Re: Benchmark: Constant List in Hash Slice

Literal keys in hash subscripts and hash slices have the the hash (as in checksum) values of their strings pre-computed, and the string part of the string constant is stored in such a way that its easy and quick to pass directly to the hash lookup API functions.

Technically: at compile time, the PV part of the const SV is converted to a HEK, with SvPV(sv) pointing to the string part within the HEK. So it looks like a string but is also a HEK.

Dave.

Comment on Re: Benchmark: Constant List in Hash Slice

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Re^2: Benchmark: Constant List in Hash Slice by LanX (Saint) on Mar 13, 2019 at 17:38 UTC
OK I thought the inlining of the constant list might happen early enough to trigger this pre-computation. But the optimizer doesn't see the inlined keys as literal strings, so it has to calculate this lookup-index dynamically at run-time. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply]
Re^2: Benchmark: Constant List in Hash Slice by LanX (Saint) on Mar 14, 2019 at 12:03 UTC
does this also explain why using a normal @array variable is faster than a constant list? Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply]
Re^3: Benchmark: Constant List in Hash Slice by dave_the_m (Monsignor) on Mar 14, 2019 at 15:01 UTC
does this also explain why using a normal @array variable is faster than a constant list? No. That's because constant arrays are implemented as constant array refs. So `use constant foo qw(a b c); @a = $h{foo()}` [download] is implemented roughly as `use constant foo [qw(a b c)]; @a = $h{@{foo()}}` [download] And neither (as far as I'm aware) get the compile-time HEK treatment Dave.	[reply] [d/l] [select]
Re^4: Benchmark: Constant List in Hash Slice by LanX (Saint) on Mar 14, 2019 at 15:33 UTC
Oh that's good to know, thanks! :) Hence constant arrays won't help much for tuning performance. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply]


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