eval: ensure debugging saved lines have an IV part

[tonycoz]

perldebguts documents that the lines stored in @{"_<$filename"}
arrays have a numeric value in addition to the text of the source,
ensure that is true for evals.

Non-zero IV values indicate the lines are breakable (they represent
the address of the COP for that line)

---

• This set of changes requires a perldelta entry, and it is included.

[bulk88]

        SV * const tmpstr = newSV_type(SVt_PVMG);
        t = (const char *)memchr(s, '\n', send - s);
        if (t)
            t++;
        else
            t = send;

        sv_setpvn_fresh(tmpstr, s, t - s);
        /* not breakable until we compile a COP for it */
        SvIV_set(tmpstr, 0);

Improvement, why is this code using SVt_PVMG and not the smaller PVIV body struct?

git blame shows Larry typed in "PVMG" for unknown reasons

STATIC void
S_save_lines(pTHX_ AV *array, SV *sv)
{
    const char *s = SvPVX_const(sv);
    const char * const send = SvPVX_const(sv) + SvCUR(sv);
    I32 line = 1;

    while (s && s < send) {
	const char *t;
	SV * const tmpstr = newSV(0);

	sv_upgrade(tmpstr, SVt_PVMG);
	t = strchr(s, '\n');
	if (t)
	    t++;

Revision: a0d0e21ea6ea90a22318550944fe6cb09ae10cda
Author: Larry Wall <[email protected]>
Date: 10/17/1994 7:00:00 PM
Message:perl 5.000

[tonycoz]

Improvement, why is this code using SVt_PVMG and not the smaller PVIV body struct?

Good point, I've added changing from PVMG to PVIV.

[tonycoz]

Fixes #23151

[bulk88]

Unroll SvUPGRADE(), add {} to SvPVCLEAR(sv);, add 2 goto skipping if(!SvPOK(sv)) test and jump right into the branch, if was < SVt_PV or we did a newSV_type(SVt_PVIV);. We know the SV* is undef and/or doesn't have POK_on, so we can skip the if(!SvPOK(sv)) conditional jump CPU opcode/opcodes.