Perl_newSLICEOP: Optimise '(caller)[0]' into 'scalar caller'

[richardleach]

A subroutine can obtain just the package of its caller in a couple of ways. Both seem somewhat common.

• caller - in scaler context, as in my $x = caller;
• (caller)[0], as in my $x = (caller)[0];

In the first, caller finds the package name, sticks it in a new SV, and puts that (or undef) on the stack:

<0> caller[t2] s

In the second, caller (a) finds the package name, filename, and line (b) creates three new SVs to hold them all (c) puts those SVs on the stack (d) does a list slice to leave just the package SV on the stack.

7        <2> lslice sK/2 ->8
-           <1> ex-list lK ->5
3              <0> pushmark s ->4
4              <$> const[IV 0] s ->5
-           <1> ex-list lK ->7
5              <0> pushmark s ->6
6              <0> caller[t2] l ->7

This commit checks for the second case inside Perl_newSLICEOP and instead of constructing a lslice OP, returns just the caller OP with scalar context applied.

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• This set of changes does not require a perldelta entry.

[bulk88]

https://grep.metacpan.org/search?size=20&_bb=86025425&q=%5C%28caller%5C%29%5C%5B&qft=*.pm%2C+*.t&qd=&qifl=

Should've been done 30 years ago. Most PP devs think (caller())[\w] is constant folded as if Perl is identical to C++. The truth is list context PP context() is like writing a 100MB core dump to a SSD everytime you execute it. caller() shouldve never ever have become Perl best practices/cargo culted. There is nothing wrong with caller's public PP API IMO, but b/c of its horrible runtime internal implementation, it shouldve been sent to the landfill, by use strict; on day 1 of use strict;, just like this Perl 5 code was sent to the landfill by use strict;

C:\Users\Owner>perl -e" push( @a, THISS); push( @a, ISS); push( @a, PERL); $, =' '; print @a;"
THISS ISS PERL
C:\Users\Owner>

Rich, would you pretty please be able to tackle adding OP tree compile time next gen G_VOID propagation to the other 12-15 list context retval indexes/retval SV*s created by pp_caller?