llvm dev - Sep 2015 - [PATCH] D12923: Add support for function attribute "notail"

To be clear, this is a debuging aid only?  It's not something required 
for correctness?  I'm somewhat bothered by that because it seems like it 
would be a useful implementation tool for higher level languages.

A couple of thoughts in no particular order:
1) Can we always annotate the call site rather than the function? That 
removes the unpredictability due to optimization.
2) Calling it something like "no-direct-tail-call" or
"prefer-no-tail"
would remove some of the confusion value.  When I see "notail", I
expect
that to always be respected; the best effort semantics come as a bit of 
a surprise.
3) This seems analogous to the "tail" marker in that it indicates a 
preference/option.  Whatever we end up with, it needs to be a verifier 
option to have a "tail" or "musttail" call site which is
also "notail".
It also needs to be an error to have a mustail callsite to a notail 
function (if such ends up existing.)
4) It somewhat feels like there are two concepts being intermixed here.  
1) A call site which will never be a tail call.  2) A function which we 
prefer not to tail call to.  Does it make sense to separate them?

Philip

On 09/21/2015 06:22 PM, Akira Hatanaka wrote:
> Several users have been asking for this function attribute to prevent 
> losing the calling functions's information in the backtrace. If we 
> attach the attribute to a function, ideally we would want to prevent 
> tail call optimization on all call sites that call the function. 
> However, the compiler cannot always tell which function is called from 
> a call site if it's an indirect call, so it's fine if an indirect
call
> to the marked function ends up being tail-call optimized. For direct 
> calls, we want the function attribute to prevent tail call 100% of the 
> time.
>
> We can also use a "notail" marker on the call instruction instead
of
> using a function attribute. The only downside of using a marker is 
> that we probably will never be able to prevent tail call optimization 
> on indirect calls even when the compiler can turn it into a direct 
> call (for example, via inlining). I'm not sure at the moment how 
> important this is.
>
> On Thu, Sep 17, 2015 at 9:47 AM, Philip Reames via llvm-commits 
> <llvm-commits at lists.llvm.org <mailto:llvm-commits at
lists.llvm.org>> wrote:
>
>     +llvm-dev
>
>     Can you give a bit of background on what you're trying to address
>     here?  After reading through the discussion and seeing that this
>     is a best effort flag, I'm not sure that a function attribute is
>     the best way to describe this.  I'm open to being convinced it is,
>     but I'd like to hear a bit more about the use case and get broader
>     visibility on the proposal first.
>
>     Philip
>
>
>     On 09/16/2015 07:27 PM, Akira Hatanaka via llvm-commits wrote:
>>     ahatanak created this revision.
>>     ahatanak added a subscriber: llvm-commits.
>>
>>     This patch adds support for a new IR function attribute
"notail". The attribute is used to disable tail call optimization on
calls to functions marked with the attribute.
>>
>>     This attribute is different from the existing attribute
"disable-tail-calls", which disables tail call optimizations on all
call sites within the marked function.
>>
>>     The patch to add support for the corresponding source-level
function attribute is here:
>>     http://reviews.llvm.org/D12922
>>
>>     http://reviews.llvm.org/D12923
>>
>>     Files:
>>        docs/LangRef.rst
>>        include/llvm/Bitcode/LLVMBitCodes.h
>>        include/llvm/IR/Attributes.h
>>        include/llvm/IR/Instructions.h
>>        lib/AsmParser/LLLexer.cpp
>>        lib/AsmParser/LLParser.cpp
>>        lib/AsmParser/LLToken.h
>>        lib/Bitcode/Reader/BitcodeReader.cpp
>>        lib/Bitcode/Writer/BitcodeWriter.cpp
>>        lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
>>        lib/IR/Attributes.cpp
>>        lib/IR/Verifier.cpp
>>        lib/Transforms/Scalar/TailRecursionElimination.cpp
>>        test/Bindings/llvm-c/Inputs/invalid.ll.bc
>>        test/Bindings/llvm-c/invalid-bitcode.test
>>        test/Bitcode/attributes.ll
>>        test/Bitcode/invalid.ll
>>        test/Bitcode/invalid.ll.bc
>>        test/CodeGen/X86/attr-notail.ll
>>        test/Transforms/TailCallElim/notail.ll
>>
>>
>>
>>     _______________________________________________
>>     llvm-commits mailing list
>>     llvm-commits at lists.llvm.org <mailto:llvm-commits at
lists.llvm.org>
>>     http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>
>
>     _______________________________________________
>     llvm-commits mailing list
>     llvm-commits at lists.llvm.org <mailto:llvm-commits at
lists.llvm.org>
>     http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20150922/23634da1/attachment.html>

On Tue, Sep 22, 2015 at 8:31 AM, Philip Reames <listmail at
philipreames.com>
wrote:
> To be clear, this is a debuging aid only?  It's not something required
for
> correctness?  I'm somewhat bothered by that because it seems like it
would
> be a useful implementation tool for higher level languages.
>
>
It's not purely a debugging aid that helps when you are using the debugger.
There are projects (that are not debuggers) that rely on not missing frames
to produce results that are useful.

> A couple of thoughts in no particular order:
> 1) Can we always annotate the call site rather than the function?  That
> removes the unpredictability due to optimization.
>
Annotating the call site should be fine. For the use cases that we care
about, it probably isn't important to prevent tail calls on indirect calls.

> 2) Calling it something like "no-direct-tail-call" or
"prefer-no-tail"
> would remove some of the confusion value.  When I see "notail", I
expect
> that to always be respected; the best effort semantics come as a bit of a
> surprise.
>
I agree. A name that indicates it's only a best effort option or it's an
option that affects only direct calls would be good.

>
> 3) This seems analogous to the "tail" marker in that it indicates
a
> preference/option.  Whatever we end up with, it needs to be a verifier
> option to have a "tail" or "musttail" call site which
is also "notail".  It
> also needs to be an error to have a mustail callsite to a notail function
> (if such ends up existing.)
>
If we are going to annotate the function, I think we should have the
verifier catch incompatibilities between the markers on the call sites and
the function attribute on the called functions.

If we are annotating the call site, the verifier check isn't needed since
the tail-call related markers are enums that are mutually exclusive.

> 4) It somewhat feels like there are two concepts being intermixed here.
> 1) A call site which will never be a tail call.  2) A function which we
> prefer not to tail call to.  Does it make sense to separate them?
>
>
Yes, it makes sense to separate them. For the use case we care about,
either 1) or 2) will do. We don't have to have support for both.

Philip
>
>
> On 09/21/2015 06:22 PM, Akira Hatanaka wrote:
>
> Several users have been asking for this function attribute to prevent
> losing the calling functions's information in the backtrace. If we
attach
> the attribute to a function, ideally we would want to prevent tail call
> optimization on all call sites that call the function. However, the
> compiler cannot always tell which function is called from a call site if
> it's an indirect call, so it's fine if an indirect call to the
marked
> function ends up being tail-call optimized. For direct calls, we want the
> function attribute to prevent tail call 100% of the time.
>
> We can also use a "notail" marker on the call instruction instead
of using
> a function attribute. The only downside of using a marker is that we
> probably will never be able to prevent tail call optimization on indirect
> calls even when the compiler can turn it into a direct call (for example,
> via inlining). I'm not sure at the moment how important this is.
>
> On Thu, Sep 17, 2015 at 9:47 AM, Philip Reames via llvm-commits <
> <llvm-commits at lists.llvm.org>llvm-commits at lists.llvm.org>
wrote:
>
>> +llvm-dev
>>
>> Can you give a bit of background on what you're trying to address
here?
>> After reading through the discussion and seeing that this is a best
effort
>> flag, I'm not sure that a function attribute is the best way to
describe
>> this.  I'm open to being convinced it is, but I'd like to hear
a bit more
>> about the use case and get broader visibility on the proposal first.
>>
>> Philip
>>
>>
>> On 09/16/2015 07:27 PM, Akira Hatanaka via llvm-commits wrote:
>>
>> ahatanak created this revision.
>> ahatanak added a subscriber: llvm-commits.
>>
>> This patch adds support for a new IR function attribute
"notail". The attribute is used to disable tail call optimization on
calls to functions marked with the attribute.
>>
>> This attribute is different from the existing attribute
"disable-tail-calls", which disables tail call optimizations on all
call sites within the marked function.
>>
>> The patch to add support for the corresponding source-level function
attribute is here:http://reviews.llvm.org/D12922
>> http://reviews.llvm.org/D12923
>>
>> Files:
>>   docs/LangRef.rst
>>   include/llvm/Bitcode/LLVMBitCodes.h
>>   include/llvm/IR/Attributes.h
>>   include/llvm/IR/Instructions.h
>>   lib/AsmParser/LLLexer.cpp
>>   lib/AsmParser/LLParser.cpp
>>   lib/AsmParser/LLToken.h
>>   lib/Bitcode/Reader/BitcodeReader.cpp
>>   lib/Bitcode/Writer/BitcodeWriter.cpp
>>   lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
>>   lib/IR/Attributes.cpp
>>   lib/IR/Verifier.cpp
>>   lib/Transforms/Scalar/TailRecursionElimination.cpp
>>   test/Bindings/llvm-c/Inputs/invalid.ll.bc
>>   test/Bindings/llvm-c/invalid-bitcode.test
>>   test/Bitcode/attributes.ll
>>   test/Bitcode/invalid.ll
>>   test/Bitcode/invalid.ll.bc
>>   test/CodeGen/X86/attr-notail.ll
>>   test/Transforms/TailCallElim/notail.ll
>>
>>
>>
>>
>> _______________________________________________
>> llvm-commits mailing listllvm-commits at
lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>
>>
>>
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20150923/3d65f367/attachment-0001.html>

On 09/23/2015 08:48 AM, Akira Hatanaka wrote:
> On Tue, Sep 22, 2015 at 8:31 AM, Philip Reames 
> <listmail at philipreames.com <mailto:listmail at
philipreames.com>> wrote:
>
>     To be clear, this is a debuging aid only?  It's not something
>     required for correctness?  I'm somewhat bothered by that because
>     it seems like it would be a useful implementation tool for higher
>     level languages.
>
>
> It's not purely a debugging aid that helps when you are using the 
> debugger. There are projects (that are not debuggers) that rely on not 
> missing frames to produce results that are useful.
If it's not simply best effort, that constrains our
choices.
>
>     A couple of thoughts in no particular order:
>     1) Can we always annotate the call site rather than the function? 
>     That removes the unpredictability due to optimization.
>
>
> Annotating the call site should be fine. For the use cases that we 
> care about, it probably isn't important to prevent tail calls on 
> indirect calls.
Given this, I would lean towards a notail value being added as an 
alternative to "tail" and "musttail".  This seems to fit the
existing
uses, doesn't have any obvious loop holes or best effort semantics, and 
solves the problem at hand.
>
>     2) Calling it something like "no-direct-tail-call" or
>     "prefer-no-tail" would remove some of the confusion value. 
When I
>     see "notail", I expect that to always be respected; the best
>     effort semantics come as a bit of a surprise.
>
>
> I agree. A name that indicates it's only a best effort option or
it's
> an option that affects only direct calls would be good.
(This only applies if we're talking about a function annotation. The 
call site annotation applies to both direct and indirect
calls.)
>
>
>     3) This seems analogous to the "tail" marker in that it
indicates
>     a preference/option.  Whatever we end up with, it needs to be a
>     verifier option to have a "tail" or "musttail" call
site which is
>     also "notail".  It also needs to be an error to have a
mustail
>     callsite to a notail function (if such ends up existing.)
>
>
> If we are going to annotate the function, I think we should have the 
> verifier catch incompatibilities between the markers on the call sites 
> and the function attribute on the called functions.
>
> If we are annotating the call site, the verifier check isn't needed 
> since the tail-call related markers are enums that are mutually exclusive.
Yep.
>
>     4) It somewhat feels like there are two concepts being intermixed
>     here.  1) A call site which will never be a tail call. 2) A
>     function which we prefer not to tail call to.  Does it make sense
>     to separate them?
>
>
> Yes, it makes sense to separate them. For the use case we care about, 
> either 1) or 2) will do. We don't have to have support for both.
I would lean toward doing (1) for now.  We can come back and implement 
(2) at a later time if we find it's needed.  After (1), each call site 
will have four states:
- "notail" - Can not be a tail call.
- "" - May be a tail call if analysis finds it legal, profitable, and 
desirable*
- "tail" - May be a tail call, profitability hinted
- "musttail" - Must be a tail call.

* (2) would basically just change the desirability of moving from ""
to
"tail".
>
>     Philip
>
>
>     On 09/21/2015 06:22 PM, Akira Hatanaka wrote:
>>     Several users have been asking for this function attribute to
>>     prevent losing the calling functions's information in the
>>     backtrace. If we attach the attribute to a function, ideally we
>>     would want to prevent tail call optimization on all call sites
>>     that call the function. However, the compiler cannot always tell
>>     which function is called from a call site if it's an indirect
>>     call, so it's fine if an indirect call to the marked function
>>     ends up being tail-call optimized. For direct calls, we want the
>>     function attribute to prevent tail call 100% of the time.
>>
>>     We can also use a "notail" marker on the call instruction
instead
>>     of using a function attribute. The only downside of using a
>>     marker is that we probably will never be able to prevent tail
>>     call optimization on indirect calls even when the compiler can
>>     turn it into a direct call (for example, via inlining). I'm not
>>     sure at the moment how important this is.
>>
>>     On Thu, Sep 17, 2015 at 9:47 AM, Philip Reames via llvm-commits
>>     <llvm-commits at lists.llvm.org
>>     <mailto:llvm-commits at lists.llvm.org>> wrote:
>>
>>         +llvm-dev
>>
>>         Can you give a bit of background on what you're trying to
>>         address here?  After reading through the discussion and
>>         seeing that this is a best effort flag, I'm not sure that a
>>         function attribute is the best way to describe this.  I'm
>>         open to being convinced it is, but I'd like to hear a bit
>>         more about the use case and get broader visibility on the
>>         proposal first.
>>
>>         Philip
>>
>>
>>         On 09/16/2015 07:27 PM, Akira Hatanaka via llvm-commits wrote:
>>>         ahatanak created this revision.
>>>         ahatanak added a subscriber: llvm-commits.
>>>
>>>         This patch adds support for a new IR function attribute
"notail". The attribute is used to disable tail call optimization on
calls to functions marked with the attribute.
>>>
>>>         This attribute is different from the existing attribute
"disable-tail-calls", which disables tail call optimizations on all
call sites within the marked function.
>>>
>>>         The patch to add support for the corresponding source-level
function attribute is here:
>>>         http://reviews.llvm.org/D12922
>>>
>>>         http://reviews.llvm.org/D12923
>>>
>>>         Files:
>>>            docs/LangRef.rst
>>>            include/llvm/Bitcode/LLVMBitCodes.h
>>>            include/llvm/IR/Attributes.h
>>>            include/llvm/IR/Instructions.h
>>>            lib/AsmParser/LLLexer.cpp
>>>            lib/AsmParser/LLParser.cpp
>>>            lib/AsmParser/LLToken.h
>>>            lib/Bitcode/Reader/BitcodeReader.cpp
>>>            lib/Bitcode/Writer/BitcodeWriter.cpp
>>>            lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
>>>            lib/IR/Attributes.cpp
>>>            lib/IR/Verifier.cpp
>>>            lib/Transforms/Scalar/TailRecursionElimination.cpp
>>>            test/Bindings/llvm-c/Inputs/invalid.ll.bc
>>>            test/Bindings/llvm-c/invalid-bitcode.test
>>>            test/Bitcode/attributes.ll
>>>            test/Bitcode/invalid.ll
>>>            test/Bitcode/invalid.ll.bc
>>>            test/CodeGen/X86/attr-notail.ll
>>>            test/Transforms/TailCallElim/notail.ll
>>>
>>>
>>>
>>>         _______________________________________________
>>>         llvm-commits mailing list
>>>         llvm-commits at lists.llvm.org <mailto:llvm-commits at
lists.llvm.org>
>>>         http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>
>>
>>         _______________________________________________
>>         llvm-commits mailing list
>>         llvm-commits at lists.llvm.org <mailto:llvm-commits at
lists.llvm.org>
>>         http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20150923/6fc927e7/attachment-0001.html>