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optimize head/tail #70

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optimize head/tail #70

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97d5000
head/tail: avoid copying return values
dur-randir Feb 24, 2018
9cb01b1
head/tail: simplify boundary detection code
dur-randir Feb 24, 2018
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46 changes: 14 additions & 32 deletions ListUtil.xs
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Original file line number Diff line number Diff line change
Expand Up @@ -623,44 +623,26 @@ PPCODE:
{
int size = 0;
int start = 0;
int end = 0;
int i = 0;

size = SvIV( ST(0) );
if (items > 1) {
size = SvIV(*(SP + 1));

if ( ix == 0 ) {
start = 1;
end = start + size;
if ( size < 0 ) {
end += items - 1;
}
if ( end > items ) {
end = items;
}
}
else {
end = items;
if ( size < 0 ) {
start = -size + 1;
}
else {
start = end - size;
}
if ( start < 1 ) {
start = 1;
if (ix) start = items - size - 1;
start = 1 + (start + items - 1) % (items - 1);

if (size > items - 1) {
size = items - 1;
} else if (size < 0) {
size = size + items - 1;
}
}

if ( end < start ) {
XSRETURN(0);
}
else {
EXTEND( SP, end - start );
for ( i = start; i <= end; i++ ) {
PUSHs( sv_2mortal( newSVsv( ST(i) ) ) );
}
XSRETURN( end - start );
while (--size >= 0) {
++SP;
*SP = *(SP + start);
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@leonerd leonerd Nov 26, 2018 •
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Hmm.. This is quite the change of behaviour - we're now returning aliases of the original input list, rather than new temporary SVs that copy it. This would have a change of behaviour in lvalue context; for example

my @n = (1 .. 10);
$_++ for tail 4, @n;

I'm undecided if that's a good thing or not.

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@Grinnz Grinnz Nov 26, 2018

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If such code wouldn't work before, I'd say this would just be a new feature...

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@leonerd leonerd Nov 26, 2018 •
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It could be considered a feature yes. But it's a somewhat surprising change of behaviour. Is there a chance of accidental collateral damage?

I've asked for further opinion on p5p@ since it's a core module.

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@haarg haarg Jun 13, 2022

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Returning aliases is the behavior I would expect.

}

PUTBACK;
}

void
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