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From "Rana Dasgupta" <rdasg...@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 16:32:40 GMT
The combined patch looks good and enriches the JIT/VM interface(
thanks Mikhail ).

Calling convention may be a seperate discussion, imho.

Xiaofeng, Could you please commit this when you have some time?

On 4/13/07, Mikhail Fursov <mike.fursov@gmail.com> wrote:
> 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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