Does the Perl compiler need to be told not to optimize away function calls with ignored return values?

Question

I am writing new Perl 5 module Class::Tiny::ConstrainedAccessor to check type constraints when you touch object attributes, either by setting or by getting a default value. I am writing the unit tests and want to run the accessors for the latter case. However, I am concerned that Perl may optimize away my accessor-function call since the return value is discarded. Will it? If so, can I tell it not to? Is the corresponding behaviour documented? If the answer is as simple as "don't worry about it," that's good enough, but a reference to the docs would be appreciated :) .

The following MCVE succeeds when I run it on my Perl 5.26.2 x64 Cygwin. However, I don't know if that is guaranteed, or if it just happens to work now and may change someday.

use 5.006; use strict; use warnings; use Test::More; use Test::Exception;

dies_ok {   # One I know works
    my $obj = Klass->new;   # Default value of "attribute" is invalid
    diag $obj->accessor;    # Dies, because the default is invalid
} 'Bad default dies';

dies_ok {
    my $obj = Klass->new;
    $obj->accessor;         # <<< THE QUESTION --- Will this always run?
} 'Dies even without diag';

done_testing();

{   package Klass;
    sub new { my $class = shift; bless {@_}, $class }
    sub check { shift; die 'oops' if @_ and $_[0] eq 'bad' }
    sub default { 'bad' }
    sub accessor {
        my $self = shift;
        if(@_) { $self->check($_[0]); return $self->{attribute} = $_[0] }   # W
        elsif(exists $self->{attribute}) { return $self->{attribute} }      # R
        else {  
            # Request to read the attribute, but no value is assigned yet.
            # Use the default.
            $self->check($self->default);    # <<<---- What I want to exercise
            return $self->{attribute} = $self->default;
        }
    } #accessor()
} #Klass

This question deals with variables, but not functions. perlperf says that Perl will optimize away various things, but other than ()-prototyped functions, it's not clear to me what.

In JavaScript, I would say void obj.accessor();, and then I would know for sure it would run but the result would be discarded. However, I can't use undef $obj->accessor; for a similar effect; compilation legitimately fails with Can't modify non-lvalue subroutine call of &Klass::accessor.

Answer 1

Perl doesn't ever optimize away sub calls, and sub calls with side effects shouldn't be optimised away in any language.

undef $obj->accessor means something similar to $obj->accessor = undef