[LLVMdev] Boehm GC + static variables?

[James Williams]

On 31 January 2010 17:08, Jeffrey Yasskin <jyasskin at google.com> wrote:

> You should look at
>
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ExecutionEngine/JITMemoryManager.h?view=markup
> and see if inheriting from that and overriding allocateGlobal() will
> do what you want.
>
Thanks - I'll take a look at it.


> I'm a little surprised the boehm gc doesn't already see the globals,
> since there's a reference to their memory from the JMM, but maybe it
> doesn't scan mmap regions by default.
>
I believe that Boehm GC will scan data sections in the main ELF executable
and in any loaded shared objects and also scan machine registers and the
stack down to (roughly) the stack top at program entry but it won't scan
anything else unless it's explictly told to. If the JMM gets the memory that
global variables are stored in via malloc or mmap then I'll need to add it
to the GC root set via GC_add_roots.

Having said that, since I know the exact set of global variables in my
compiled program it may be more efficient for me to give the GC this exact
set rather than the entire JMM area(s), provided I can ensure that the GC
knows of the existence of each global before any reference to a garbage
collected object is stored in in.

-- James

On Sun, Jan 31, 2010 at 6:12 AM, James Williams <junk at giantblob.com> wrote:
> > I've implemented this by adding calls to GC_add_roots(<first global in
> > module>,<last global in module>+1) to the llvm.global_ctors before any
> other
> > static initialization code for the module.
> >
> > This should be safe assuming that:
> > - global variables are laid out in memory in the order they appear in
> their
> > module (and ideally contiguously without being interleaved with any other
> > values)
>
> This isn't necessarily the case. The default JMM allocates slabs with
> mmap, which doesn't guarantee that they're in order.
>
> > - llvm.global_ctors for a given module is run before any other code can
> > reference static variables belonging to that module.
>
> llvm.global_ctors runs when you call
> runStaticConstructorsDestructors(false), which lli does before running
> any other user code. There could be other global constructors that
> access the globals, though, before your constructor runs.
>
> > Can anyone confirm if I can rely on these assumptions?
> >
> > -- James
> >
> >
> > On 31 January 2010 12:53, James Williams <junk at giantblob.com> wrote:
> >>
> >> Hi,
> >>
> >> I'm running LLVM bitcode generated by my compiler under lli. The bitcode
> >> is linked against Boehm GC (lli -load=/usr/lib/libgc.so).
> >>
> >> It looks like Boehm GC isn't scanning global variables and as a result
> >> objects referenced only through globals are being prematurely collected.
> I
> >> understand that Boehm GC needs to see the data segment containing my
> global
> >> variables as a root. For native executables It's smart enough to pick
> this
> >> up from the sections in the ELF executable but this doesn't apply when
> LLVM
> >> bitcode is running under lli.
> >>
> >> Is there some way I can hook into the code generator's placement of new
> >> data segments as bitcode is compiled to native code so I can add roots
> for
> >> the segments as required by Boehm GC?
> >>
> >> Thanks in advance,
> >> -- James Williams
> >
> >
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> >
> >
>
>
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