eval: ensure debugging saved lines have an IV part

MANIFEST

@@ -6305,6 +6305,7 @@ t/op/cproto.t				Check builtin prototypes
 t/op/crypt.t				See if crypt works
 t/op/current_sub.t			__SUB__ tests
 t/op/dbm.t				See if dbmopen/dbmclose work
+t/op/debug.t				Test mechanisms used by the debugger
 t/op/decl-refs.t			See if my \$foo works
 t/op/defer.t				See if defer blocks work
 t/op/defined.t				See if defined() edge cases work

pod/perldelta.pod

@@ -366,7 +366,16 @@ manager will later use a regex to expand these into links.
 
 =item *
 
-XXX
+In some cases an C<eval> would not add integer parts to the source
+lines saved by the debugger.  [GH #23151]
+
+=item *
+
+Save debugger lines as C<PVIV> SVs rather than as C<PVMG> SVs as they
+don't need magic, aren't blessed and don't need to store a floating
+point part.  This should save 24 bytes per stored line for 64-bit
+systems, more for C<-Duselongdouble> or C<-Dusequadmath> builds.
+Discussed in [GH #23171].
 
 =back
 

pp_ctl.c

@@ -3735,7 +3735,7 @@ S_save_lines(pTHX_ AV *array, SV *sv)
 
     while (s && s < send) {
         const char *t;
-        SV * const tmpstr = newSV_type(SVt_PVMG);
+        SV * const tmpstr = newSV_type(SVt_PVIV);
 
         t = (const char *)memchr(s, '\n', send - s);
         if (t)
@@ -3744,6 +3744,9 @@ S_save_lines(pTHX_ AV *array, SV *sv)
             t = send;
 
         sv_setpvn_fresh(tmpstr, s, t - s);
+        /* not breakable until we compile a COP for it */
+        SvIV_set(tmpstr, 0);
+        SvIOK_on(tmpstr);
         av_store(array, line++, tmpstr);
         s = t;
     }

t/op/debug.t

@@ -0,0 +1,124 @@
+#!./perl
+
+# intended for testing language mechanisms that debuggers use not for
+# testing the debugger itself
+
+BEGIN {
+    chdir 't' if -d 't';
+    require "./test.pl";
+    set_up_inc( qw(. ../lib) );
+}
+
+use strict;
+use warnings;
+
+SKIP:
+{
+    skip_if_miniperl("need XS", 1);
+    # github 23151
+    # trivial debugger
+    local $ENV{PERL5DB} = 'sub DB::DB {}';
+    # eval code trimmed from code generated by Sub::Quote
+    fresh_perl_is(<<'CODE', <<'EXPECT',
+use B qw(SVf_IOK);
+
+sub _do_eval {
+  eval $_[0] or die $!;
+}
+
+_do_eval(<<'EVAL');
+{
+  sub table {
+  }
+}
+1;
+EVAL
+
+# look for lines that don't have an IV set
+my ($f) = grep /\(eval/, keys %::;
+my $x = $::{$f};
+my $lineno = 0;
+for my $l (@$x) {
+  if ($l) {
+    my $b = B::svref_2object(\$l);
+    if (!($b->FLAGS & SVf_IOK)) {
+      print "No IV for $f line $lineno: $l\n";
+      last
+    }
+  }
+  ++$lineno;
+}
+
+print "Done\n";
+CODE
+Done
+EXPECT
+                  {
+                      switches => [ '-d' ],
+                      stderr => 1,
+                  },
+                  "saved lines all have an IV"
+                  );
+}
+
+SKIP:
+{
+    # Historically lines were stored as PVMG, but we don't need
+    # magic on these lines.
+    #
+    # This checks that none of these lines get upgraded, ie. that
+    # we don't need them to be PVMG
+    #
+    # If this test fails perhaps we do need to make them PVMG
+    # and toke.c:S_update_debugger_info and pp_ctl.c:S_save_lines
+    # can be switched back to using SVt_PVMG and this test
+    # removed.
+    #
+    # See https://github.com/Perl/perl5/pull/23171#issuecomment-2780007725
+    skip_if_miniperl("need B");
+    local $ENV{PERL5DB} = 'sub DB::DB {}';
+    fresh_perl_is(<<'CODE', <<'EXPECT',
+use B;
+
+sub _do_eval {
+  eval $_[0] or die $!;
+}
+
+_do_eval(<<'EVAL');
+
+sub some_code {
+   print "Hello";
+}
+
+1;
+EVAL
+
+# check if any lines have been upgraded from PVIV
+my @files = grep /^_</, keys %::;
+for my $f (@files) {
+    my $lineno = 0;
+
+    for my $l (@{$f}) {
+        if ($l) {
+            my $b = B::svref_2object(\$l);
+            if (ref $b ne "B::PVIV") {
+               print "Not PVIV for $f:$lineno: $l\n";
+               last
+            }
+        }
+        ++$lineno;
+    }
+}
+print "Done\n";
+CODE
+Done
+EXPECT
+                  {
+                      switches => [ '-d' ],
+                      stderr => 1,
+                  },
+                  "saved lines are all PVIV"
+                  );
+}
+
+done_testing();

toke.c

@@ -2012,10 +2012,11 @@ S_update_debugger_info(pTHX_ SV *orig_sv, const char *const buf, STRLEN len)
     AV *av = CopFILEAVx(PL_curcop);
     if (av) {
         SV * sv;
-        if (PL_parser->preambling == NOLINE) sv = newSV_type(SVt_PVMG);
+        if (PL_parser->preambling == NOLINE)
+            sv = newSV_type(SVt_PVIV);
         else {
             sv = *av_fetch(av, 0, 1);
-            SvUPGRADE(sv, SVt_PVMG);
+            SvUPGRADE(sv, SVt_PVIV);
         }
         if (!SvPOK(sv)) SvPVCLEAR(sv);
         if (orig_sv)