Copy_on_Write COW isn't working when the constant is the result of compile-time constant-folding

[LorenzoTa]

This is a bug report for perl from LorenzoTa,
generated with the help of perlbug 1.40 running under perl 5.26.0.

---

Hello esteemed ones,

it seems (in every version of perl and on every OS) that COW does not work as expected when the constant is the result of compile-time constant-folding.

In short:

my $x = 'a' x (2**30);

Allocates 2.104.612 K of OS memory while Devel::Size::total_size($x) reports 1.00 Gb

We spotted this wrong behaviour at perlmonks in the thread:

www.perlmonks.org/?node_id=11147727

where you can find many more details and test run on different envs, with the very same (unexpected) result, alongside comments of more experienced programmers: notably this one

More details:

# WRONG behaviour as it doubles the memory
my $x = 'a' x (2**30);

# RIGHT behaviour
foreach my $order ( qw(20 24 30 32) ) { my $x = 'a' x ( 2 ** $order ) }

# also RIGHT forcing RHS to runtime
$x = 'a' x (2**${\32})

Thanks for your work!

lorenzo

[Please do not change anything below this line]

---

Flags:
category=core
severity=medium

Site configuration information for perl 5.26.0:

Configured by strawberry-perl at Sat Sep 2 16:28:54 2017.

Summary of my perl5 (revision 5 version 26 subversion 0) configuration:

Platform:
osname=MSWin32
osvers=6.3
archname=MSWin32-x64-multi-thread
uname='Win32 strawberry-perl 5.26.0.2 #1 Sat Sep 2 16:25:32 2017 x64'
config_args='undef'
hint=recommended
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bincompat5005=undef
Compiler:
cc='gcc'
ccflags =' -s -O2 -DWIN32 -DWIN64 -DCONSERVATIVE -D__USE_MINGW_ANSI_STDIO -DPERL_TEXTMODE_SCRIPTS -DPERL_IMPLICIT_CONTEXT -DPERL_IMPLICIT_SYS -DUSE_PERLIO -fwrapv -fno-strict-aliasing -mms-bitfields'
optimize='-s -O2'
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ccversion=''
gccversion='7.1.0'
gccosandvers=''
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Linker and Libraries:
ld='g++.exe'
ldflags ='-s -L"C:\EX_D\ulisseDUE\perl5.26.64bit\perl\lib\CORE" -L"C:\EX_D\ulisseDUE\perl5.26.64bit\c\lib"'
libpth=C:\EX_D\ulisseDUE\perl5.26.64bit\c\lib C:\EX_D\ulisseDUE\perl5.26.64bit\c\x86_64-w64-mingw32\lib C:\EX_D\ulisseDUE\perl5.26.64bit\c\lib\gcc\x86_64-w64-mingw32\7.1.0
libs= -lmoldname -lkernel32 -luser32 -lgdi32 -lwinspool -lcomdlg32 -ladvapi32 -lshell32 -lole32 -loleaut32 -lnetapi32 -luuid -lws2_32 -lmpr -lwinmm -lversion -lodbc32 -lodbccp32 -lcomctl32
perllibs= -lmoldname -lkernel32 -luser32 -lgdi32 -lwinspool -lcomdlg32 -ladvapi32 -lshell32 -lole32 -loleaut32 -lnetapi32 -luuid -lws2_32 -lmpr -lwinmm -lversion -lodbc32 -lodbccp32 -lcomctl32
libc=
so=dll
useshrplib=true
libperl=libperl526.a
gnulibc_version=''
Dynamic Linking:
dlsrc=dl_win32.xs
dlext=xs.dll
d_dlsymun=undef
ccdlflags=' '
cccdlflags=' '
lddlflags='-mdll -s -L"C:\EX_D\ulisseDUE\perl5.26.64bit\perl\lib\CORE" -L"C:\EX_D\ulisseDUE\perl5.26.64bit\c\lib"'

---

@inc for perl 5.26.0:
C:/EX_D/ulisseDUE/perl5.26.64bit/perl/site/lib/MSWin32-x64-multi-thread
C:/EX_D/ulisseDUE/perl5.26.64bit/perl/site/lib
C:/EX_D/ulisseDUE/perl5.26.64bit/perl/vendor/lib
C:/EX_D/ulisseDUE/perl5.26.64bit/perl/lib

---

Environment for perl 5.26.0:
HOME (unset)
LANG (unset)
LANGUAGE (unset)
LD_LIBRARY_PATH (unset)
LOGDIR (unset)
PATH=C:\EX_D\ulisseDUE\perl5.26.64bit\perl\site\bin;C:\EX_D\ulisseDUE\perl5.26.64bit\perl\bin;C:\EX_D\ulisseDUE\perl5.26.64bit\c\bin;C:\EX_D\ulisseDUE\bin\UnxUtils\usr\local\wbin;C:\WINDOWS;C:\WINDOWS\system32;
PERL_BADLANG (unset)
PERL_JSON_BACKEND=JSON::XS
PERL_RL=Perl
PERL_YAML_BACKEND=YAML
SHELL (unset)

[demerphq]

This is a result of the constant optimization kicking in. The expression is evaluated at compile time and stored in the optree, and the result is then copied into the SV. It is not so much a bug as malbehavior of the optimization as we do not make any specific commitments about how much memory a given string will take. (I am being legalistic here, I agree this is undesirable.)

If COW is involved, it is that this is not a COW style copy. But i believe we do not store SV's in the optree, so we cannot use COW.

[richardleach]

But i believe we do not store SV's in the optree, so we cannot use COW.

CONST ops do store pointers to SVs and so, at least on the face of it, COW might be possible. No COW flag on this one:

$ perl -MDevel::Peek -e'Dump("a"x10)'
SV = PV(0x562f4beb40f0) at 0x562f4bee0528
  REFCNT = 1
  FLAGS = (PADTMP,POK,READONLY,PROTECT,pPOK)
  PV = 0x562f4bee93b0 "aaaaaaaaaa"\0
  CUR = 10
  LEN = 16

[richardleach]

Can get:

 ./perl -Ilib -MDevel::Peek -e 'Dump("a"x10)'
SV = PV(0x55f64b2aa070) at 0x55f64b2cd0e0
  REFCNT = 1
  FLAGS = (PADTMP,POK,IsCOW,READONLY,PROTECT,pPOK)
  PV = 0x55f64b2f61c0 "aaaaaaaaaa"\0
  CUR = 10
  LEN = 16
  COW_REFCNT = 0

by messing with S_fold_constants, but I dunno if this is appropriate or not:

diff --git a/op.c b/op.c
index 8851dce046..8706d1349f 100644
--- a/op.c
+++ b/op.c
@@ -4976,6 +4976,8 @@ S_fold_constants(pTHX_ OP *const o)
     is_stringify = type == OP_STRINGIFY && !o->op_folded;
     op_free(o);
     assert(sv);
+    if (SvPOK(sv))
+        SvIsCOW_on(sv);
     if (is_stringify)
         SvPADTMP_off(sv);
     else if (!SvIMMORTAL(sv)) {

Running tests to see if anything fails....

[richardleach]

With blead:

$ /usr/bin/time -v ./perl -e 'my $x = "a"x (2**32)' 2>&1 | grep "Maximum resident"
	Maximum resident set size (kbytes): 7696288

With the above patch:

$ /usr/bin/time -v ./perl -e 'my $x = "a"x (2**32)' 2>&1 | grep "Maximum resident"
	Maximum resident set size (kbytes): 4199680

Two failing test files, but a quick look suggests it might just because the Dump() output doesn't match what is currently hardcoded:

• ../ext/Devel-Peek/t/Peek.t
• op/undef.t

But as mentioned, I don't have the experience to know whether this is a suitable patch or not. If longer standing committers think this is a decent approach, I'm happy to make a proper PR out of it.

[demerphq]

I think its worthy to investigate more. But, I am not clear that that is the correct approach. I think the IsCOW flag should only happen when the data is copied from the const op, not when it is created. (Maybe i read the output wrong tho.)

BTW, I am confused, i thought we had a hard rule that SV's cant be stored in the optree as it means we cant share optrees between threads. Eg, an optree can be compiled in one thread, thus allocating the SV's out of that threads pool, and then destroyed in another thread, leading to "SV freed to wrong pool" errors. Hence me thinking we had no SV's in the optree. Obviously the situation is a bit more complex than I understood.

[demerphq]

Also, note that even using COW, one could only create up to 254 copies of this string before it was duplicated in memory. (IMO that is something we should fix.)

[iabyn]

To reply to various points in this ticket so far. (I first diagnosed the issue on perlmonks). The issue is that, while string constants are usually marked as COW-able, string constants created as a result of constant folding aren't: $ p -MDevel::Peek -e'my $x = "aaaaaaaaaa"; Dump $x' 2>&1 | grep COW FLAGS = (POK,IsCOW,pPOK) COW_REFCNT = 1 $ p -MDevel::Peek -e'my $x = "aaaaa" . "aaaaa"; Dump $x' 2>&1 | grep COW $ p -MDevel::Peek -e'my $x = "a" x 10; Dump $x' 2>&1 | grep COW $ Normal string constants are marked as COW-able by Perl_ck_svconst(). Either that function needs calling against a newly-created const op which is the result of constant folding, or at least the actions in that function need carrying out. On threaded builds, the pointer from an OP_CONST op to the constant SV is stored in the pad (similarly to GV pointers for OP_GVSV etc), and there's a different pad for each thread. As for the 255 COW copy limit, that doesn't bother me much. Most of the time the COW count is going to be a small number, often just 1, as a string gets "passed along" between various variables, like in my ($string, ...) = @_; On the rare occasions where the 254 limit is exceeded, then the string gets copied each time, which is no worse than when we didn't have COW at all. Finally, a comment about constant folding and the 'x' operator. At the moment we just blindly fold, regardless of the (constant) value of the RHS. So all these get constant-folded: if (rare_condition_1) { $x = 'a' x (1<<30); } elsif (rare_condition_2) { $x = 'b' x (1<<30); } elsif ... so even though they may never be executed, multiple 1Gb constant strings have been created at compile time. Similarly, @A = (1) x (1<<30) creates (at compile time) an AV with a billion elements. Perhaps we should only constant fold 'x' when the RHS is below a certain threshold?

…

-- The Enterprise successfully ferries an alien VIP from one place to another without serious incident. -- Things That Never Happen in "Star Trek" #7

[richardleach]

Normal string constants are marked as COW-able by Perl_ck_svconst(). Either that function needs calling against a newly-created const op which is the result of constant folding, or at least the actions in that function need carrying out.

Perl_ck_svconst does fire when S_fold_constants creates the new const:

    newop = newSVOP(OP_CONST, 0, MUTABLE_SV(sv));

However, by that time, sv already has the READONLY flag set:
FLAGS = (PADTMP,POK,READONLY,PROTECT,pPOK)
so the code branch which would make it COW-able does not get followed: https://github.com/Perl/perl5/blob/blead/op.c#L14425

So possibly we need this instead? (Perl_ck_svconst will add the READONLY flag.)

diff --git a/op.c b/op.c
index 8851dce046..a3ad0c823f 100644
--- a/op.c
+++ b/op.c
@@ -4980,7 +4980,9 @@ S_fold_constants(pTHX_ OP *const o)
         SvPADTMP_off(sv);
     else if (!SvIMMORTAL(sv)) {
         SvPADTMP_on(sv);
-        SvREADONLY_on(sv);
+        /* Used to SvREADONLY_on(sv); here, but this will be done
+           by newSVOP anyway and doing it now stops Perl_ck_svconst
+           from marking the SV as COW-able. */
     }
     newop = newSVOP(OP_CONST, 0, MUTABLE_SV(sv));
     if (!is_stringify) newop->op_folded = 1;

[richardleach]

So possibly we need this instead?

The above seemed pretty reasonable to me, so I've turned it into a PR for review: #20595

[demerphq]

@richardleach I agree that PR should be standalone, but will you follow up with one for "x" with large multipliers? I think that should happen, but i have a feeling you are better placed to do it than me.

Thanks for taking this on. Its been a nice learning experience.

[richardleach]

I agree that PR should be standalone, but will you follow up with one for "x" with large multipliers? I think that should happen, but i have a feeling you are better placed to do it than me.

I don't know how to do this off the top of my head but am happy to poke. We'd have to pick an arbitrary large multiplier, so would we want it to be controllable through e.g. an env var? (If so, I haven't looked at how to do that before either.)

Its been a nice learning experience.

For me too. :)

[richardleach]

one for "x" with large multipliers

I don't know how to do this off the top of my head

Building on case OP_REPEAT: in S_fold_constants() looks like a likely possibility.