SV PV COW 255 doesn't support strings under 9 SvCUR() bytes long

[bulk88]

Description
from sv.c

#ifndef SV_COW_MAX_WASTE_FACTOR_THRESHOLD
#    define SV_COW_MAX_WASTE_FACTOR_THRESHOLD   2    /* COW iff len < (cur * K) */
#endif
#ifndef SV_COWBUF_WASTE_FACTOR_THRESHOLD
#    define SV_COWBUF_WASTE_FACTOR_THRESHOLD    2    /* COW iff len < (cur * K) */
#endif

8 bytes or smaller strings can't 255 COW because of ^^^^^ added in 5.19.12 in

#13800
e8c6a47

Author: Yves Orton [email protected]
Date: 5/11/2014 6:37:33 AM
Message:
Implement "max waste" thresholds to avoid problems with COW and deliberately overallocated pvs
https://rt.perl.org/Ticket/Display.html?id=121796

Steps to Reproduce

use Devel::Peek;
 #use Inline  ('force', 'noclean');

use Inline C => Config =>
 PRE_HEAD => '#define PERL_NO_GET_CONTEXT 1';
use Inline C => Config => BUILD_NOISY => 1;

use Inline C => <<'END_OF_C_CODE';


SV* do3(SV* ssv) {
 dTHX;
 U32 flags = 0;
    SV* dsv;
    if ((SvFLAGS(ssv) & ((SVf_OK|SVs_GMG) &~(SVp_NOK))) == SVf_NOK)
        dsv = newSVnv(SvNVX(ssv));
    else if ((SvFLAGS(ssv) & ((SVf_OK|SVs_GMG) &~(SVp_IOK))) == SVf_IOK) {
        IV iv = SvIVX(ssv);
        dsv = SvUOK(ssv) ? newSVuv((UV)iv) : newSViv(iv);
    }
    else {
        U32 type = SvPOK(ssv) ? SVt_PV : SvNOK(ssv) ? SVt_NV : SVt_IV;
        if (flags & SVs_TEMP)
            dsv = newSV_type_mortal(type);
        else
            dsv = newSV_type(type);
        sv_dump_depth(dsv, 3);
        sv_dump_depth(ssv, 3);
        sv_setsv_flags(dsv, ssv, SV_GMAGIC | SV_NOSTEAL
            | SV_COW_SHARED_HASH_KEYS | SV_COW_OTHER_PVS | SV_DO_COW_SVSETSV);
        sv_dump_depth(ssv, 3);
        sv_dump_depth(dsv, 3);
        return dsv;
    }
    if (flags & SVs_TEMP)
        dsv = sv_2mortal(dsv);
    return dsv;
}


END_OF_C_CODE


$, = "\n";

use v5.30;
use strict;
use warnings;
#$DB::single = 1;

#$DB::single = 1;

use version;
my $s1;
my $s2;
my $s3;
my $s4;
warn "\nwith 8 bytes\n\n";
$s1 = substr(int(rand(10)),0,1).'.345678';
$s2 = do3($s1);
warn "\nnow with 9 bytes\n\n";
$s1 = substr(int(rand(10)),0,1).'.3456789';
$s3 = do3($s1);
warn "\nwith 4 bytes\n\n";
$s1 = substr(int(rand(10)),0,1).'.34';
$s4 = do3($s1);

with 8 bytes

SV = PV(0x2abb18) at 0x2ab0f0
  REFCNT = 1
  FLAGS = ()
  PV = 0
SV = PV(0x2abae8) at 0x244ce88
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x231e048 "4.345678"\0
  CUR = 8
  LEN = 16
SV = PV(0x2abae8) at 0x244ce88
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x231e048 "4.345678"\0
  CUR = 8
  LEN = 16
SV = PV(0x2abb18) at 0x2ab0f0
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x2496e98 "4.345678"\0
  CUR = 8
  LEN = 16

now with 9 bytes

SV = PV(0x2abb48) at 0x2ab0f0
  REFCNT = 1
  FLAGS = ()
  PV = 0
SV = PV(0x2abae8) at 0x244ce88
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x231e048 "7.3456789"\0
  CUR = 9
  LEN = 16
SV = PV(0x2abae8) at 0x244ce88
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0x231e048 "7.3456789"\0
  CUR = 9
  LEN = 16
  COW_REFCNT = 1
SV = PV(0x2abb48) at 0x2ab0f0
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0x231e048 "7.3456789"\0
  CUR = 9
  LEN = 16
  COW_REFCNT = 1

with 4 bytes

SV = PV(0x2abb68) at 0x2ab0f0
  REFCNT = 1
  FLAGS = ()
  PV = 0
SV = PV(0x2abae8) at 0x244ce88
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x2495098 "8.34"\0
  CUR = 4
  LEN = 16
SV = PV(0x2abae8) at 0x244ce88
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x2495098 "8.34"\0
  CUR = 4
  LEN = 16
SV = PV(0x2abb68) at 0x2ab0f0
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x2497018 "8.34"\0
  CUR = 4
  LEN = 16

Expected behavior
That tiny strings can SVPV 255 COW. 8 bytes and under are often tokens/fields/keys/prop names/string-ified integers/stringified 1 or 0 bools/api func calls or var names/json prop names. All of those are very high frequency to see bounce around at runtime on end user code.

Perl_sv_grow()/PERL_STRLEN_ROUNDUP/PERL_STRLEN_EXPAND_SHIFT/PERL_STRLEN_NEW_MIN/newSVpvn/etc should not be discriminating against short strings held in Perl API's Newx() backed, API min buf length of 0x10==SvLEN().

Perl configuration

Summary of my perl5 (revision 5 version 41 subversion 7) configuration:
  Derived from: 73172a67eaae5671dffc06b427f005810d151472
  Platform:
    osname=MSWin32
    osvers=6.1.7601
    archname=MSWin32-x64-multi-thread
    uname=''
    config_args='undef'
    hint=recommended
    useposix=true
    d_sigaction=undef
    useithreads=define
    usemultiplicity=define
    use64bitint=define
    use64bitall=undef
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
  Compiler:
    cc='cl'
    ccflags ='-nologo -GF -W3 -MD -TC -DWIN32 -D_CONSOLE -DNO_STRICT -DWIN64 -D_
CRT_SECURE_NO_DEPRECATE -D_CRT_NONSTDC_NO_DEPRECATE -D_WINSOCK_DEPRECATED_NO_WAR
NINGS -DPERL_TEXTMODE_SCRIPTS -DMULTIPLICITY -DPERL_IMPLICIT_SYS -DWIN32_NO_REGI
STRY -DUSE_PERLIO'
    optimize='-O1 -Zi -GL -fp:precise'
    cppflags='-DWIN32'
    ccversion='19.36.32535'
    gccversion=''
    gccosandvers=''
    intsize=4
    longsize=4
    ptrsize=8
    doublesize=8
    byteorder=12345678
    doublekind=3
    d_longlong=undef
    longlongsize=8
    d_longdbl=define
    longdblsize=8
    longdblkind=0
    ivtype='__int64'
    ivsize=8
    nvtype='double'
    nvsize=8
    Off_t='__int64'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='link'
    ldflags ='-nologo -nodefaultlib -debug -opt:ref,icf -ltcg -libpath:"c:\pb64\
lib\CORE" -machine:AMD64 -subsystem:console,"5.02"'
    libpth="C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSV
C\14.36.32532\lib\x64"
    libs=oldnames.lib kernel32.lib user32.lib gdi32.lib winspool.lib comdlg32.li
b advapi32.lib shell32.lib ole32.lib oleaut32.lib netapi32.lib uuid.lib ws2_32.l
ib mpr.lib winmm.lib version.lib odbc32.lib odbccp32.lib comctl32.lib msvcrt.lib
 vcruntime.lib ucrt.lib
    perllibs=oldnames.lib kernel32.lib user32.lib gdi32.lib winspool.lib comdlg3
2.lib advapi32.lib shell32.lib ole32.lib oleaut32.lib netapi32.lib uuid.lib ws2_
32.lib mpr.lib winmm.lib version.lib odbc32.lib odbccp32.lib comctl32.lib msvcrt
.lib vcruntime.lib ucrt.lib
    libc=ucrt.lib
    so=dll
    useshrplib=true
    libperl=perl541.lib
    gnulibc_version=''
  Dynamic Linking:
    dlsrc=dl_win32.xs
    dlext=dll
    d_dlsymun=undef
    ccdlflags=' '
    cccdlflags=' '
    lddlflags='-dll -nologo -nodefaultlib -debug -opt:ref,icf -ltcg -libpath:"c:
\pb64\lib\CORE" -machine:AMD64 -subsystem:console,"5.02"'


Characteristics of this binary (from libperl):
  Compile-time options:
    HAS_LONG_DOUBLE
    HAS_TIMES
    HAVE_INTERP_INTERN
    MULTIPLICITY
    PERLIO_LAYERS
    PERL_COPY_ON_WRITE
    PERL_DONT_CREATE_GVSV
    PERL_HASH_FUNC_SIPHASH13
    PERL_HASH_USE_SBOX32
    PERL_IMPLICIT_SYS
    PERL_MALLOC_WRAP
    PERL_OP_PARENT
    PERL_PRESERVE_IVUV
    PERL_USE_SAFE_PUTENV
    USE_64_BIT_INT
    USE_ITHREADS
    USE_LARGE_FILES
    USE_LOCALE
    USE_LOCALE_COLLATE
    USE_LOCALE_CTYPE
    USE_LOCALE_NUMERIC
    USE_LOCALE_TIME
    USE_NO_REGISTRY
    USE_PERLIO
    USE_PERL_ATOF
    USE_THREAD_SAFE_LOCALE
  Locally applied patches:
    uncommitted-changes
  Built under MSWin32
  Compiled at Dec 20 2024 10:03:46
  %ENV:
    PERL_JSON_BACKEND="Cpanel::JSON::XS"
    PERL_YAML_BACKEND="YAML::XS"
  @INC:
    C:/pb64/site/lib/MSWin32-x64-multi-thread
    C:/pb64/site/lib
    C:/pb64/lib