llvm dev - Jul 2016 - RFC: To add __attribute__((regmask("preserve/clobbered list here"))) in clang

Hello Clang and LLVM Devs,

I have been working to add support for an attribute in clang and LLVM that
helps
user to guide interprocedural register allocation. But the use case I am
having
is very limited and thus I belieave it is good to have discussion on this
before
sending a patch.

So for IPRA we have a situation where a function is calling a function
which is
written in assembly and it is not defined in current module. IPRA's scope is
limited to a module so for such externally defined function it uses default
calling convention but here as the function is written in assembly user can
provide exact register usage detials. So we dicided to mark declration of
such
function with __attribute__((regmask("clobbered list here"))) so LLVM
can
construct regmask out of it and use it with IPRA to improve register
allocation.

For this purpose I added support for this attribute in clang and clang
codegen
this attribute as target dependent attribute and add to declaration. Then
IPRA
constructs regmask from this and use during the optimization.

How ever after disussion on IRC with Joerg Sonnenberger and jroelofs we
think
that this may effect other areas too. So it would be better to discuss this
and
implement this as we all agree upon it.

Sincerely,
Vivek
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20160715/e0176bf1/attachment.html>

Sorry I forget to add clang mailing list.

-Vivek

On Fri, Jul 15, 2016 at 11:57 PM, vivek pandya <vivekvpandya at gmail.com>
wrote:
> Hello Clang and LLVM Devs,
>
> I have been working to add support for an attribute in clang and LLVM that
> helps
> user to guide interprocedural register allocation. But the use case I am
> having
> is very limited and thus I belieave it is good to have discussion on this
> before
> sending a patch.
>
> So for IPRA we have a situation where a function is calling a function
> which is
> written in assembly and it is not defined in current module. IPRA's
scope
> is
> limited to a module so for such externally defined function it uses default
> calling convention but here as the function is written in assembly user can
> provide exact register usage detials. So we dicided to mark declration of
> such
> function with __attribute__((regmask("clobbered list here"))) so
LLVM can
> construct regmask out of it and use it with IPRA to improve register
> allocation.
>
> For this purpose I added support for this attribute in clang and clang
> codegen
> this attribute as target dependent attribute and add to declaration. Then
> IPRA
> constructs regmask from this and use during the optimization.
>
> How ever after disussion on IRC with Joerg Sonnenberger and jroelofs we
> think
> that this may effect other areas too. So it would be better to discuss
> this and
> implement this as we all agree upon it.
>
> Sincerely,
> Vivek
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20160716/08619168/attachment.html>

On Fri, Jul 15, 2016 at 11:57:35PM +0530, vivek pandya via llvm-dev
wrote:
> So for IPRA we have a situation where a function is calling a function
> which is written in assembly and it is not defined in current module.
IPRA's scope is
> limited to a module so for such externally defined function it uses default
> calling convention but here as the function is written in assembly user can
> provide exact register usage detials. So we dicided to mark declration of
> such
> function with __attribute__((regmask("clobbered list here"))) so
LLVM can
> construct regmask out of it and use it with IPRA to improve register
> allocation.
This situation is actually far more common and not restricted to
assembly at all. There are a number of functions already that have
special ABIs with much larger set of preserved registers. A typical
is __tls_get_addr in many ABIs. At the moment, we need hacks in the
target specific part of LLVM for handling this. Related (limited)
approaches for this are the preserve_most and preserve_all calling
conventions.

As mentioned in the IRC discussion, there are two important issues to be
considered here from my perspective.

(1) I really dislike an attribute providing a clobber list. Whether a
given register is clobbered or not is an implementation detail of a
specific version and can easily change. It is also something difficult
to reason about. The invariance that should be put into the ABI contract
is the inverse -- what registers a function is going to preserve. That
is even more important when looking at long time ABI stability. New
registers are introduced every so often. That shouldn't change the
meaning of a declaration.

The main reason for using a clobber list seems to be a concern about
verbosity. I think that can be mostly avoided by allowing the use of
register classes in the specifier, e.g. all-fp for i387 register,
all-sse2 for the SSE2 register set, all-avx for the AVX register etc.
At the same time, I consider a certain verbosity to be useful, since
ultimately, implementation and interface definition need to be carefully
compared.

(2) Should the attribute extend or replace the normal preserved
registers? Randomly clobbering registers is going to create all kinds of
fun issues with the backend assumptions. We already have such fun with
inline assembler. Extend-only semantic is much easier to support. It can
also be combined with a special CC with minimal default preservation and
well defined meanings e.g. for arguments passed in registers.

Joerg

> On Jul 15, 2016, at 1:48 PM, Joerg Sonnenberger via llvm-dev <llvm-dev
at lists.llvm.org> wrote:
> 
> On Fri, Jul 15, 2016 at 11:57:35PM +0530, vivek pandya via llvm-dev wrote:
>> So for IPRA we have a situation where a function is calling a function
>> which is written in assembly and it is not defined in current module.
IPRA's scope is
>> limited to a module so for such externally defined function it uses
default
>> calling convention but here as the function is written in assembly user
can
>> provide exact register usage detials. So we dicided to mark declration
of
>> such
>> function with __attribute__((regmask("clobbered list here")))
so LLVM can
>> construct regmask out of it and use it with IPRA to improve register
>> allocation.
> 
> This situation is actually far more common and not restricted to
> assembly at all. There are a number of functions already that have
> special ABIs with much larger set of preserved registers. A typical
> is __tls_get_addr in many ABIs. At the moment, we need hacks in the
> target specific part of LLVM for handling this. Related (limited)
> approaches for this are the preserve_most and preserve_all calling
> conventions.
> 
> As mentioned in the IRC discussion, there are two important issues to be
> considered here from my perspective.
> 
> (1) I really dislike an attribute providing a clobber list. Whether a
> given register is clobbered or not is an implementation detail of a
> specific version and can easily change. It is also something difficult
> to reason about. The invariance that should be put into the ABI contract
> is the inverse -- what registers a function is going to preserve. That
> is even more important when looking at long time ABI stability. New
> registers are introduced every so often. That shouldn't change the
> meaning of a declaration.
Interestingly your last point is the reason why I'd think a clobber list
could be more appropriate for some cases: if I have a hand-written assembly
function, and it is clobbering some registers, the fact that the client code
enables AVX2 won’t make my routine clobbering these.

Maybe a syntax with +/- could be used to express things like “all vector
registers but these”.
> The main reason for using a clobber list seems to be a concern about
> verbosity. I think that can be mostly avoided by allowing the use of
> register classes in the specifier, e.g. all-fp for i387 register,
> all-sse2 for the SSE2 register set, all-avx for the AVX register etc.
> At the same time, I consider a certain verbosity to be useful, since
> ultimately, implementation and interface definition need to be carefully
> compared.
> 
> (2) Should the attribute extend or replace the normal preserved
> registers? Randomly clobbering registers is going to create all kinds of
> fun issues with the backend assumptions. We already have such fun with
> inline assembler. Extend-only semantic is much easier to support. It can
> also be combined with a special CC with minimal default preservation and
> well defined meanings e.g. for arguments passed in registers.
Agree.

Overall I’m unsure how much applicability this attribute feature will have in
practice though, or if it is worth the complexity to support it.

— 
Mehdi

I think that to use this information no pass from IPRA is required but we
can do this earlier that IPRA as we have this information from the source
file we can use it to build regmask for relevant call insertion while
generating MI. Thus register allocators will be benefited automatically
even with out IPRA.

-Vivek

On Sat, Jul 16, 2016 at 12:03 AM, vivek pandya <vivekvpandya at gmail.com>
wrote:
> Sorry I forget to add clang mailing list.
>
> -Vivek
>
> On Fri, Jul 15, 2016 at 11:57 PM, vivek pandya <vivekvpandya at
gmail.com>
> wrote:
>
>> Hello Clang and LLVM Devs,
>>
>> I have been working to add support for an attribute in clang and LLVM
>> that helps
>> user to guide interprocedural register allocation. But the use case I
am
>> having
>> is very limited and thus I belieave it is good to have discussion on
this
>> before
>> sending a patch.
>>
>> So for IPRA we have a situation where a function is calling a function
>> which is
>> written in assembly and it is not defined in current module. IPRA's
scope
>> is
>> limited to a module so for such externally defined function it uses
>> default
>> calling convention but here as the function is written in assembly user
>> can
>> provide exact register usage detials. So we dicided to mark declration
of
>> such
>> function with __attribute__((regmask("clobbered list here")))
so LLVM can
>> construct regmask out of it and use it with IPRA to improve register
>> allocation.
>>
>> For this purpose I added support for this attribute in clang and clang
>> codegen
>> this attribute as target dependent attribute and add to declaration.
Then
>> IPRA
>> constructs regmask from this and use during the optimization.
>>
>> How ever after disussion on IRC with Joerg Sonnenberger and jroelofs we
>> think
>> that this may effect other areas too. So it would be better to discuss
>> this and
>> implement this as we all agree upon it.
>>
>> Sincerely,
>> Vivek
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20160718/20ae3e01/attachment.html>