[llvm-dev] Memory scope proposal
Philip Reames via llvm-dev
llvm-dev at lists.llvm.org
Mon Mar 28 19:17:45 PDT 2016
Ke,
I'll be the bearer of bad news here. The radio silence this proposal
has gotten probably means there is not enough interest in the community
in this proposal to see it land.
One concern I have with the current proposal is that the optimization
value of these scopes is not clear to me. Is it only the backend which
is expected to support optimizations over these scopes? Or are you
expecting the middle end optimizer to understand them? If so, I'd
suspect we'd need a refined definition which allows us to discuss
relative strengths of memory scopes.
More fundamentally, it's not clear to me that "scope" is even the right
model for this. I could see a case where we'd want something along the
lines of "acquire semantics on memory space 1, release semantics on
memory space 2, cst_seq semantics on address space 3".
Also, unless I'm misreading on my skim of your proposal, the current
definition of scope is slightly off from what you've specified. A
"seq_cst singlethread" fence is a much weaker fence than a "seq_cst
crossthread". It's probably easiest to reason about the current scheme
as having the cross product of {singlethread, crossthread} x
{orderings...} distinct orderings rather than a set of orderings with
two overlapping scopes.
Philip
On 03/22/2016 01:42 PM, Ke Bai via llvm-dev wrote:
>
> Dear all,
>
> Here is the plain text version of the proposal:
>
> Currently, the LLVM IR uses a binary value (SingleThread/CrossThread)
> to represent synchronization scope on atomic instructions. We would
> like to enhance the representation of memory scopes in LLVM IR to
> allow more values than just the current two. The intention of this
> email is to invite comments on our proposal. There are some discussion
> before and it can be found here:
> https://groups.google.com/forum/#!searchin/llvm-dev/hsail/llvm-dev/46eEpS5h0E4/i3T9xw-DNVYJ
> <https://groups.google.com/forum/#%21searchin/llvm-dev/hsail/llvm-dev/46eEpS5h0E4/i3T9xw-DNVYJ>
>
> Here is our new proposal:
>
> =================================================================
> We still let the bitcode store memory scopes as "unsigned integers",
> since that is the easiest way to maintain compatibility. The values 0
> and 1 are special. All other values are meaningful only within that bc
> file. In addition, "a global metadata in the file" will provide a map
> from unsigned integers to string symbols which should be used to
> interpret all the non-standard integers. If the global metadata is
> empty or non-existent, then all non-zero values will be mapped to
> "system", which is the current behavior.
>
> The proposed syntax for synchronization scope is as follows:
> * Synchronization scopes are of arbitrary width, but implemented as
> unsigned in the bitcode, just like address spaces.
> * Cross-thread is default.
> * Keyword "singlethread" is unchanged
> * New syntax "synchscope(n)" for other target-specific scopes.
> * There is no keyword for cross-thread, but it can be specified as
> "synchscope(0)".
>
> The proposed new integer implementation expanded synchronization
> scopes are as follows:
> ****************************************************************
> Format
>
> Single Thread
>
> System (renamed)
>
> Intermediate
> Bitcode
>
> zero
>
> one
>
> unsigned n
> Assembly
>
> singlethread,
>
> empty (default),
>
> synchscope(n-1)
>
> synchscope(~0U)
>
> synchscope(0)
> In-memory
>
> ~0U
> z
> ero
>
> unsigned n-1
> SelectionDAG
>
> ~0U
>
> zero
>
> unsigned n-1
> ****************************************************************
>
> The choice of “~0U” for singlethread makes it easy to maintain
> backward compatibility in the bitcode. The values 0 and 1 remain
> unchanged in the bitcode, and the reader simply decrements them by one
> to compute the correct value in the in-memory data-structure.
>
> Name Mapping
>
> Now we comes to name mapping from integers to strings. If a CLANG
> front end wants to map a language that has memory scopes (e.g. OpenCL)
> to LLVM IR, how does it determine what syncscopes to use? Without any
> rules, each target can define its own meaning for the scopes, can give
> them any name, and can map them to the LLVM-IR unit values in any way.
> In this case, I think each target have to provide a mapping function
> that maps a specific language’s name for a scope into that targets
> name for a scope that has conservatively the same semantics. Namely,
> the act of supporting a new language that has memory scopes requires
> every target to support that language to be updated accordingly.
>
> Therefore, in order to allow front end writers to share memory scope
> definitions when they match to avoid the effort of updating all
> targets for each language,it's better to define standard memory scope
> names. A target is free to implement them or not, but if a target does
> implement them they must have the defined relational semantics (e.g.,
> hierarchical nesting). If a target does implement them then it will be
> able to support any language that uses them, including languages not
> yet invented. A new memory scope name can be added if the existing
> ones are insufficient.
>
> With the first try, we can define the standard scopes with what a
> common language that has memory scopes needs, e.g., OpenCL uses
> system, device, workgroup, workitem. It uses the same approach as LLVM
> has done for debug information. There are standard debug entities
> (that a common language (C) needs), and each new language uses those
> standard entities where there is a match, and subsequently defines
> only the delta.
>
> A bitcode example with the proposal
> *****************************************************************
> define void <at> test(i32* %addr) {
> ; forward compatibility
> cmpxchg i32* %addr, i32 42, i32 0 singlethread monotonic monotonic
>
> ; new synchscope that will be defined by each backend
> cmpxchg i32* %addr, i32 42, i32 0 synchscope(2) monotonic monotonic, 2
> cmpxchg i32* %addr, i32 42, i32 0 synchscope(3) monotonic monotonic, 3
>
> ret void
> }
>
> !synchscope = metadata !{{i32 0, !"SingleThread"}, {i32 2,
> !"WorkGroup"}, ...}
> *****************************************************************
>
> =================================================================
>
> On Thu, Jan 28, 2016 at 12:27 PM, Ke Bai <kebai613 at gmail.com
> <mailto:kebai613 at gmail.com>> wrote:
>
> Hi all,
>
> Currently, the LLVM IR uses a binary value
> (SingleThread/CrossThread) to represent synchronization scope on
> atomic instructions. We would like to enhance the representation
> of memory scopes in LLVM IR to allow more values than just the
> current two. The intention of this email is to invite comments on
> our proposal. There are some discussion before and it can be found
> here:
>
> https://groups.google.com/forum/#!searchin/llvm-dev/hsail/llvm-dev/46eEpS5h0E4/i3T9xw-DNVYJ
> <https://groups.google.com/forum/#%21searchin/llvm-dev/hsail/llvm-dev/46eEpS5h0E4/i3T9xw-DNVYJ>
>
>
> Here is our new proposal:
>
> =================================================================
> We still let the bitcode store memory scopes as *unsigned
> integers*, since that is the easiest way to maintain
> compatibility. The values 0 and 1 are special. All other values
> are meaningful only within that bc file. In addition, *a global
> metadata in the **file will provide a map* from unsigned integers
> to string symbols which should be used to interpret all the
> non-standard integers. If the global metadata is empty or
> non-existent, then all non-zero values will be mapped to "system",
> which is the current behavior.
> The proposed syntax for synchronization scope is as follows:
>
> * Synchronization scopes are of arbitrary width, but implemented
> as unsigned in the bitcode, just like address spaces.
> * Cross-thread is default.
> * Keyword "singlethread" is unchanged
> * New syntax "synchscope(n)" for other target-specific scopes.
> * There is no keyword for cross-thread, but it can be specified
> as "synchscope(0)".
>
> The proposed new integer implementation expanded synchronization
> scopes are as follows:
> *Format*
>
> *Single Thread*
>
> *System (renamed)*
>
> *Intermediate*
> *Bitcode*
>
> zero
>
> one
>
> unsigned n
> *Assembly*
>
> singlethread,
> synchscope(~0U)
>
> empty (default),
> synchscope(0)
>
> synchscope(n-1)
> *In-memory*
>
> ~0U
>
> zero
>
> unsigned n-1
> *SelectionDAG*
>
> ~0U
>
> zero
>
> unsigned n-1
>
> The choice of “~0U” for singlethread makes it easy to maintain
> backward compatibility in the bitcode. The values 0 and 1 remain
> unchanged in the bitcode, and the reader simply decrements them by
> one to compute the correct value in the in-memory data-structure.
>
> Name Mapping
>
> Now we comes to name mapping from integers to strings. If a CLANG
> front end wants to map a language that has memory scopes (e.g.
> OpenCL) to LLVM IR, how does it determine what syncscopes to use?
> Without any rules, each target can define its own meaning for the
> scopes, can give them any name, and can map them to the LLVM-IR
> unit values in any way. In this case, I think each target have to
> provide a mapping function that maps a specific language’s name
> for a scope into that targets name for a scope that has
> conservatively the same semantics. Namely, the act of supporting a
> new language that has memory scopes requires every target to
> support that language to be updated accordingly.
>
> Therefore, in order to allow front end writers to share memory
> scope definitions when they match to avoid the effort of updating
> all targets for each language,it's better to define standard
> memory scope names. A target is free to implement them or not, but
> if a target does implement them they must have the defined
> relational semantics (e.g., hierarchical nesting). If a target
> does implement them then it will be able to support any language
> that uses them, including languages not yet invented. A new memory
> scope name can be added if the existing ones are insufficient.
>
> With the first try, we can define the standard scopes with what a
> common language that has memory scopes needs, e.g., OpenCL uses
> system, device, workgroup, workitem. It uses the same approach as
> LLVM has done for debug information. There are standard debug
> entities (that a common language (C) needs), and each new language
> uses those standard entities where there is a match, and
> subsequently defines only the delta.
>
> *A **bitcode example with the proposal*
> define void <at> test(i32* %addr) {
> ; forward compatibility
> cmpxchg i32* %addr, i32 42, i32 0 singlethread monotonic monotonic
>
> ; new synchscope that will be defined by each backend
> cmpxchg i32* %addr, i32 42, i32 0 synchscope(2) monotonic
> monotonic, 2
> cmpxchg i32* %addr, i32 42, i32 0 synchscope(3) monotonic
> monotonic, 3
>
> ret void
> }
>
> !synchscope = metadata !{{i32 0, !"SingleThread"}, {i32 2,
> !"WorkGroup"}, ...}
> =================================================================
> Thank you!
> ---
> Best regards,
> Ke
>
>
>
>
> --
> Best Regard,
> Ke Bai, Ph.D.
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160328/950ac85b/attachment-0001.html>
More information about the llvm-dev
mailing list