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From "Mikhail Fursov" <mike.fur...@gmail.com>
Subject Re: [drlvm][jit] New method in VM-JIT interface: check if vm helper is GC suspension point
Date Fri, 13 Apr 2007 08:14:21 GMT
I've attached combined JIT+VM patch to JIRA. The test failed before passes
now.
VM-guru, please check VMEXPORT Boolean
vm_helper_is_gc_interruptible(VM_RT_SUPPORT f) method in this patch.

On 4/13/07, Alexey Varlamov <alexey.v.varlamov@gmail.com> wrote:
>
> 2007/4/13, Mikhail Fursov <mike.fursov@gmail.com>:
> > Rana,
> > thank you for the fast update.
> > I'll integrate this function usage into JIT and report if the problem I
> have
> > disappeares today.
> >
> > >>BTW, how does the jit knows which helpers throw exceptions?
> > JIT generates stack info for every call instruction. So we can
> > unwind stack frame from every call.
> > Actually this is not enough: JIT must generate stackinfo for every push
> inst
> > also to handle SOE correctly. But this is one of the TODO list items.
> >
> >
> > Another problem with helper calls I know is their calling conventions.
> > Now the calling convention of every helper is hardcoded to JIT.
> > I think it worth creating another JIRA to solve the problem one day.
> Today I
> > know no bugs about it, so it's design only issue.
>
> There is one already: HARMONY-2702. Actually Jitrino provides such
> interface internally,
> CompilationInterface::getRuntimeHelperCallingConvention(), so fix
> should be easy enough (yet is blocked by HARMONY-3553).
>
> >
> >
> > On 4/13/07, Rana Dasgupta <rdasgupt@gmail.com> wrote:
> > >
> > > Yes, that make sense. I'll wait a bit to see if Mikhail has any more
> > > suggestions and change it to is_gc_interruptible.
> > >
> > > On 4/12/07, Xiao-Feng Li <xiaofeng.li@gmail.com> wrote:
> > > > Agree that this information better be provided by VM than JIT
> internals.
> > > >
> > > > Rana, Thanks for the quick work. A minor comment:
> > > >
> > > > The interface name might be better use "has_gc_safepoint" or
> > > > "is_gc_interruptible" than "is_suspension_point", since the helper
> is
> > > > a function (not a point), and "suspension" is implementation detail.
> > > >
> > > > Thanks,
> > > > xiaofeng
> > > >
> > > > On 4/13/07, Rana Dasgupta <rdasgupt@gmail.com> wrote:
> > > > > Mikhail,
> > > > >    I extended the vm-jit interface with a function:
> > > > > boolean vm_is_helper_suspension_point( VM_RT_SUPPORT f)
> > > > >
> > > > > and attached it to your JIRA.
> > > > >
> > > > >   Right now it returns true for almost every helper. But you may
> want
> > > > > to update this instead of hardcoding in the jit. At some point, we
> > > > > need some analysis to determine which helpers are interruptible.
> > > > >
> > > > >   BTW, how does the jit knows which helpers throw exceptions?
> > > > >
> > > > > Rana
> > > > >
> > > > >
> > > > >
> > > > > On 4/11/07, Mikhail Fursov <mike.fursov@gmail.com> wrote:
> > > > > > On 4/12/07, Rana Dasgupta <rdasgupt@gmail.com> wrote:
> > > > > > >
> > > > > > > On 4/11/07, Xiao-Feng Li <xiaofeng.li@gmail.com>
wrote:
> > > > > > > > On 4/12/07, Rana Dasgupta <rdasgupt@gmail.com>
wrote:
> > > > > > > > > A silly question possibly. But other than inlined
> fastpaths,
> > > why not
> > > > > > > > > gcm treat all helper calls as interruptible?
> > > > > > > >
> > > > > > > > I think this needs case by case study to decide which
ones
> can
> > > be
> > > > > > > > interruptible. In most cases, the helper is only an
> extension of
> > > a
> > > > > > > > certain bytecode operation, which can be virtually
treated
> as a
> > > > > > > > bytecode (or part of a bytecode semantics). Normally,
the
> > > safepoint
> > > > > > > > can be before or after it, but not in middle of it.
If we
> think
> > > of the
> > > > > > > > stack map info, the helper itself may not stand for
a formal
> > > method
> > > > > > > > frame for GC or exception handling.
> > > > > > >
> > > > > > > This is a good way to look at helper calls. My point,
> > > however,  was
> > > > > > > not really that helper calls should be interruptible. But
that
> > > other
> > > > > > > than the fastpath for which the JIT sees the IR, why not
treat
> the
> > > > > > > calls themselves as ineligible for code motion etc. like
other
> > > > > > > operators with side effects? Is the cost too high?
> > > > > > > Implementing the interface extension is not hard for the
VM,
> but
> > > looks
> > > > > > > like implementation dependant info passed from the VM to
the
> JIT.
> > > > > > >
> > > > > > > Here is more details:
> > > > > > The example of a helper that can be moved by GCM are number
> > > conversion helpers.
> > > > > > 'i2d' never throws exceptions and this knowledge is hardcoded
in
> > > HLO. On the
> > > > > > other hand this HIR opcode is translated into LIR vm-helper
> call, so
> > > instead
> > > > > > of simple Java opcode it becomes a
> > > > > > call. I can hardcode which vm-helper call is interruptable and
> which
> > > is
> > > > > > not on the
> > > > > > JIT side, but think retrieving this information from VM is a
> better
> > > solution.
> > > > > > Another example of uninterruptable vm-helper is
> > > > > > TLS accessor. This helper is placed into the middle of the
> inlined
> > > > > > 'alloc' helper body because 'alloc' reqires TLS access.
> > > > > > Now  managed<->unmanaged convertion checker in GCMap pass
has
> > > hardcoded
> > > > > > knowledge that TLS access will never be suspended.
> > > > > >
> > > > > > JET compiler uses different code then OPT and maintains special
> > > > > > GC-bitmasks for object operands runtime.
> > > > > > The per-call gc-mask maintainance is not free and can also be
> > > > > > optimized if VM provides a method to describe which call is
real
> > > > > > suspension point and which is not.
> > > > > >
> > > > > >
> > > > > >
> > > > > > + Suspension and exception throwing are different attributes.
> SOE
> > > can be
> > > > > > thrown even if method is not a suspension point. So JIT must
> prepare
> > > stack
> > > > > > info for every call it generates to be able to unwind  the
> frame.
> > > > > >
> > > > > > --
> > > > > > Mikhail Fursov
> > > > > >
> > > > >
> > > >
> > > >
> > > > --
> > > > http://xiao-feng.blogspot.com
> > > >
> > >
> >
> >
> >
> > --
> > Mikhail Fursov
> >
>



-- 
Mikhail Fursov

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