llvm dev - Dec 2014 - [LLVMdev] Future plans for GC in LLVM

Now that the statepoint changes have landed, I wanted to start a 
discussion about what's next for GC support in LLVM.  I'm going to 
sketch out a strawman proposal, but I'm not set on any of this.  I 
mostly just want to draw interested parties out of the woodwork.  :)

Overall Direction:
In the short term, my intent is to preserve the functionality of the 
existing code, but migrate towards a position where the gcroot specific 
pieces are optional and well separated.  I also plan to start updating 
the documentation to reflect a separation between the general support 
for garbage collection (function attributes, identifying references, 
load and store barrier lowering, generating stack maps) and the 
implementation choices (gcroot & it's lowering vs statepoints & addr
spaces for identifying references).

Longer term, I plan to *EVENTUALLY DELETE* the existing gcroot lowering 
code and in tree GCStrategies unless an interesting party speaks up.  I 
have no problem with retaining some of the existing pieces for legacy 
support or helping users to migrate, but as of right now, I don't know 
of any such active users.  The only exception to this might be the 
shadow stack GC.  Eventually in this context is at least six months from 
now, but likely less than 18 months.  Hopefully, that's vague enough.  :)

HELP - If anyone knows which Ocaml implementation and which Erlang 
implementation triggered the in tree GC strategies, please let me know!


Near Term Changes:
- Migrate ownership of GCStrategy objects from GCModuleInfo to 
LLVMContext.  In theory, this looses the ability for two different 
Modules to have the same collector with different state, but I know of 
no use case for this.
- Modify the primary Function::getGC/setGC interface to return a 
reference the GCStrategy object, not a string.  I will provide a 
Function::setGCString and getGCString.
- Extend the GCStrategy class to include a notion of which compilation 
strategy is being used.  The two choices right now will be Legacy and 
Statepoint.  (Longer term, this will likely become a more fine grained 
choice.)
- Separate GCStategy and related pieces from the 
GCFunctionInfo/GCModuleInfo/GCMetadataPrinter lowering code.  At first, 
this will simply mean clarifying documentation and rearranging code a bit.
- Document/clarify the callbacks used to customize the lowering. Decide 
which of these make sense to preserve and document.

(Lest anyone get the wrong idea, the above changes are intended to be 
minor cleanup.  I'm not looking to do anything controversial yet.)

Questions:
- Is proving the ability to generate a custom binary stack map format a 
valuable feature?  Adapting the new statepoint infrastructure to work 
with the existing GCMetadataPrinter classes wouldn't be particularly hard.
- Are there any GCs out there that need gcroot's single stack slot per 
value implementation?   By default, statepoints may generate a different 
stackmap for every safepoint in a function.
- Is using gcroot and allocas to mark pointers as garbage collected 
references valuable?  (As opposed to using address spaces on the SSA 
values themselves?)  Long term, should we retain the gcroot marker 
intrinsics at all?


Philip

Appendix: The Current Implementations Key Classes:

GCStrategy - Provides a configurable description of the collector. The 
strategy can also override parts of the default GC root lowering 
strategy.  The concept of such a collector description is very valuable, 
but the current implementation could use some cleanup.  In particular, 
the custom lowering hooks are a bit of a mess.

GCMetadataPrinter - Provides a means to dump a custom binary format 
describing each functions safepoints.  All safepoints in a function must 
share a single root Value to stack slot mapping.

GCModuleInfo/GCFunctionInfo - These contain the metadata which is saved 
to enable GCMetadataPrinter.

Excellent direction. My input, although I'm verrrrry far removed from the
project at this point:

• I vehemently support replacement of the gcroot intrinsic. It makes the system
make unnecessarily conservative decisions by default, which is incompatible with
producing a high quality compiler for garbage collected languages. I  just never
got ambitious enough to replace it.
• The Ocaml-specific code can be jettisoned. Ocaml was hostile to contributors
outside of France and INRIA in specific, so no published callers use it that
I'm aware of. (The ocaml runtime also requires an alternative calling
convention, so it's highly unlikely that there's a stray caller!)
• Shadow-stack is probably useful for bootstrapping up new runtimes/new
languages because it's crazy simple to interoperate with. It's not
threadsafe or anything tho, so don't let it stand in the way of a high
quality implementation.
• Generating a different stack map at every safepoint is highly desirable.
Anything that depends on the contrary deserves to be broken!
• It was my experience that porting a compiler was simplified by not
concurrently porting the runtime. So the ability to customize serialization of
the stack map was valuable, particularly if it is inexpensive to preserve that
behavior. (If LLVM has gotten into the business of providing collector runtimes
and memory allocators and …, then dump it!)
• Interior pointers pose a particular challenge once the gcroot intrinsic is
removed. I was totally punting on that problem domain.

Psyched to see movement here. :)

- Gordon

P.S. Please keep tagged pointers in mind somehow. *coughcoughlispmachinecough*
> On Dec 4, 2014, at 20:50, Philip Reames <listmail at
philipreames.com> wrote:
> 
> Now that the statepoint changes have landed, I wanted to start a discussion
about what's next for GC support in LLVM.  I'm going to sketch out a
strawman proposal, but I'm not set on any of this.  I mostly just want to
draw interested parties out of the woodwork.  :)
> 
> Overall Direction:
> In the short term, my intent is to preserve the functionality of the
existing code, but migrate towards a position where the gcroot specific pieces
are optional and well separated.  I also plan to start updating the
documentation to reflect a separation between the general support for garbage
collection (function attributes, identifying references, load and store barrier
lowering, generating stack maps) and the implementation choices (gcroot &
it's lowering vs statepoints & addr spaces for identifying references).
> 
> Longer term, I plan to *EVENTUALLY DELETE* the existing gcroot lowering
code and in tree GCStrategies unless an interesting party speaks up.  I have no
problem with retaining some of the existing pieces for legacy support or helping
users to migrate, but as of right now, I don't know of any such active
users.  The only exception to this might be the shadow stack GC.  Eventually in
this context is at least six months from now, but likely less than 18 months. 
Hopefully, that's vague enough.  :)
> 
> HELP - If anyone knows which Ocaml implementation and which Erlang
implementation triggered the in tree GC strategies, please let me know!
> 
> 
> Near Term Changes:
> - Migrate ownership of GCStrategy objects from GCModuleInfo to LLVMContext.
In theory, this looses the ability for two different Modules to have the same
collector with different state, but I know of no use case for this.
> - Modify the primary Function::getGC/setGC interface to return a reference
the GCStrategy object, not a string.  I will provide a Function::setGCString and
getGCString.
> - Extend the GCStrategy class to include a notion of which compilation
strategy is being used.  The two choices right now will be Legacy and
Statepoint.  (Longer term, this will likely become a more fine grained choice.)
> - Separate GCStategy and related pieces from the
GCFunctionInfo/GCModuleInfo/GCMetadataPrinter lowering code.  At first, this
will simply mean clarifying documentation and rearranging code a bit.
> - Document/clarify the callbacks used to customize the lowering. Decide
which of these make sense to preserve and document.
> 
> (Lest anyone get the wrong idea, the above changes are intended to be minor
cleanup.  I'm not looking to do anything controversial yet.)
> 
> Questions:
> - Is proving the ability to generate a custom binary stack map format a
valuable feature?  Adapting the new statepoint infrastructure to work with the
existing GCMetadataPrinter classes wouldn't be particularly hard.
> - Are there any GCs out there that need gcroot's single stack slot per
value implementation?   By default, statepoints may generate a different
stackmap for every safepoint in a function.
> - Is using gcroot and allocas to mark pointers as garbage collected
references valuable?  (As opposed to using address spaces on the SSA values
themselves?)  Long term, should we retain the gcroot marker intrinsics at all?
> 
> 
> Philip
> 
> Appendix: The Current Implementations Key Classes:
> 
> GCStrategy - Provides a configurable description of the collector. The
strategy can also override parts of the default GC root lowering strategy.  The
concept of such a collector description is very valuable, but the current
implementation could use some cleanup.  In particular, the custom lowering hooks
are a bit of a mess.
> 
> GCMetadataPrinter - Provides a means to dump a custom binary format
describing each functions safepoints.  All safepoints in a function must share a
single root Value to stack slot mapping.
> 
> GCModuleInfo/GCFunctionInfo - These contain the metadata which is saved to
enable GCMetadataPrinter.
>

On 12/04/2014 07:38 PM, Gordon Henriksen wrote:
> Excellent direction. My input, although I'm verrrrry far removed from
the project at this point:
>
> • I vehemently support replacement of the gcroot intrinsic. It makes the
system make unnecessarily conservative decisions by default, which is
incompatible with producing a high quality compiler for garbage collected
languages. I  just never got ambitious enough to replace it.
> • The Ocaml-specific code can be jettisoned. Ocaml was hostile to
contributors outside of France and INRIA in specific, so no published callers
use it that I'm aware of. (The ocaml runtime also requires an alternative
calling convention, so it's highly unlikely that there's a stray
caller!)
Great!  Glad to hear.  I suspected this was dead, but I wasn't
sure.
> • Shadow-stack is probably useful for bootstrapping up new runtimes/new
languages because it's crazy simple to interoperate with. It's not
threadsafe or anything tho, so don't let it stand in the way of a high
quality implementation.
So far, I see no reason not to keep it.  It's fairly separate from the 
problematic bits of gcroot and should be easy-ish to keep
working.
> • Generating a different stack map at every safepoint is highly desirable.
Anything that depends on the contrary deserves to be broken!
> • It was my experience that porting a compiler was simplified by not
concurrently porting the runtime. So the ability to customize serialization of
the stack map was valuable, particularly if it is inexpensive to preserve that
behavior. (If LLVM has gotten into the business of providing collector runtimes
and memory allocators and …, then dump it!)
We haven't.  But we might be in the business for shipping a parser for 
the stack map format.  Would that solve the problem?
> • Interior pointers pose a particular challenge once the gcroot intrinsic
is removed. I was totally punting on that problem domain.
Interior pointers (what I've been calling derived pointers) have first 
class support with gc.statepoint.  I have no plans to try to make them 
work with gcroot.
>
> Psyched to see movement here. :)
Thanks for the feedback.
>
> - Gordon
>
> P.S. Please keep tagged pointers in mind somehow.
*coughcoughlispmachinecough*
So far, I haven't deliberately designed them in (or out.)  Now that 
gc.statepoints has landed, I'm open to ideas and proposals if you want 
to extend them.  I've given them some thought before, but I have no 
current plans to make a proposal in this space.
>
>> On Dec 4, 2014, at 20:50, Philip Reames <listmail at
philipreames.com> wrote:
>>
>> Now that the statepoint changes have landed, I wanted to start a
discussion about what's next for GC support in LLVM.  I'm going to
sketch out a strawman proposal, but I'm not set on any of this.  I mostly
just want to draw interested parties out of the woodwork.  :)
>>
>> Overall Direction:
>> In the short term, my intent is to preserve the functionality of the
existing code, but migrate towards a position where the gcroot specific pieces
are optional and well separated.  I also plan to start updating the
documentation to reflect a separation between the general support for garbage
collection (function attributes, identifying references, load and store barrier
lowering, generating stack maps) and the implementation choices (gcroot &
it's lowering vs statepoints & addr spaces for identifying references).
>>
>> Longer term, I plan to *EVENTUALLY DELETE* the existing gcroot lowering
code and in tree GCStrategies unless an interesting party speaks up.  I have no
problem with retaining some of the existing pieces for legacy support or helping
users to migrate, but as of right now, I don't know of any such active
users.  The only exception to this might be the shadow stack GC.  Eventually in
this context is at least six months from now, but likely less than 18 months. 
Hopefully, that's vague enough.  :)
>>
>> HELP - If anyone knows which Ocaml implementation and which Erlang
implementation triggered the in tree GC strategies, please let me know!
>>
>>
>> Near Term Changes:
>> - Migrate ownership of GCStrategy objects from GCModuleInfo to
LLVMContext.  In theory, this looses the ability for two different Modules to
have the same collector with different state, but I know of no use case for
this.
>> - Modify the primary Function::getGC/setGC interface to return a
reference the GCStrategy object, not a string.  I will provide a
Function::setGCString and getGCString.
>> - Extend the GCStrategy class to include a notion of which compilation
strategy is being used.  The two choices right now will be Legacy and
Statepoint.  (Longer term, this will likely become a more fine grained choice.)
>> - Separate GCStategy and related pieces from the
GCFunctionInfo/GCModuleInfo/GCMetadataPrinter lowering code.  At first, this
will simply mean clarifying documentation and rearranging code a bit.
>> - Document/clarify the callbacks used to customize the lowering. Decide
which of these make sense to preserve and document.
>>
>> (Lest anyone get the wrong idea, the above changes are intended to be
minor cleanup.  I'm not looking to do anything controversial yet.)
>>
>> Questions:
>> - Is proving the ability to generate a custom binary stack map format a
valuable feature?  Adapting the new statepoint infrastructure to work with the
existing GCMetadataPrinter classes wouldn't be particularly hard.
>> - Are there any GCs out there that need gcroot's single stack slot
per value implementation?   By default, statepoints may generate a different
stackmap for every safepoint in a function.
>> - Is using gcroot and allocas to mark pointers as garbage collected
references valuable?  (As opposed to using address spaces on the SSA values
themselves?)  Long term, should we retain the gcroot marker intrinsics at all?
>>
>>
>> Philip
>>
>> Appendix: The Current Implementations Key Classes:
>>
>> GCStrategy - Provides a configurable description of the collector. The
strategy can also override parts of the default GC root lowering strategy.  The
concept of such a collector description is very valuable, but the current
implementation could use some cleanup.  In particular, the custom lowering hooks
are a bit of a mess.
>>
>> GCMetadataPrinter - Provides a means to dump a custom binary format
describing each functions safepoints.  All safepoints in a function must share a
single root Value to stack slot mapping.
>>
>> GCModuleInfo/GCFunctionInfo - These contain the metadata which is saved
to enable GCMetadataPrinter.
>>

Hi,

This looks great.  I'm excited to see this come in and the direction looks
right to me.  I've started to look at how to use LLVM to target the MS CLR
GC (now being open sourced) and the statepoints are much closer to the current
implementation so that could make my life easier.  On your questions, add my
vote to the ability to implement a custom binary format for safepoints since it
means I can just target the CLRs format.    I've been trying to keep up with
your progress but since I'm coming out of lurk mode I have a few questions:

- From the documentation it looks like you're using the patchpoint stackmap
format (http://llvm.org/docs/StackMaps.html#stackmap-format).  In that format
you can describe register locations - but from the overview
(http://llvm.org/docs/Statepoints.html#overview) it implies that all gc pointers
are spilled to the stack.  Is the spilling to memory required?  Or is the plan
to allow gc pointers to reside in register as well.  (I'm hoping that a
store/load at safepoinsts won't be required and that they can stack register
resident)
- I'm still fuzzy how code motion is blocked from moving SSA uses past the
safepoint once they've been inserted?   I'm likely just missing some
invariant in LLVM or the design since I can't seem to noodle it out from
what I've seen.
- In the CLR GC we don't require the base object pointer to be kept alive
for a derived managed pointer (interior pointer) but in your design there is 
the requirement to maintain a base, derived pairing.  (If I remember right this
is a Java requirement) Is this a hard requirement?  Or is there the potential
for other collectors to deal just with managed pointers

Thanks,

-R

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Philip Reames
Sent: Thursday, December 4, 2014 5:50 PM
To: LLVM Developers Mailing List
Cc: rayiner at gmail.com; gordonhenriksen at mac.com
Subject: [LLVMdev] Future plans for GC in LLVM

Now that the statepoint changes have landed, I wanted to start a discussion
about what's next for GC support in LLVM.  I'm going to sketch out a
strawman proposal, but I'm not set on any of this.  I mostly just want to
draw interested parties out of the woodwork.  :)

Overall Direction:
In the short term, my intent is to preserve the functionality of the existing
code, but migrate towards a position where the gcroot specific pieces are
optional and well separated.  I also plan to start updating the documentation to
reflect a separation between the general support for garbage collection
(function attributes, identifying references, load and store barrier lowering,
generating stack maps) and the implementation choices (gcroot & it's
lowering vs statepoints & addr spaces for identifying references).

Longer term, I plan to *EVENTUALLY DELETE* the existing gcroot lowering code and
in tree GCStrategies unless an interesting party speaks up.  I have no problem
with retaining some of the existing pieces for legacy support or helping users
to migrate, but as of right now, I don't know of any such active users.  The
only exception to this might be the shadow stack GC.  Eventually in this context
is at least six months from now, but likely less than 18 months.  Hopefully,
that's vague enough.  :)

HELP - If anyone knows which Ocaml implementation and which Erlang
implementation triggered the in tree GC strategies, please let me know!


Near Term Changes:
- Migrate ownership of GCStrategy objects from GCModuleInfo to 
LLVMContext.  In theory, this looses the ability for two different 
Modules to have the same collector with different state, but I know of 
no use case for this.
- Modify the primary Function::getGC/setGC interface to return a 
reference the GCStrategy object, not a string.  I will provide a 
Function::setGCString and getGCString.
- Extend the GCStrategy class to include a notion of which compilation 
strategy is being used.  The two choices right now will be Legacy and 
Statepoint.  (Longer term, this will likely become a more fine grained 
choice.)
- Separate GCStategy and related pieces from the 
GCFunctionInfo/GCModuleInfo/GCMetadataPrinter lowering code.  At first, 
this will simply mean clarifying documentation and rearranging code a bit.
- Document/clarify the callbacks used to customize the lowering. Decide 
which of these make sense to preserve and document.

(Lest anyone get the wrong idea, the above changes are intended to be 
minor cleanup.  I'm not looking to do anything controversial yet.)

Questions:
- Is proving the ability to generate a custom binary stack map format a 
valuable feature?  Adapting the new statepoint infrastructure to work 
with the existing GCMetadataPrinter classes wouldn't be particularly hard.
- Are there any GCs out there that need gcroot's single stack slot per 
value implementation?   By default, statepoints may generate a different 
stackmap for every safepoint in a function.
- Is using gcroot and allocas to mark pointers as garbage collected 
references valuable?  (As opposed to using address spaces on the SSA 
values themselves?)  Long term, should we retain the gcroot marker 
intrinsics at all?


Philip

Appendix: The Current Implementations Key Classes:

GCStrategy - Provides a configurable description of the collector. The 
strategy can also override parts of the default GC root lowering 
strategy.  The concept of such a collector description is very valuable, 
but the current implementation could use some cleanup.  In particular, 
the custom lowering hooks are a bit of a mess.

GCMetadataPrinter - Provides a means to dump a custom binary format 
describing each functions safepoints.  All safepoints in a function must 
share a single root Value to stack slot mapping.

GCModuleInfo/GCFunctionInfo - These contain the metadata which is saved 
to enable GCMetadataPrinter.

_______________________________________________
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LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Hello Philip,

I am an active user of LLVM's GC infrastructure. Here are some notes on how
I currently use gcroot():

1) I use several IRs. A high-level (CPS/SSA hybrid) IR is the target of
inlining and other such high-level optimization; most allocation is
implicit in the high level IR. Closure conversion/lambda-lifting produces a
variant of that IR, extended with explicit allocation constructs. A
separate pass introduces the allocations themselves.
2) Next, a dataflow-based root insertion pass inserts the minimal(ish) set
of roots and root reloads to preserve the invariant that every live GCable
pointer is in a root at each potential GC point. The root insertion pass
also computes liveness and uses it to minimize the set of roots via slot
coloring. The result of root insertion is (very close to) LLVM IR.
3) Potential GC points are determined inter-procedurally, and call sites
which are statically known to (transitively) not GC do not break GCable
pointer live ranges.
4) I use a lightly-modified variant of the OCaml plugin's stackmap format.
5) I don't currently use load or store barriers, but do eventually plan to.
6) I do support GCing through global variables.
7) I wrote a custom LLVM pass to verify that values loaded from GC roots
aren't used across GC points.

When I first started using gcroot(), I used the naïve approach of reloading
roots after every call, and encountered significant overhead. The
optimizations sketched above significantly reduced that overhead.
Unfortunately, I don't know of any meaningful language-independent
benchmark suites for measuring that sort of overhead, and of course the
overhead on any given program will strongly depend on details of how that
program is written...

Also, FWIW when I first started, I got up and running with the shadow
stack, just as Gordon described, before transitioning to the "real"
infrastructure. As long as it doesn't carry a significant burden on the
implementation side, I think it's worth having, because it significantly
improves the learning curve for new users.

OK, direct answers to some of your questions:

* I think custom stack map formats have small but non-zero value. There
have been a few papers (most in the early 90's, I think) which showed that
stack maps can make up a non-trivial fraction of a binary's size, and thus
are increasingly desirable to optimize as programs grow larger. My verdict:
if support for custom formats are ever actively impeding forward progress,
toss 'em; otherwise, there should (eventually) be a more detailed look at
the costs and benefits.

* As above, I think the primary benefit of one-stackmap-per-safepoint is
saving space at the cost of time (and root traffic). AFAIK the primary
virtue of the one-stack-slot-per-gcroot() implementation is implementation
simplicity, nothing more.

* The ultimate strength & weakness of gcroot() is that basically everything
is left to the frontend. There's very little unavoidable overhead imposed
on a frontend that is willing to go the distance to generate non-naïve code
-- any knowledge that the frontend has can be used to generate better code.
Unfortunately, this also places a rather heavy burden on the frontend to
generate valid & efficient code.


Since I haven't had the chance to look beyond mailing list & blog posts
on
statepoints, I can't comment much on their tradeoffs vs gcroots. The
high-level impression I get is that statepoints will allow a
less-sophisticated frontend to get better results than naïve usage of
gcroot(). It also looks like statepoints will do a better job of steering
frontends away from the pitfalls of using GC-invalidated pointer values.  I
have no idea whether there will be any codegen benefit for more
sophisticated frontends, but I'll be happy to port my implementation to
statepoints once they've gotten a chance to settle down somewhat, and
provide more detailed feedback to help determine what to eventually do with
gcroot(). I can currently only do ad-hoc benchmarking, but hopefully by the
time gcroot() is on the chopping block, I'll have a more extensive
automated performance regression test suite ;-)


One quick question: I think I understand why address spaces are needed to
distinguish GCable pointers for late safepoint placement, but are they also
needed if the frontend is inserting all safepoints itself?


Finally, thank you for taking on the burden of improving LLVM's GC
functionality!


On Thu, Dec 4, 2014 at 8:50 PM, Philip Reames <listmail at
philipreames.com>
wrote:
> Now that the statepoint changes have landed, I wanted to start a
> discussion about what's next for GC support in LLVM.  I'm going to
sketch
> out a strawman proposal, but I'm not set on any of this.  I mostly just
> want to draw interested parties out of the woodwork.  :)
>
> Overall Direction:
> In the short term, my intent is to preserve the functionality of the
> existing code, but migrate towards a position where the gcroot specific
> pieces are optional and well separated.  I also plan to start updating the
> documentation to reflect a separation between the general support for
> garbage collection (function attributes, identifying references, load and
> store barrier lowering, generating stack maps) and the implementation
> choices (gcroot & it's lowering vs statepoints & addr spaces
for
> identifying references).
>
> Longer term, I plan to *EVENTUALLY DELETE* the existing gcroot lowering
> code and in tree GCStrategies unless an interesting party speaks up.  I
> have no problem with retaining some of the existing pieces for legacy
> support or helping users to migrate, but as of right now, I don't know
of
> any such active users.  The only exception to this might be the shadow
> stack GC.  Eventually in this context is at least six months from now, but
> likely less than 18 months.  Hopefully, that's vague enough.  :)
>
> HELP - If anyone knows which Ocaml implementation and which Erlang
> implementation triggered the in tree GC strategies, please let me know!
>
>
> Near Term Changes:
> - Migrate ownership of GCStrategy objects from GCModuleInfo to
> LLVMContext.  In theory, this looses the ability for two different Modules
> to have the same collector with different state, but I know of no use case
> for this.
> - Modify the primary Function::getGC/setGC interface to return a reference
> the GCStrategy object, not a string.  I will provide a
> Function::setGCString and getGCString.
> - Extend the GCStrategy class to include a notion of which compilation
> strategy is being used.  The two choices right now will be Legacy and
> Statepoint.  (Longer term, this will likely become a more fine grained
> choice.)
> - Separate GCStategy and related pieces from the
> GCFunctionInfo/GCModuleInfo/GCMetadataPrinter lowering code.  At first,
> this will simply mean clarifying documentation and rearranging code a bit.
> - Document/clarify the callbacks used to customize the lowering. Decide
> which of these make sense to preserve and document.
>
> (Lest anyone get the wrong idea, the above changes are intended to be
> minor cleanup.  I'm not looking to do anything controversial yet.)
>
> Questions:
> - Is proving the ability to generate a custom binary stack map format a
> valuable feature?  Adapting the new statepoint infrastructure to work with
> the existing GCMetadataPrinter classes wouldn't be particularly hard.
> - Are there any GCs out there that need gcroot's single stack slot per
> value implementation?   By default, statepoints may generate a different
> stackmap for every safepoint in a function.
> - Is using gcroot and allocas to mark pointers as garbage collected
> references valuable?  (As opposed to using address spaces on the SSA values
> themselves?)  Long term, should we retain the gcroot marker intrinsics at
> all?
>
>
> Philip
>
> Appendix: The Current Implementations Key Classes:
>
> GCStrategy - Provides a configurable description of the collector. The
> strategy can also override parts of the default GC root lowering strategy.
> The concept of such a collector description is very valuable, but the
> current implementation could use some cleanup.  In particular, the custom
> lowering hooks are a bit of a mess.
>
> GCMetadataPrinter - Provides a means to dump a custom binary format
> describing each functions safepoints.  All safepoints in a function must
> share a single root Value to stack slot mapping.
>
> GCModuleInfo/GCFunctionInfo - These contain the metadata which is saved to
> enable GCMetadataPrinter.
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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> - From the documentation it looks like you're using the patchpoint
>   stackmap format
>   (http://llvm.org/docs/StackMaps.html#stackmap-format).  In that
>   format you can describe register locations - but from the overview
>   (http://llvm.org/docs/Statepoints.html#overview) it implies that all
>   gc pointers are spilled to the stack.  Is the spilling to memory
>   required?  Or is the plan to allow gc pointers to reside in register
>   as well.  (I'm hoping that a store/load at safepoinsts won't be
>   required and that they can stack register resident)
We're currently spilling to stack to keep the implementation simple.
Ideally we should be able to lower the complete gc.statepoint
construct to a no-op; and have the GC deal with whatever decision the
register allocator made.
> - I'm still fuzzy how code motion is blocked from moving SSA uses past
>   the safepoint once they've been inserted?  I'm likely just
missing
>   some invariant in LLVM or the design since I can't seem to noodle it
>   out from what I've seen.
The representation only prevents "observable" uses of the GC pointers
from being moved across safepoints.  The semantics of gc.statepoint is
not that all uses of `%ptr' automatically become uses of the latest,
most relocated value of the object `%ptr' points to; but that the
gc.statepoint *explicitly* returns a `%ptr.reloc' that you're supposed
to use instead of `%ptr' once you've dynamically passed the
gc.statepoint.  Ensuring this can involve inserting phi nodes.

So, the two following pieces of code are semantically equivalent (in
pseudo-llvm):

  %cmp = (%ptr == null)
  tok = statepoint(relocate %ptr)
  %ptr.reloc = relocate(tok, %ptr)

Vs.

  tok = statepoint(relocate %ptr)
  %ptr.reloc = relocate(tok, %ptr)
  %cmp = (%ptr == null)

In both the code segments, `%cmp' holds true if the *unrelocated* %ptr
is null.  In both the code segments, nothing looks at where %ptr was
relocated to.

Since gc.statepoint is specified to possibly have arbitrary
side-effects and can read/write arbitrary memory, the following two
are *not* equivalent:

  %cmp = (%ptr == null)
  if (%cmp) *global = 42;
  tok = statepoint(relocate %ptr)
  %ptr.reloc = relocate(tok, %ptr)

Vs.

  tok = statepoint(relocate %ptr)
  %ptr.reloc = relocate(tok, %ptr)
  %cmp = (%ptr == null)
  if (%cmp) *global = 42;

but the second one is equivalent to

  %cmp = (%ptr == null)
  tok = statepoint(relocate %ptr)
  %ptr.reloc = relocate(tok, %ptr)
  if (%cmp) *global = 42;
> - In the CLR GC we don't require the base object pointer to be kept
>   alive for a derived managed pointer (interior pointer) but in your
>   design there is the requirement to maintain a base, derived pairing.
>   (If I remember right this is a Java requirement) Is this a hard
>   requirement?  Or is there the potential for other collectors to deal
>   just with managed pointers
I'm not familiar with the CLR GC, won't you need base pointers to
be able to relocate stack roots?  In any case, if you don't need
derived pointers, you can just have the identity map as the
base-derived relationship (i.e. every pointer is a base pointer for
itself).

-- Sanjoy

On 12/08/2014 04:02 PM, Russell Hadley wrote:
> Hi,
>
> This looks great.  I'm excited to see this come in and the direction
looks right to me.  I've started to look at how to use LLVM to target the MS
CLR GC (now being open sourced) and the statepoints are much closer to the
current implementation so that could make my life easier.  On your questions,
add my vote to the ability to implement a custom binary format for safepoints
since it means I can just target the CLRs format.
It seems like there's a strong desire to preserve the custom binary 
format mechanism.  I wasn't really expecting that, but I see no real 
downsides other than some minor code complexity.
>   I've been trying to keep up with your progress but since I'm
coming out of lurk mode I have a few questions:
>
> - From the documentation it looks like you're using the patchpoint
stackmap format (http://llvm.org/docs/StackMaps.html#stackmap-format).  In that
format you can describe register locations - but from the overview
(http://llvm.org/docs/Statepoints.html#overview) it implies that all gc pointers
are spilled to the stack.  Is the spilling to memory required?  Or is the plan
to allow gc pointers to reside in register as well.  (I'm hoping that a
store/load at safepoinsts won't be required and that they can stack register
resident)
At the moment, we will eagerly spill and the stack map will only contain 
stack slots.  My hope is in the not too distant future to extend the 
backend infrastructure to allow accurate reporting of gc pointers in 
registers.  The format specification already supports this, we're just 
not able to actually exploit that in the backend yet.
> - I'm still fuzzy how code motion is blocked from moving SSA uses past
the safepoint once they've been inserted?   I'm likely just missing some
invariant in LLVM or the design since I can't seem to noodle it out from
what I've seen.
I think Sanjoy's response did a pretty good job on this one.  If it's 
still unclear, let me know.
> - In the CLR GC we don't require the base object pointer to be kept
alive for a derived managed pointer (interior pointer) but in your design there
is  the requirement to maintain a base, derived pairing.  (If I remember right
this is a Java requirement) Is this a hard requirement?  Or is there the
potential for other collectors to deal just with managed pointers
I'm a little unclear on what you're trying to ask here.  A pointer 
associated with an object (say, the address of a field) must keep the 
object alive; doing otherwise would create use-after-free errors.  I'm 
guessing that you simply don't keep interior pointers live across a 
safepoint?  That's fine and everything should work normally.

If - at the safepoint - all of your pointers are base pointers, then you 
can simply list it for both the base and derived fields in the 
gc.relocate.  This mechanism definitely works; it's a pretty common case 
for any compiled code.

The other case you might be referring to is so-called 'contained 
objects'.  (i.e. one gc managed object embedded within another, but 
whose lifetimes are distinct)  This is a more complicated case, so 
unless this is actually what you're getting at, I'm going to avoid 
explaining all the complexities.  :)
>
> Thanks,
>
> -R
>
> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu] On Behalf Of Philip Reames
> Sent: Thursday, December 4, 2014 5:50 PM
> To: LLVM Developers Mailing List
> Cc: rayiner at gmail.com; gordonhenriksen at mac.com
> Subject: [LLVMdev] Future plans for GC in LLVM
>
> Now that the statepoint changes have landed, I wanted to start a discussion
about what's next for GC support in LLVM.  I'm going to sketch out a
strawman proposal, but I'm not set on any of this.  I mostly just want to
draw interested parties out of the woodwork.  :)
>
> Overall Direction:
> In the short term, my intent is to preserve the functionality of the
existing code, but migrate towards a position where the gcroot specific pieces
are optional and well separated.  I also plan to start updating the
documentation to reflect a separation between the general support for garbage
collection (function attributes, identifying references, load and store barrier
lowering, generating stack maps) and the implementation choices (gcroot &
it's lowering vs statepoints & addr spaces for identifying references).
>
> Longer term, I plan to *EVENTUALLY DELETE* the existing gcroot lowering
code and in tree GCStrategies unless an interesting party speaks up.  I have no
problem with retaining some of the existing pieces for legacy support or helping
users to migrate, but as of right now, I don't know of any such active
users.  The only exception to this might be the shadow stack GC.  Eventually in
this context is at least six months from now, but likely less than 18 months. 
Hopefully, that's vague enough.  :)
>
> HELP - If anyone knows which Ocaml implementation and which Erlang
implementation triggered the in tree GC strategies, please let me know!
>
>
> Near Term Changes:
> - Migrate ownership of GCStrategy objects from GCModuleInfo to
> LLVMContext.  In theory, this looses the ability for two different
> Modules to have the same collector with different state, but I know of
> no use case for this.
> - Modify the primary Function::getGC/setGC interface to return a
> reference the GCStrategy object, not a string.  I will provide a
> Function::setGCString and getGCString.
> - Extend the GCStrategy class to include a notion of which compilation
> strategy is being used.  The two choices right now will be Legacy and
> Statepoint.  (Longer term, this will likely become a more fine grained
> choice.)
> - Separate GCStategy and related pieces from the
> GCFunctionInfo/GCModuleInfo/GCMetadataPrinter lowering code.  At first,
> this will simply mean clarifying documentation and rearranging code a bit.
> - Document/clarify the callbacks used to customize the lowering. Decide
> which of these make sense to preserve and document.
>
> (Lest anyone get the wrong idea, the above changes are intended to be
> minor cleanup.  I'm not looking to do anything controversial yet.)
>
> Questions:
> - Is proving the ability to generate a custom binary stack map format a
> valuable feature?  Adapting the new statepoint infrastructure to work
> with the existing GCMetadataPrinter classes wouldn't be particularly
hard.
> - Are there any GCs out there that need gcroot's single stack slot per
> value implementation?   By default, statepoints may generate a different
> stackmap for every safepoint in a function.
> - Is using gcroot and allocas to mark pointers as garbage collected
> references valuable?  (As opposed to using address spaces on the SSA
> values themselves?)  Long term, should we retain the gcroot marker
> intrinsics at all?
>
>
> Philip
>
> Appendix: The Current Implementations Key Classes:
>
> GCStrategy - Provides a configurable description of the collector. The
> strategy can also override parts of the default GC root lowering
> strategy.  The concept of such a collector description is very valuable,
> but the current implementation could use some cleanup.  In particular,
> the custom lowering hooks are a bit of a mess.
>
> GCMetadataPrinter - Provides a means to dump a custom binary format
> describing each functions safepoints.  All safepoints in a function must
> share a single root Value to stack slot mapping.
>
> GCModuleInfo/GCFunctionInfo - These contain the metadata which is saved
> to enable GCMetadataPrinter.
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

On 12/09/2014 03:12 AM, Ben Karel wrote:
> Hello Philip,
>
> I am an active user of LLVM's GC infrastructure. Here are some notes 
> on how I currently use gcroot():
Wow, thank you for sharing!  Your usage is far beyond anything else I've 
heard of.  Can I ask which project/language this is?  Or is that 
proprietary?
>
> 1) I use several IRs. A high-level (CPS/SSA hybrid) IR is the target 
> of inlining and other such high-level optimization; most allocation is 
> implicit in the high level IR. Closure conversion/lambda-lifting 
> produces a variant of that IR, extended with explicit allocation 
> constructs. A separate pass introduces the allocations themselves.
> 2) Next, a dataflow-based root insertion pass inserts the minimal(ish) 
> set of roots and root reloads to preserve the invariant that every 
> live GCable pointer is in a root at each potential GC point. The root 
> insertion pass also computes liveness and uses it to minimize the set 
> of roots via slot coloring. The result of root insertion is (very 
> close to) LLVM IR.
I would be really interested in hearing more about your implementation 
for this part.  We need to solve a similar problem in the statepoint 
lowering code.  What we have to date is a simply greedy scheme that 
works reasonable well in practice, but leaves lots of room for
improvement.
> 3) Potential GC points are determined inter-procedurally, and call 
> sites which are statically known to (transitively) not GC do not break 
> GCable pointer live ranges.
I was planning something like this for LLVM at some point.  It's 
relatively low on my priority list since IPO tends to be of minimal 
interest when JITing which is my primary use case.
> 4) I use a lightly-modified variant of the OCaml plugin's stackmap
format.
> 5) I don't currently use load or store barriers, but do eventually 
> plan to.
> 6) I do support GCing through global variables.
> 7) I wrote a custom LLVM pass to verify that values loaded from GC 
> roots aren't used across GC points.
Cool.  We have a similar one for statepoints.  (If you can share, 
getting both into the common tree would be nice.)
>
> When I first started using gcroot(), I used the naïve approach of 
> reloading roots after every call, and encountered significant 
> overhead. The optimizations sketched above significantly reduced that 
> overhead.  Unfortunately, I don't know of any meaningful 
> language-independent benchmark suites for measuring that sort of 
> overhead, and of course the overhead on any given program will 
> strongly depend on details of how that program is written...
>
> Also, FWIW when I first started, I got up and running with the shadow 
> stack, just as Gordon described, before transitioning to the
"real"
> infrastructure. As long as it doesn't carry a significant burden on 
> the implementation side, I think it's worth having, because it 
> significantly improves the learning curve for new users.
I'm planning on retaining the shadow stack
mechanism.
>
> OK, direct answers to some of your questions:
>
> * I think custom stack map formats have small but non-zero value. 
> There have been a few papers (most in the early 90's, I think) which 
> showed that stack maps can make up a non-trivial fraction of a 
> binary's size, and thus are increasingly desirable to optimize as 
> programs grow larger. My verdict: if support for custom formats are 
> ever actively impeding forward progress, toss 'em; otherwise, there 
> should (eventually) be a more detailed look at the costs and benefits.
My preferred usage model would be: LLVM generates standard format, 
runtime parses and saves in custom binary format.

Having said that, retaining the capability doesn't seem to involve too 
much complexity.  I see no reason to kill it since multiple folks seem 
to want it.
>
> * As above, I think the primary benefit of one-stackmap-per-safepoint 
> is saving space at the cost of time (and root traffic). AFAIK the 
> primary virtue of the one-stack-slot-per-gcroot() implementation is 
> implementation simplicity, nothing more.
>
> * The ultimate strength & weakness of gcroot() is that basically 
> everything is left to the frontend. There's very little unavoidable 
> overhead imposed on a frontend that is willing to go the distance to 
> generate non-naïve code -- any knowledge that the frontend has can be 
> used to generate better code. Unfortunately, this also places a rather 
> heavy burden on the frontend to generate valid & efficient code.
>
>
> Since I haven't had the chance to look beyond mailing list & blog 
> posts on statepoints, I can't comment much on their tradeoffs vs 
> gcroots. The high-level impression I get is that statepoints will 
> allow a less-sophisticated frontend to get better results than naïve 
> usage of gcroot(). It also looks like statepoints will do a better job 
> of steering frontends away from the pitfalls of using GC-invalidated 
> pointer values.  I have no idea whether there will be any codegen 
> benefit for more sophisticated frontends, but I'll be happy to port my 
> implementation to statepoints once they've gotten a chance to settle 
> down somewhat, and provide more detailed feedback to help determine 
> what to eventually do with gcroot(). I can currently only do ad-hoc 
> benchmarking, but hopefully by the time gcroot() is on the chopping 
> block, I'll have a more extensive automated performance regression 
> test suite ;-)
I'll be very interested in your results.  I'm quite sure you'll find
stability and performance bugs.  The existing code 'works' but has 
really only been hammered in one particular use case (source language, 
virtual machine).  Every new consumer will help to make the code more 
robust.  Let me know when you're ready to start prototyping this.  I'm 
happy to answer questions and help guide you around any bugs you might 
find.

I suspect from what you've said above that we might want to extend the 
mechanism slightly to allow you to retain your preassigned stack slots.  
You may be getting better spilling code than the current implementation 
would give you by default.  This seems like it might be a generally 
useful mechanism.  We could either use a call attribute on the 
gc.relocate, a bit of metadata, or possible an optional third argument 
that specifies an 'abstract slot'.  It would depend on your initial 
results and how much we've managed to improve the lowering code by then. 
:)
>
>
> One quick question: I think I understand why address spaces are needed 
> to distinguish GCable pointers for late safepoint placement, but are 
> they also needed if the frontend is inserting all safepoints itself?
Nope.  They might allow some additional sanity checks, but nothing that 
is currently checked in relies on address spaces in any way. My plan is 
to make the pointer distinction mechanism (addrspace, gcroot, vs custom) 
a property of the GCStrategy with easily control extension
points.
>
>
> Finally, thank you for taking on the burden of improving LLVM's GC 
> functionality!
Thank you for sharing your experience!
>
>
> On Thu, Dec 4, 2014 at 8:50 PM, Philip Reames 
> <listmail at philipreames.com <mailto:listmail at
philipreames.com>> wrote:
>
>     Now that the statepoint changes have landed, I wanted to start a
>     discussion about what's next for GC support in LLVM.  I'm going
to
>     sketch out a strawman proposal, but I'm not set on any of this.  I
>     mostly just want to draw interested parties out of the woodwork.  :)
>
>     Overall Direction:
>     In the short term, my intent is to preserve the functionality of
>     the existing code, but migrate towards a position where the gcroot
>     specific pieces are optional and well separated.  I also plan to
>     start updating the documentation to reflect a separation between
>     the general support for garbage collection (function attributes,
>     identifying references, load and store barrier lowering,
>     generating stack maps) and the implementation choices (gcroot &
>     it's lowering vs statepoints & addr spaces for identifying
>     references).
>
>     Longer term, I plan to *EVENTUALLY DELETE* the existing gcroot
>     lowering code and in tree GCStrategies unless an interesting party
>     speaks up.  I have no problem with retaining some of the existing
>     pieces for legacy support or helping users to migrate, but as of
>     right now, I don't know of any such active users.  The only
>     exception to this might be the shadow stack GC.  Eventually in
>     this context is at least six months from now, but likely less than
>     18 months. Hopefully, that's vague enough.  :)
>
>     HELP - If anyone knows which Ocaml implementation and which Erlang
>     implementation triggered the in tree GC strategies, please let me
>     know!
>
>
>     Near Term Changes:
>     - Migrate ownership of GCStrategy objects from GCModuleInfo to
>     LLVMContext.  In theory, this looses the ability for two different
>     Modules to have the same collector with different state, but I
>     know of no use case for this.
>     - Modify the primary Function::getGC/setGC interface to return a
>     reference the GCStrategy object, not a string.  I will provide a
>     Function::setGCString and getGCString.
>     - Extend the GCStrategy class to include a notion of which
>     compilation strategy is being used.  The two choices right now
>     will be Legacy and Statepoint.  (Longer term, this will likely
>     become a more fine grained choice.)
>     - Separate GCStategy and related pieces from the
>     GCFunctionInfo/GCModuleInfo/GCMetadataPrinter lowering code.  At
>     first, this will simply mean clarifying documentation and
>     rearranging code a bit.
>     - Document/clarify the callbacks used to customize the lowering.
>     Decide which of these make sense to preserve and document.
>
>     (Lest anyone get the wrong idea, the above changes are intended to
>     be minor cleanup.  I'm not looking to do anything controversial
yet.)
>
>     Questions:
>     - Is proving the ability to generate a custom binary stack map
>     format a valuable feature?  Adapting the new statepoint
>     infrastructure to work with the existing GCMetadataPrinter classes
>     wouldn't be particularly hard.
>     - Are there any GCs out there that need gcroot's single stack slot
>     per value implementation?   By default, statepoints may generate a
>     different stackmap for every safepoint in a function.
>     - Is using gcroot and allocas to mark pointers as garbage
>     collected references valuable?  (As opposed to using address
>     spaces on the SSA values themselves?)  Long term, should we retain
>     the gcroot marker intrinsics at all?
>
>
>     Philip
>
>     Appendix: The Current Implementations Key Classes:
>
>     GCStrategy - Provides a configurable description of the collector.
>     The strategy can also override parts of the default GC root
>     lowering strategy.  The concept of such a collector description is
>     very valuable, but the current implementation could use some
>     cleanup.  In particular, the custom lowering hooks are a bit of a
>     mess.
>
>     GCMetadataPrinter - Provides a means to dump a custom binary
>     format describing each functions safepoints.  All safepoints in a
>     function must share a single root Value to stack slot mapping.
>
>     GCModuleInfo/GCFunctionInfo - These contain the metadata which is
>     saved to enable GCMetadataPrinter.
>
>     _______________________________________________
>     LLVM Developers mailing list
>     LLVMdev at cs.uiuc.edu <mailto:LLVMdev at cs.uiuc.edu>
>     http://llvm.cs.uiuc.edu
>     http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
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Hi,

On 2014-12-05 02:50, Philip Reames wrote:
> Questions:
> - Is proving the ability to generate a custom binary stack map format
> a valuable feature?  Adapting the new statepoint infrastructure to
> work with the existing GCMetadataPrinter classes wouldn't be
> particularly hard.
I think custom binary stack map formats are valuable (smaller and easier 
to parse).  I'm also willing to implement this after the code around 
GCStrategy is a bit more stable.
> - Are there any GCs out there that need gcroot's single stack slot per
> value implementation?   By default, statepoints may generate a
> different stackmap for every safepoint in a function.
Probably not, since the GC runtime looks only at one safepoint per stack 
frame and doesn't care about the other safepoints in the function.  For 
saving space it's probably a better idea to have a custom stack map 
format with some form of compression (e.g. a bitmap).
> - Is using gcroot and allocas to mark pointers as garbage collected
> references valuable?  (As opposed to using address spaces on the SSA
> values themselves?)  Long term, should we retain the gcroot marker
> intrinsics at all?
According to [1], the gcroot intrinsic was made for frontends which rely 
on allocas and mem2reg to bring their code in a SSA form.  In this case 
the code emitted by the frontend has the following properties:

1) The GC root is loaded from the alloca before each use.
2) The GC root is stored into the alloca after each assignment.
3) No interior pointers are passed across safepoints.

The resulting code should always be correct albeit slow.  It shouldn't 
be too hard to add a case to mem2reg which transforms gcroot'ed allocas 
into something the late safepoint placement pass understands as being a 
GC root.

-Manuel

[1] http://llvm.org/docs/GarbageCollection.html#gcroot

On 01/25/2015 06:56 AM, Manuel Jacob wrote:
> Hi,
>
> On 2014-12-05 02:50, Philip Reames wrote:
>> Questions:
>> - Is proving the ability to generate a custom binary stack map format
>> a valuable feature?  Adapting the new statepoint infrastructure to
>> work with the existing GCMetadataPrinter classes wouldn't be
>> particularly hard.
>
> I think custom binary stack map formats are valuable (smaller and 
> easier to parse).  I'm also willing to implement this after the code 
> around GCStrategy is a bit more stable.
Great to hear.
>
>> - Are there any GCs out there that need gcroot's single stack slot
per
>> value implementation?   By default, statepoints may generate a
>> different stackmap for every safepoint in a function.
>
> Probably not, since the GC runtime looks only at one safepoint per 
> stack frame and doesn't care about the other safepoints in the 
> function.  For saving space it's probably a better idea to have a 
> custom stack map format with some form of compression (e.g. a bitmap).
The default lowering for gc.root definitely supports multiple safepoints 
per frame.  They're required to have the same *layout* but multiple 
safepoints were supported.

p.s. Your use of the phrase "gc runtime" above is mildly ambiguous. 
Hopefully I inferred the right meaning.
>
>> - Is using gcroot and allocas to mark pointers as garbage collected
>> references valuable?  (As opposed to using address spaces on the SSA
>> values themselves?)  Long term, should we retain the gcroot marker
>> intrinsics at all?
>
> According to [1], the gcroot intrinsic was made for frontends which 
> rely on allocas and mem2reg to bring their code in a SSA form.  In 
> this case the code emitted by the frontend has the following properties:
>
> 1) The GC root is loaded from the alloca before each use.
> 2) The GC root is stored into the alloca after each assignment.
> 3) No interior pointers are passed across safepoints.
>
> The resulting code should always be correct albeit slow.  It shouldn't 
> be too hard to add a case to mem2reg which transforms gcroot'ed 
> allocas into something the late safepoint placement pass understands 
> as being a GC root.
This is harder than it might first seem, but might be doable.  I would 
strongly suggest that it not be part of mem2reg though. 
:)
>
> -Manuel
>
> [1] http://llvm.org/docs/GarbageCollection.html#gcroot