llvm dev - Sep 2015 - RFC: alloca -- specify address space for allocation

Thanks,

this makes the use case much more clear.
Now though, as far as I would like actually to see supported in LLVM the
capability of not having any special meaning assigned to address space 0 your
proposal goes slightly in contrast with how I always thought of address spaces
in LLVM.
I also have to say that I don’t know deeply how address spaces are meant to be
intended in LLVM so my vision of them might be actually off the LLVM-way.

This is how I see them though:
If in OpenCL for example we demark private memory with addrspace(0) and global
memory with addrspace(1) what addrspace 0 or 1 tells me is an information about
the memory pointed by the pointer (whether it is private or global memory). So
it tells me something about the “pointee” and not the pointer.
What you are proposing here though is the opposite if I understood correctly,
which is that actually addrspace(1) is telling us an information about the
pointer (the fact that is a managed pointer) and nothing about the pointee
(which is just some address on the stack I assume).

Still my idea of address spaces and my understanding of what you are trying to
do could still be completely wrong … :P

Marcello
 
> On 30 Aug 2015, at 19:45, Joseph Tremoulet via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> MM> It is not clear to me if these GC pointers are actually living in a
different address space when allocated on the stack (aka. you have two different
stacks)
> HF> Is the mental model here that a function using this feature actually
has multiple stacks, in different address spaces, and this allows creating local
stack allocations in those non-addrspace-0 stacks?
> PR> the reason we originally went with addrspaces to mark GC pointers
was that the managed and non-managed heaps for us are semantically
non-overlapping
> PR> we might be better off introducing an orthogonal notion for tracking
gc references entirely.  The addrspace mechanism has worked, but it is a little
bit of a hack
> CC> these use cases seem really straight forward for address spaces...
But this is very different from the idea of using an alloca with an address
spaces for GC pointers to stack objects
> 
> 
> I think it's an important question whether address spaces are a good
fit for what we're trying to model here.  So I'll explain my mental
model of LLILC's address spaces, and I'd be very interested in feedback
on whether this seems like a good fit for LLVM's address space concept, or a
bastardization thereof:
> 
> [preface: it's been my understanding that dereferences of pointers in
different address spaces can alias is semantically meaningful ways; i.e. that
it's appropriate to use different address spaces to model different means of
indexing the same memory.  Some of the comments/questions on this thread seemed
to imply instead an expectation that distinct address spaces always reference
semantically disjoint storage; if that's a hard assumption, then nothing
I'm about to say will make sense and we'll almost certainly need a
different mechanism to model GC pointers]
> 
> 1. The value of an unmanaged/addrspace(0) pointer is an address (in the
virtual memory available to the process)
> 2. The value of a managed/addrspace(1) pointer is conceptually an (ID,
offset) pair.  The first component is the "identity" of the object
that the offset component is relative to.  Identity is distinct from address;
you could imagine the GC heap allocator having a counter that it increments with
each allocation, and that an object's identity is the value the counter had
when it was allocated.
> 3. There are two special reserved IDs for pseudo-objects:
>  3a. one reserved ID for the null pseudo-object (i.e. what nullptr points
to)
>  3b. one reserved ID for the "everything-outside-the-GC-heap"
pseudo-object.  Conceptually this is an infinitely-large object and the data at
offset N in this object is the data at address N (in the virtual memory
available to the process, and with a requirement that N does not correspond to
an address in the GC heap)
> 4. The benefit of this conceptual model is that garbage collections are
value-preserving, which is why we don't need to model safepoints as
rewriting GC pointers (until the RewriteStatepointsForGC pass where we rewrite
the IR in terms of addresses rather than object identity)
> 5. We know that the representation of an addrspace(1) pointer into the
outside-the-GC-heap pseudo-object is bit-identical to the representation of an
addrspace(0) pointer to the corresponding address  (question: Doesn't that
mean we can/should be using bitcast instead of addrspacecast when we know the
value in question is an outside-the-GC-heap pointer?)
> 6. Our source language includes constructs that expose a managed
pointer's address (which have well-defined semantics only if the object in
question is "pinned").  These naturally correspond to addrspace casts
(or maybe need to be additionally constrained?) and are a function of not only
the input pointer but also of the layout of the GC heap and the runtime's
pinning state.  This is where we can get a mix of addrspace(0) and addrspace(1)
pointers whose dereferences alias (in semantically well-defined ways).
> 
> Again, I'd love feedback on how sane that sounds to those of you
familiar with LLVM's address space notion.
> 
> I think that 3b is the part that seems to generate the most
surprise/consternation.  The reason our source language includes the
"outside-the-GC-heap" pseudo-object is that it allows more powerful
code (you could think of it as allowing polymorphism over GC-heap-ness of
inputs) with no adverse effects to the system's typesafety guarantees.
> 
> Relating this back to the question of what address space an alloca belongs
in (which I'm doing to contextualize, not in an attempt to continue the
debate), as the stack (of which there is only one) is outside-the-GC-heap, the
question is one of whether you want to conceptualize an alloca as producing an
unmanaged address or as producing a pointer into the outside-the-GC-heap
pseudo-object.  I'd argue that our source language conceptualizes it as the
latter[1], which (as Swaroop points out below) is why it would feel natural to
model it that way in LLVM, but of course (as Swaroop also points out below) we
could also decompose the source construct into two steps in LLVM IR.  As far as
compiler-introduced allocas, they of course wouldn't be referenced in the
source, and so we wouldn't have a hard requirement either way.
> 
> Thanks
> -Joseph
> 
> [1] - To be more pedantic/precise:
> - Static allocas are implied by local variable declarations, and those
declarations are not annotated with a pointer type
> - Our source language includes a compact form that can be used to describe
a load or store of a local variable; these compact forms are not annotated with
a pointer type
> - Everywhere else that our source language refers to the address of a local
variable, it uses the managed pointer type
> 
> 
> 
> 
> -----Original Message-----
> From: Swaroop Sridhar 
> Sent: Saturday, August 29, 2015 12:30 AM
> To: Philip Reames <listmail at philipreames.com>; llvm-dev
<llvm-dev at lists.llvm.org>; Sanjoy Das <sanjoy at
playingwithpointers.com>
> Cc: Joseph Tremoulet <jotrem at microsoft.com>; Andy Ayers <andya
at microsoft.com>; Russell Hadley <rhadley at microsoft.com>
> Subject: RE: RFC: alloca -- specify address space for allocation
> 
>> -----Original Message-----
>> From: Philip Reames [mailto:listmail at philipreames.com]
>> Sent: Friday, August 28, 2015 9:38 AM
>> To: Swaroop Sridhar <Swaroop.Sridhar at microsoft.com>; llvm-dev
<llvm-
>> dev at lists.llvm.org>; Sanjoy Das <sanjoy at
playingwithpointers.com>
>> Cc: Joseph Tremoulet <jotrem at microsoft.com>; Andy Ayers 
>> <andya at microsoft.com>; Russell Hadley <rhadley at
microsoft.com>
>> Subject: Re: RFC: alloca -- specify address space for allocation
>> 
> 
>>> I think for the use case you are outlining, an addrspacecast is the
>>> correct IR model -- you're specifically saying that it is OK in
this
>>> case to turn a pointer from addrspace 0 into one for addrspace N 
>>> because N is your "managed pointer" set that can be
*either* a GC-pointer or a non-GC-pointer.
> 
>>> What the FE is saying is that this is an *acceptable* transition of
>>> addrspace, because your language and runtime semantics have
provided for it.
>>> I think the proper way to say that is with a cast.
> 
>> The key bit here is that I think Chandler is right.  You are 
>> effectively casting a stack allocation *into* a managed pointer. 
>> Having something to mark that transition seems reasonable.
> 
> I think there are two views here:
> 
> (1) MSIL level view:
> In CLR, the stack, is a part of "managed memory" (which is not
the same as gc-heap, which is managed and garbage-collected memory).
> Therefore, all *references* to stack locations are "managed
addresses,"  in the sense that the compiler/runtime exercises certain
control over (values that are) managed-address:
> For example: it enforces certain restrictions to guarantee safety -- ex:
lifetime restrictions, non-null requirement in certain contexts, etc.
> 
> This is different from a notion of "unmanaged memory" which is
for interoperability with native code.
> *Pointers* to unmanaged memory are not controlled by the runtime (ex: do
not provide any safety guarantees).
> 
> So, from the language semantics point of view, stack addresses are created
as managed pointers.
> Which is why the proposal is to have alloca directly in the managed
address-space seemed natural.
> 
> Joseph has written more details in the document that Philip shared out in
this thread.
> 
> (2) A more Lower level IR view:
> LLVM creates all stack locations in addrespace(0) for all code, whether it
comes from managed-code or native code.
> Of these, Stack locations corresponding to the managed-stack are promoted
to managed-addresses via addrspacecast.
> As an optimization, the FrontEnd inserts the addrspace casts only for those
stack locations that are actually address-taken.
> 
> If I understand correctly, the recommendation (by Philip, Chandler and
David) is approach (2) because:
> (a) No change to Instruction-Set is necessary when the semantics is
achievable via existing instructions.
> (b) It saves changing the optimizer to allocate in the correct
address-space.
> Looks like the problem here is that: the optimizer is expected to create
type-preserving transformation by allocating in the correct address-space, but
blindly allocates in the default address space today.
> I don't know the LLVM optimizer well enough to have a good estimate of
the magnitude of changes necessary here. But, I agree that (avoiding)
substantial changes to the optimizer is a strong consideration.
> 
>>> You might need N to be a distinct address space from the one used
for
>>> GC-pointers and to have similar casts emitted by the frontend.
> 
> Yes, eventually we'll need to differentiate between:
> (i) Pointers to unmanaged memory -- which will never be reported to the
runtime
> (ii) Pointers to GC-heap objects -- which will always be reported to the
runtime
> (iii) Generic managed pointer -- which may need to be reported if we cannot
establish that it points outside the GC heap.
> 
> Currently we report all pointers to the runtime as managed pointers.
> This is inefficient because the GC then needs to do extra work to figure
out what kind of pointer it is:
> Pointer to a heap object, pointer within a heap object, or outside the
heap.
> 
>> Of course, having said that all, I'm back to thinking that having a
>> marker on the alloca would be somewhat reasonable too.  However, I 
>> think we need a much stronger justification to change the IR than has 
>> been provided.  If you can show that the cast based model doesn't
work
>> for some reason, we can re-evaluate.
> 
> I don't think we can say that the cast-based model will not work. 
> The question is whether alloca addrspace(1)* is a better fit for MSIL
semantics, analysis phases, and managed-code specific optimizations.
> 
> I'm OK if we conclude that we'll keep using the cast model until we
hit a concrete case where it does not work, or seems architecturally misfit.
> 
>> Worth noting is that we might be better off introducing an orthogonal 
>> notion for tracking gc references entirely.  The addrspace mechanism 
>> has worked, but it is a little bit of a hack. We've talked about
the
>> need for an opaque pointer type.  Maybe when we actually get around to 
>> defining that, the alloca case is one we should consider.
> 
> Yes, I'm mainly concerned about getting the right types on the
different kinds of pointers. If adders space annotation implies more constraints
(ex: on layout) than what's already necessitated by the type distinction, we
should use a separate mechanism.
> 
> Again, I'm OK if we want to keep using addrspacecast until we hit a
concrete case where it breaks down.
> 
> Swaroop.
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Thanks for your reply!

It sounds like you're understanding correctly what we're trying to do.

Reading your take prompted me to look back over how I came to my understanding
of address spaces.  I think one part of it was reading a recent RFC[1] which
explicitly discusses both aliased and disjoint address spaces.  Another aspect
was the notion/name of the 'addrspacecast' operation.  I wonder: in your
usage where address spaces demark private and global memory, is addrspacecast
illegal, or does it require copying data around (and if so how would one know
how much to copy), or can some memory be both private and global, or ... ?  The
LangRef does state that "if the address space conversion is legal then both
result and operand refer to the same memory location"[2].

At any rate, looking this up led me to the LangRef's section on data layout
[3], which indicates that pointers into different address spaces can have
different sizes, which is a property of the pointer and not the pointee.  So it
must be possible that address spaces can tell us something about both pointer
and pointee.  Sometimes (as in your case) the pointers are the same and only the
pointee differs, and I wonder whether it's novel that LLILC's plan is to
have the pointees the same and only the pointers differ.

Reading further, I see both that addrspacecast "can be a no-op cast or a
complex value modification"[2] and that bitcast "may only be [used on
pointers] with the same address space"[4].

So I'm getting the impression that it's ok to have a model with
semantically meaningful aliasing between address spaces, but also that anywhere
we want to reference a local's address with an addrspace(1) pointer (which
is everywhere our source language takes its address), as things stand now we
need either to use an addrspace cast which will be assumed to possibly have
side-effects, or to round-trip through ptrtoint/inttoptr which I presume will
obscure the aliasing information.  It certainly gives us a correct place to
start from, but (unless I'm misunderstanding and the "complex value
modification" type of addrspacecast isn't assumed to have side-effects)
I wouldn't be surprised if we come back to this wanting a way to represent a
cast across address spaces that's as transparent as a bitcast.

Thanks
-Joseph

[1] - http://lists.llvm.org/pipermail/llvm-dev/2015-August/thread.html#89023
[2] - http://llvm.org/docs/LangRef.html#addrspacecast-to-instruction
[3] - http://llvm.org/docs/LangRef.html#data-layout
[4] - http://llvm.org/docs/LangRef.html#i-bitcast


-----Original Message-----
From: Marcello Maggioni [mailto:mmaggioni at apple.com] 
Sent: Monday, August 31, 2015 10:49 PM
To: Joseph Tremoulet <jotrem at microsoft.com>
Cc: Swaroop Sridhar <Swaroop.Sridhar at microsoft.com>; Philip Reames
<listmail at philipreames.com>; llvm-dev <llvm-dev at
lists.llvm.org>; Sanjoy Das <sanjoy at playingwithpointers.com>
Subject: Re: [llvm-dev] RFC: alloca -- specify address space for allocation

Thanks,

this makes the use case much more clear.
Now though, as far as I would like actually to see supported in LLVM the
capability of not having any special meaning assigned to address space 0 your
proposal goes slightly in contrast with how I always thought of address spaces
in LLVM.
I also have to say that I don’t know deeply how address spaces are meant to be
intended in LLVM so my vision of them might be actually off the LLVM-way.

This is how I see them though:
If in OpenCL for example we demark private memory with addrspace(0) and global
memory with addrspace(1) what addrspace 0 or 1 tells me is an information about
the memory pointed by the pointer (whether it is private or global memory). So
it tells me something about the “pointee” and not the pointer.
What you are proposing here though is the opposite if I understood correctly,
which is that actually addrspace(1) is telling us an information about the
pointer (the fact that is a managed pointer) and nothing about the pointee
(which is just some address on the stack I assume).

Still my idea of address spaces and my understanding of what you are trying to
do could still be completely wrong … :P

Marcello
 
> On 30 Aug 2015, at 19:45, Joseph Tremoulet via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> MM> It is not clear to me if these GC pointers are actually living in 
> MM> a different address space when allocated on the stack (aka. you 
> MM> have two different stacks)
> HF> Is the mental model here that a function using this feature actually
has multiple stacks, in different address spaces, and this allows creating local
stack allocations in those non-addrspace-0 stacks?
> PR> the reason we originally went with addrspaces to mark GC pointers 
> PR> was that the managed and non-managed heaps for us are semantically 
> PR> non-overlapping we might be better off introducing an orthogonal 
> PR> notion for tracking gc references entirely.  The addrspace 
> PR> mechanism has worked, but it is a little bit of a hack
> CC> these use cases seem really straight forward for address spaces... 
> CC> But this is very different from the idea of using an alloca with 
> CC> an address spaces for GC pointers to stack objects
> 
> 
> I think it's an important question whether address spaces are a good
fit for what we're trying to model here.  So I'll explain my mental
model of LLILC's address spaces, and I'd be very interested in feedback
on whether this seems like a good fit for LLVM's address space concept, or a
bastardization thereof:
> 
> [preface: it's been my understanding that dereferences of pointers in 
> different address spaces can alias is semantically meaningful ways; 
> i.e. that it's appropriate to use different address spaces to model 
> different means of indexing the same memory.  Some of the 
> comments/questions on this thread seemed to imply instead an 
> expectation that distinct address spaces always reference semantically 
> disjoint storage; if that's a hard assumption, then nothing I'm
about
> to say will make sense and we'll almost certainly need a different 
> mechanism to model GC pointers]
> 
> 1. The value of an unmanaged/addrspace(0) pointer is an address (in 
> the virtual memory available to the process) 2. The value of a
managed/addrspace(1) pointer is conceptually an (ID, offset) pair.  The first
component is the "identity" of the object that the offset component is
relative to.  Identity is distinct from address; you could imagine the GC heap
allocator having a counter that it increments with each allocation, and that an
object's identity is the value the counter had when it was allocated.
> 3. There are two special reserved IDs for pseudo-objects:
>  3a. one reserved ID for the null pseudo-object (i.e. what nullptr 
> points to)  3b. one reserved ID for the 
> "everything-outside-the-GC-heap" pseudo-object.  Conceptually
this is
> an infinitely-large object and the data at offset N in this object is 
> the data at address N (in the virtual memory available to the process, and
with a requirement that N does not correspond to an address in the GC heap) 4.
The benefit of this conceptual model is that garbage collections are
value-preserving, which is why we don't need to model safepoints as
rewriting GC pointers (until the RewriteStatepointsForGC pass where we rewrite
the IR in terms of addresses rather than object identity) 5. We know that the
representation of an addrspace(1) pointer into the outside-the-GC-heap
pseudo-object is bit-identical to the representation of an addrspace(0) pointer
to the corresponding address  (question: Doesn't that mean we can/should be
using bitcast instead of addrspacecast when we know the value in question is an
outside-the-GC-heap pointer?) 6. Our source language includes constructs that
expose a managed pointer's address (which have well-defined semantics only
if the object in question is "pinned").  These naturally correspond to
addrspace casts (or maybe need to be additionally constrained?) and are a
function of not only the input pointer but also of the layout of the GC heap and
the runtime's pinning state.  This is where we can get a mix of addrspace(0)
and addrspace(1) pointers whose dereferences alias (in semantically well-defined
ways).
> 
> Again, I'd love feedback on how sane that sounds to those of you
familiar with LLVM's address space notion.
> 
> I think that 3b is the part that seems to generate the most
surprise/consternation.  The reason our source language includes the
"outside-the-GC-heap" pseudo-object is that it allows more powerful
code (you could think of it as allowing polymorphism over GC-heap-ness of
inputs) with no adverse effects to the system's typesafety guarantees.
> 
> Relating this back to the question of what address space an alloca belongs
in (which I'm doing to contextualize, not in an attempt to continue the
debate), as the stack (of which there is only one) is outside-the-GC-heap, the
question is one of whether you want to conceptualize an alloca as producing an
unmanaged address or as producing a pointer into the outside-the-GC-heap
pseudo-object.  I'd argue that our source language conceptualizes it as the
latter[1], which (as Swaroop points out below) is why it would feel natural to
model it that way in LLVM, but of course (as Swaroop also points out below) we
could also decompose the source construct into two steps in LLVM IR.  As far as
compiler-introduced allocas, they of course wouldn't be referenced in the
source, and so we wouldn't have a hard requirement either way.
> 
> Thanks
> -Joseph
> 
> [1] - To be more pedantic/precise:
> - Static allocas are implied by local variable declarations, and those 
> declarations are not annotated with a pointer type
> - Our source language includes a compact form that can be used to 
> describe a load or store of a local variable; these compact forms are 
> not annotated with a pointer type
> - Everywhere else that our source language refers to the address of a 
> local variable, it uses the managed pointer type
> 
> 
> 
> 
> -----Original Message-----
> From: Swaroop Sridhar
> Sent: Saturday, August 29, 2015 12:30 AM
> To: Philip Reames <listmail at philipreames.com>; llvm-dev 
> <llvm-dev at lists.llvm.org>; Sanjoy Das <sanjoy at
playingwithpointers.com>
> Cc: Joseph Tremoulet <jotrem at microsoft.com>; Andy Ayers 
> <andya at microsoft.com>; Russell Hadley <rhadley at
microsoft.com>
> Subject: RE: RFC: alloca -- specify address space for allocation
> 
>> -----Original Message-----
>> From: Philip Reames [mailto:listmail at philipreames.com]
>> Sent: Friday, August 28, 2015 9:38 AM
>> To: Swaroop Sridhar <Swaroop.Sridhar at microsoft.com>; llvm-dev
<llvm-
>> dev at lists.llvm.org>; Sanjoy Das <sanjoy at
playingwithpointers.com>
>> Cc: Joseph Tremoulet <jotrem at microsoft.com>; Andy Ayers 
>> <andya at microsoft.com>; Russell Hadley <rhadley at
microsoft.com>
>> Subject: Re: RFC: alloca -- specify address space for allocation
>> 
> 
>>> I think for the use case you are outlining, an addrspacecast is the
>>> correct IR model -- you're specifically saying that it is OK in
this
>>> case to turn a pointer from addrspace 0 into one for addrspace N 
>>> because N is your "managed pointer" set that can be
*either* a GC-pointer or a non-GC-pointer.
> 
>>> What the FE is saying is that this is an *acceptable* transition of
>>> addrspace, because your language and runtime semantics have
provided for it.
>>> I think the proper way to say that is with a cast.
> 
>> The key bit here is that I think Chandler is right.  You are 
>> effectively casting a stack allocation *into* a managed pointer.
>> Having something to mark that transition seems reasonable.
> 
> I think there are two views here:
> 
> (1) MSIL level view:
> In CLR, the stack, is a part of "managed memory" (which is not
the same as gc-heap, which is managed and garbage-collected memory).
> Therefore, all *references* to stack locations are "managed
addresses,"  in the sense that the compiler/runtime exercises certain
control over (values that are) managed-address:
> For example: it enforces certain restrictions to guarantee safety -- ex:
lifetime restrictions, non-null requirement in certain contexts, etc.
> 
> This is different from a notion of "unmanaged memory" which is
for interoperability with native code.
> *Pointers* to unmanaged memory are not controlled by the runtime (ex: do
not provide any safety guarantees).
> 
> So, from the language semantics point of view, stack addresses are created
as managed pointers.
> Which is why the proposal is to have alloca directly in the managed
address-space seemed natural.
> 
> Joseph has written more details in the document that Philip shared out in
this thread.
> 
> (2) A more Lower level IR view:
> LLVM creates all stack locations in addrespace(0) for all code, whether it
comes from managed-code or native code.
> Of these, Stack locations corresponding to the managed-stack are promoted
to managed-addresses via addrspacecast.
> As an optimization, the FrontEnd inserts the addrspace casts only for those
stack locations that are actually address-taken.
> 
> If I understand correctly, the recommendation (by Philip, Chandler and
David) is approach (2) because:
> (a) No change to Instruction-Set is necessary when the semantics is
achievable via existing instructions.
> (b) It saves changing the optimizer to allocate in the correct
address-space.
> Looks like the problem here is that: the optimizer is expected to create
type-preserving transformation by allocating in the correct address-space, but
blindly allocates in the default address space today.
> I don't know the LLVM optimizer well enough to have a good estimate of
the magnitude of changes necessary here. But, I agree that (avoiding)
substantial changes to the optimizer is a strong consideration.
> 
>>> You might need N to be a distinct address space from the one used 
>>> for GC-pointers and to have similar casts emitted by the frontend.
> 
> Yes, eventually we'll need to differentiate between:
> (i) Pointers to unmanaged memory -- which will never be reported to 
> the runtime
> (ii) Pointers to GC-heap objects -- which will always be reported to 
> the runtime
> (iii) Generic managed pointer -- which may need to be reported if we cannot
establish that it points outside the GC heap.
> 
> Currently we report all pointers to the runtime as managed pointers.
> This is inefficient because the GC then needs to do extra work to figure
out what kind of pointer it is:
> Pointer to a heap object, pointer within a heap object, or outside the
heap.
> 
>> Of course, having said that all, I'm back to thinking that having a
>> marker on the alloca would be somewhat reasonable too.  However, I 
>> think we need a much stronger justification to change the IR than has 
>> been provided.  If you can show that the cast based model doesn't 
>> work for some reason, we can re-evaluate.
> 
> I don't think we can say that the cast-based model will not work. 
> The question is whether alloca addrspace(1)* is a better fit for MSIL
semantics, analysis phases, and managed-code specific optimizations.
> 
> I'm OK if we conclude that we'll keep using the cast model until we
hit a concrete case where it does not work, or seems architecturally misfit.
> 
>> Worth noting is that we might be better off introducing an orthogonal 
>> notion for tracking gc references entirely.  The addrspace mechanism 
>> has worked, but it is a little bit of a hack. We've talked about
the
>> need for an opaque pointer type.  Maybe when we actually get around 
>> to defining that, the alloca case is one we should consider.
> 
> Yes, I'm mainly concerned about getting the right types on the
different kinds of pointers. If adders space annotation implies more constraints
(ex: on layout) than what's already necessitated by the type distinction, we
should use a separate mechanism.
> 
> Again, I'm OK if we want to keep using addrspacecast until we hit a
concrete case where it breaks down.
> 
> Swaroop.
> 
> 
> _______________________________________________
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> af91ab2d7cd011db47%7c1&sdata=zYbhjIodbTD9pi5YrJouAqZi4ciWLvX2Ai8%2bVJS
> LDbE%3d

On 2 Sep 2015, at 02:54, Joseph Tremoulet via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Reading further, I see both that addrspacecast "can be a no-op cast or
a complex value modification"[2] and that bitcast "may only be [used
on pointers] with the same address space"[4].
> 
> So I'm getting the impression that it's ok to have a model with
semantically meaningful aliasing between address spaces, but also that anywhere
we want to reference a local's address with an addrspace(1) pointer (which
is everywhere our source language takes its address), as things stand now we
need either to use an addrspace cast which will be assumed to possibly have
side-effects, or to round-trip through ptrtoint/inttoptr which I presume will
obscure the aliasing information.  It certainly gives us a correct place to
start from, but (unless I'm misunderstanding and the "complex value
modification" type of addrspacecast isn't assumed to have side-effects)
I wouldn't be surprised if we come back to this wanting a way to represent a
cast across address spaces that's as transparent as a bitcast.
To give a bit of background on that:

The use case for introducing AS casts as distinct from bitcasts (and not going
via inttoptr / ptrtoint) is architectures that have different pointer
representations.  For example, some microcontrollers have a 16-bit PC and 32-bit
address registers, allowing code pointers to be smaller than data pointers. 
Some GPUs (used to?) use different sized pointers for the various different
places in the memory hierarchy.  In our architecture, this is even more
complicated, because we support two different pointer representations:

- 256-bit (or 128-bit, on newer revisions) memory capabilities, that both
identify and grant access to a region of memory and have unforgeability
guaranteed by the hardware.  In LLVM, we represent these as pointers with AS 0.

- 64-bit legacy-compabible pointers that are implicitly relative to a global
capability (and so are only dereferenceable within a restricted range of the
process’ virtual address space).  In LLVM, we represent these as pointers with
AS 0.

For us, an AS cast between AS 0 and AS 200 will succeed if and only if the
address is within the current range of the global capability.  Any address in AS
0 may alias any address in AS 200 (except in some trivial cases, it’s impossible
to determine statically that they don’t), but one value is an integer
interpreted as an address, whereas the other is a fat pointer with bounds and
permissions enforced in hardware.

David