends up with
> constructions like this, and would benefit

I do wonder, sometimes, if we worry entirely too much about just when
tie and overload calls or done.  Would it break actual real-world code
to not retrieve the value of $a (tie) or not boolify it (overload)
when the value will be thrown away anyway?  Clearly, we can't do this
in a maintance release, but perhaps we should add warnings that we are
planning on doing it to 5.14.0?  It seems to me that doing this would
allow all sorts of optimizations that we currently think of, and then
say "that'd change overloading", and throw out, with very little
impact on real-world code, which either doesn't use overloading, or
would be happy if overloading were made faster by avoiding it where

   -=- James Mastros / theorbtwo

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