Re[4]: [Gc] boehm port to Native Client

[Ivan Maidanski]

Hi Elijah,

I've ported libatomic_ops to NaCl.
The patch attached.

NaCl target requires -DAO_USE_NO_SIGNALS -DAO_USE_NANOSLEEP

ChangeLog entries:
2011-02-09 Ivan Maidanski <***@mail.ru>

* src/atomic_ops.c (AO_USE_NO_SIGNALS, AO_USE_NANOSLEEP): New
macros.
* src/atomic_ops.c (AO_USE_WIN32_PTHREADS): Imply
AO_USE_NO_SIGNALS.
* src/atomic_ops.c: Don't include signal.h if AO_USE_NO_SIGNALS.
* src/atomic_ops.c: Include time.h if AO_USE_NANOSLEEP.
* src/atomic_ops.c (AO_locks, AO_pause): Reformat the code.
* src/atomic_ops.c (AO_pause): Use nanosleep() if
AO_USE_NANOSLEEP.
* src/atomic_ops.c (all_sigs, initialized,
AO_compare_and_swap_emulation,
AO_compare_double_and_swap_double_emulation): Use
AO_USE_NO_SIGNALS instead of AO_USE_WIN32_PTHREADS.

Regards.

Fri, 14 Jan 2011 15:36:34 -0800 ÐÉÓØÍÏ ÏÔ Elijah Taylor <

Hi Ivan,
Thanks for taking a look, I'm pleasantly surprised by the level of

detail

- the patch in naclports repository contains a typo in a macro

definition

(MAC_TYPE -> MACH_TYPE);

Oops, will fix.

Why not to leave MACH_TYPE as-is (e.g. "I386", etc.)? NaCl is a kind of

OS not

a kind of machine hardware.
Is eg. I386 defined for x86?
Also, please add a mapping comment in gcconfig.h (around "Feel free to

add

more clauses here").

- the patch in naclports repository looks more suitable for gc v72

than

that for mono/libgc;

This patch is meant to be applied to vanilla gc6.8. The mono/libgc

port is

already patched directly into mono's code repository.

Yes, I only meant the vanilla gc6.8 patch contains some more code (eg,

for

PARALLEL_MARK) not present in mono/libgc.

- gc_pthread_redirects.h (which is a public one) should not test NACL

macro

(or, at least, while less desirable, it should be prefixed with GC_);

Ok, makes sense, I think I didn't realize this was a public header.

Though

that explains why I had to add a test for __native_client__ (which is
defined in our toolchain). I'll fix this.

- it's not clear why you need to explicitly undef STACK_GRAN,

USE_M[UN]MAP,

etc. in gcconfig.h;

I'll do some investigation, but IIRC these were needed at one point for

me.

There's a good chance these may be unnecessary and vestigial.

There should none "undef" (no other target undefining them).
If mmap is supported by NaCl then it might be possible to support
USE_M[UN]MAP.

- is MPROTECT_VDB supported or not?;

Is MPROTECT_VDB equivalent to catching protection violations in the GC

code?

If so, then no, we don't support anything like that right now.

Protection

violation in NaCl == instant death.

Ok, so MPROTECT_VDB (i.e., incremental/generation collection) is not
supported.

- if you you want to port gc72 please use the recent CVS snapshot (it

would

be easier to me to review and commit it);

I've been grabbing source from
http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/

(http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/) ... can you

point

me

to where I should be getting the latest? It's not immediately obvious

to

me.

http://bdwgc.cvs.sourceforge.net/viewvc/bdwgc/bdwgc/
(For convenience, I have a recent snapshot as a tarball which I use for

my

project -
http://www.ivmaisoft.com/_mirror/hpl/bdwgc-7_2alpha5-20110107.tar.bz2)

- not sure that HEURISTIC1 really works reliably there (in short,

HEURISTIC1

means you treat stack pointer at GC_init call as stack bottom - is it
guaranteed that GC_init call is always done at higher stack addresses

than

any other GC call);

HEURISTIC2 will definitely not work for us as it wants to use a

segfault to

detect running over the stack. I've set STACK_GRAN to 64K, so as long

as

the stack doesn't grow beyond that size before GC_init, we should be

ok,as

right? The stack for the main thread right now in NaCl lives at a

fixed

address usually, but that isn't guaranteed for all future time, so I'd
prefer not to hard code magic numbers here.

No, HEURISTIC2 won't work without signals, but there are other
- if threads-support is on then is it possible to use
USE_GET_STACKBASE_FOR_MAIN?;
- is it possible to LINUX_STACKBOTTOM (if we are on real Linux)?
- for NaCl on Cygwin, it might be possible to use

GC_get_[main_]stack_base

based on __asm__ ("%fs:4").

- is the GC port compilable (and working) on other (non-Linux)

platforms

(eg., Cygwin);

Native Client is meant to be portable, so it should run on any x86 or

x86-64

machine once it's built. In terms of building, I haven't built this gc

port

personally on Mac or Windows, but I just checked our build bot logs and

they

seem to be building ok on Mac and in Cygwin.

So, eg. DARWIN, GC_DARWIN_THREADS, WIN32, CYGWIN, GC_WIN32_THREADS won't

ever

be defined when building NaCl, right?
Is GC_LINUX_THREADS defined when building NaCl with multi-threaded

support?

I have very little knowledge of NaCl - could you briefly explain what

does

stand for NaCl portability - is it possible to call Win32 API if I'm

compiling

on Cygwin or should I use the NaCl API (and, thus, the compiled binary

code

will run on any x86 target)?
If NaCl is some kind of OS then LINUX, DARWIN, WIN32, etc shouldn't be

defined

(even if __linux__ defined) if NACL.
Same for GC_xxx_THREADS - I think GC_NACL_THREADS could be defined

instead of

GC_LINUX_THREADS, etc.
I also think that I386 and X86_64 should stay defined for respectively

the

corresponding CPU type (I guess it is already for NaCl but i haven't

checked

yet)
I think there should be 2 ifdef NACL define OS_TYPE "NACL" ... sections

(one

for every supported CPU).

- for non-static GC-internal symbols use GC_ prefix (eg. for
nacl_thread_parked);
- define SIG_SUSPEND to -1 (instead of 0) as it is returned by
GC_get_suspend_signal;
- GC functions called from NaCl it self (eg, nacl_pre_syscall_hook)

shoud

be tagged with some attribute (like public GC functions are) both for

code

readability and to prevent that symbols stripping when compiled as a

shared

lib with -DGC_DLL);

I'll address these issues. (note that NaCl currently doesn't support

shared

libs yet so your dll example won't happen, but I agree that these

should be

treated like other public GC functions)

Ok. But what is eg. nacl_pre_syscall_hook() - a callback from the NaCl
subsystem? (I guess this should be treated as GC public API)
Of course, use STATIC or static where possible (all STATIC symbols start

with

GC_, while static typically not).
More tips: use GC_INNER and GC_EXTERN for internal global variables; use
GC_INNER for internal functions.

- libatomic_ops does not use signals API (except for CAS emulation

which

is

not used for x86/x64).

I think I saw sigprocmask and related functions and assumed the worst,

but I

see now that's windows code. Looking at the x86 variants it looks like

a

NaCl port of libatomic_ops is probably not going to be too bad. I'll

look

into this eventually.

Most probably, it work w/o any porting afforts but it would be good to

port

atomic_ops.c (similar to what I did for Win32-pthreads targets - see
AO_USE_WIN32_PTHREADS, I guess you should add AO_USE_NACL macro testing

in

that file (looks easy to add). I think it's worth doing first (and submit

me a

separate patch for libatomics_op when done).
What's about GC_HAVE_BUILTIN_BACKTRACE and GC_CAN_SAVE_CALL_STACKS? At

least,

gc.h should be consistent with the GC implementation (I mean eg. if
GC_HAVE_BUILTIN_BACKTRACE not supported then it shouldn't be defined in

gc.h

regardless of __linux__, _MSC_VER, etc. provided __native_client__).

Same for

GC_ADD_CALLER, GC_RETURN_ADDR.
Regards.

PS. Let me not do the benefits analysis (probably someone else can do
this).

Well, if the gc7.2 port is as easy as it's looking now, I think it's
probably worth doing it. I would still love to hear anyone chime in on

the

benefits of gc7.2 vs 6.8 though

Regards.
Thu, 13 Jan 2011 10:21:03 -0800 Elijah Taylor <

Hi GC folks,
I saw a little chatter in the archives related to porting libgc to

Native

Client, so I joined this list to share some details. I'm the engineer

at

Google who ported of libgc to Native Client for Mono. I've also

included a

http://code.google.com/p/naclports/ (

http://code.google.com/p/naclports/)

. This version will be available to

users that want to use libgc as part of their Native Client projects.

Before porting gc6.8 I had attempted to port one of the newer

versions,

gc7.2alpha4, but ran into snags. The largest snag right now I think

is

that

gc 7+ includes libatomic_ops which will require some non-trivial

effort in

order to work under Native Client. Most notably we don't support

signals;

that was the biggest effort in porting libgc in the first place for

NaCl,

and I assume that will require the most work in porting libatomic_ops

too.

Can someone give me the high level details of what kind of things

we

might be missing if we only support gc6.8 instead of the latest

version?

Because of our thread stopping implementation, we may not even

benefit

from

some of the newer features. I just wanted to get a sense of what the
benefits are of getting a newer version available for users.

-Elijah