llvm dev - Jun 2016 - Suggestion / Help regarding new calling convention

Dear Community,

To improve current interprocedural register allocation (IPRA) , we have
planned to set callee saved registers to none for local functions,
currently I am doing it in following way:

if (F->hasLocalLinkage()  && !F->hasAddressTaken()) {
   DEBUG(dbgs() << "Function has LocalLinkage \n");
F->setCallingConv(CallingConv::GHC);
 }

but we think threre should be clean and properway to do this perhaps like:

if (F->hasLocalLinkage() && !F->hasAddressTaken()) {
    DEBUG(dbgs() << "Function has LocalLinkage \n");
    F->setCallingConv(CallingConv::NO_Callee_Saved);
  }

So I would like to know any better suggestions and if it is better to add a
new CC for this purpose then what aspects should be considered while
defining a new CC. Actually in this case the new CC does not really
required to define how parameters should be passed or any special rule for
return value etc , it just required to set callee saved registers to be
none. So what are the minimal things required to define such a CC?

Other alternative that I have thought was to add new attribute for function
and use it like following in TargetFrameLowering::determineCalleeSaves()

 // In Naked functions we aren't going to save any registers.
  if (MF.getFunction()->hasFnAttribute(Attribute::Naked))
    return;

Any suggestions / thoughts are welcomed !

Sincerely,
Vivek
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On 6/20/16 9:39 AM, vivek pandya via llvm-dev wrote:
> Dear Community,
>
> To improve current interprocedural register allocation (IPRA) , we 
> have planned to set callee saved registers to none for local 
> functions, currently I am doing it in following way:
>
> if (F->hasLocalLinkage() && !F->hasAddressTaken()) {
As an aside, you might want to analyze how many functions have both 
local linkage and are not address taken.  I recall that many functions 
returned false for hasAddressTaken() because some direct calls casted 
the function to a different function type before calling it.  Such 
functions are still not address taken, but the simple hasAddressTaken() 
method can't determine it.

If you see that happening, you can simply scan through a function's 
def-use chains and see if any "indirect calls" are really direct calls
that cast the function pointer.  I believe SAFECode has some code 
somewhere that does this if you need it.

>    DEBUG(dbgs() << "Function has LocalLinkage \n");
> F->setCallingConv(CallingConv::GHC);
>  }
>
> but we think threre should be clean and properway to do this perhaps like:
>
> if (F->hasLocalLinkage() && !F->hasAddressTaken()) {
>     DEBUG(dbgs() << "Function has LocalLinkage \n");
>     F->setCallingConv(CallingConv::NO_Callee_Saved);
>   }
>
> So I would like to know any better suggestions and if it is better to 
> add a new CC for this purpose then what aspects should be considered 
> while defining a new CC. Actually in this case the new CC does not 
> really required to define how parameters should be passed or any 
> special rule for return value etc , it just required to set callee 
> saved registers to be none. So what are the minimal things required to 
> define such a CC?
>
> Other alternative that I have thought was to add new attribute for 
> function and use it like following in 
> TargetFrameLowering::determineCalleeSaves()
>
>  // In Naked functions we aren't going to save any registers.
>   if (MF.getFunction()->hasFnAttribute(Attribute::Naked))
>     return;
>
> Any suggestions / thoughts are welcomed !
My humble opinion is that you should avoid hacks as they will likely 
break as LLVM changes.  If the GHC calling convention or the naked 
function attribute guarantee that you will always get the behavior that 
you want on all architectures, then go ahead and use them; just make 
sure to add a clear and conspicuous comment explaining why are you using 
them as it is not obvious.

If the GHC calling convention or the naked attribute does not guarantee 
to give you what you need, I'd add an attribute or a calling convention.

That said, I'm not familiar enough with the code generator or these 
attributes/calling conventions to tell you what you should do. You'll 
need input from others more familiar with them.

My two cents,

John Criswell
>
> Sincerely,
> Vivek
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell

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GlobalOpt already does this with fastcc, use it as a guide if you want to
do this:
https://github.com/llvm-mirror/llvm/blob/63b34cdf342b8265d98357008765b2563dd04e6c/lib/Transforms/IPO/GlobalOpt.cpp#L2023

You can't just change the convention of F, you need to update the
convention used at F's call sites.

Unfortunately, while LLVM has PreserveMost and PreservesAll calling
conventions, it does not appear to have an equivalent PreservesNone/Least.

On Mon, Jun 20, 2016 at 7:39 AM, vivek pandya via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Dear Community,
>
> To improve current interprocedural register allocation (IPRA) , we have
> planned to set callee saved registers to none for local functions,
> currently I am doing it in following way:
>
> if (F->hasLocalLinkage()  && !F->hasAddressTaken()) {
>    DEBUG(dbgs() << "Function has LocalLinkage \n");
> F->setCallingConv(CallingConv::GHC);
>  }
>
> but we think threre should be clean and properway to do this perhaps like:
>
> if (F->hasLocalLinkage() && !F->hasAddressTaken()) {
>     DEBUG(dbgs() << "Function has LocalLinkage \n");
>     F->setCallingConv(CallingConv::NO_Callee_Saved);
>   }
>
> So I would like to know any better suggestions and if it is better to add
> a new CC for this purpose then what aspects should be considered while
> defining a new CC. Actually in this case the new CC does not really
> required to define how parameters should be passed or any special rule for
> return value etc , it just required to set callee saved registers to be
> none. So what are the minimal things required to define such a CC?
>
> Other alternative that I have thought was to add new attribute for
> function and use it like following in
> TargetFrameLowering::determineCalleeSaves()
>
>  // In Naked functions we aren't going to save any registers.
>   if (MF.getFunction()->hasFnAttribute(Attribute::Naked))
>     return;
>
> Any suggestions / thoughts are welcomed !
>
> Sincerely,
> Vivek
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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On Mon, Jun 20, 2016 at 8:55 PM, Reid Kleckner <rnk at google.com> wrote:
Hello Reid,
Yes actually idea to identify local function is taken from GlobalOpt : )
> GlobalOpt already does this with fastcc, use it as a guide if you want to
> do this:
>
>
https://github.com/llvm-mirror/llvm/blob/63b34cdf342b8265d98357008765b2563dd04e6c/lib/Transforms/IPO/GlobalOpt.cpp#L2023
>
> You can't just change the convention of F, you need to update the
> convention used at F's call sites.
>
 Thanks for pointing this out , and looking at RemoveNestAttribute() in
GlobalOpt I think even if I add new Function attribute then also it needs
to be updated at every call sites.
> Unfortunately, while LLVM has PreserveMost and PreservesAll calling
> conventions, it does not appear to have an equivalent PreservesNone/Least.
>
Actually we are in favor of adding new CC like No_CSR but what I want to
discuss is how much minimal changes are required and also as I mentioned
for the purpose of IPRA we don't really have any new rules for parameter
passing or return value etc So will it be good to have new CC for this
purpose and if new CC is added is it ok to keep parameter passing and other
rules same as common CC?

I hope my question is clear.

Sincerely,
Vivek
>
> On Mon, Jun 20, 2016 at 7:39 AM, vivek pandya via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Dear Community,
>>
>> To improve current interprocedural register allocation (IPRA) , we have
>> planned to set callee saved registers to none for local functions,
>> currently I am doing it in following way:
>>
>> if (F->hasLocalLinkage()  && !F->hasAddressTaken()) {
>>    DEBUG(dbgs() << "Function has LocalLinkage \n");
>> F->setCallingConv(CallingConv::GHC);
>>  }
>>
>> but we think threre should be clean and properway to do this perhaps
like:
>>
>> if (F->hasLocalLinkage() && !F->hasAddressTaken()) {
>>     DEBUG(dbgs() << "Function has LocalLinkage \n");
>>     F->setCallingConv(CallingConv::NO_Callee_Saved);
>>   }
>>
>> So I would like to know any better suggestions and if it is better to
add
>> a new CC for this purpose then what aspects should be considered while
>> defining a new CC. Actually in this case the new CC does not really
>> required to define how parameters should be passed or any special rule
for
>> return value etc , it just required to set callee saved registers to be
>> none. So what are the minimal things required to define such a CC?
>>
>> Other alternative that I have thought was to add new attribute for
>> function and use it like following in
>> TargetFrameLowering::determineCalleeSaves()
>>
>>  // In Naked functions we aren't going to save any registers.
>>   if (MF.getFunction()->hasFnAttribute(Attribute::Naked))
>>     return;
>>
>> Any suggestions / thoughts are welcomed !
>>
>> Sincerely,
>> Vivek
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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On Mon, Jun 20, 2016 at 8:42 PM, John Criswell <jtcriswel at gmail.com>
wrote:
> On 6/20/16 9:39 AM, vivek pandya via llvm-dev wrote:
>
> Dear Community,
>
> To improve current interprocedural register allocation (IPRA) , we have
> planned to set callee saved registers to none for local functions,
> currently I am doing it in following way:
>
> if (F->hasLocalLinkage()  && !F->hasAddressTaken()) {
>
>
> As an aside, you might want to analyze how many functions have both local
> linkage and are not address taken.  I recall that many functions returned
> false for hasAddressTaken() because some direct calls casted the function
> to a different function type before calling it.  Such functions are still
> not address taken, but the simple hasAddressTaken() method can't
determine
> it.
>
> If you see that happening, you can simply scan through a function's
> def-use chains and see if any "indirect calls" are really direct
calls that
> cast the function pointer.  I believe SAFECode has some code somewhere that
> does this if you need it.
>
> Dear Professor John,
Thanks for pointing out this , but I wonder that how many such cases may be
there on average in a module? Is it too frequently seen?
>
>    DEBUG(dbgs() << "Function has LocalLinkage \n");
> F->setCallingConv(CallingConv::GHC);
>  }
>
> but we think threre should be clean and properway to do this perhaps like:
>
> if (F->hasLocalLinkage() && !F->hasAddressTaken()) {
>     DEBUG(dbgs() << "Function has LocalLinkage \n");
>     F->setCallingConv(CallingConv::NO_Callee_Saved);
>   }
>
> So I would like to know any better suggestions and if it is better to add
> a new CC for this purpose then what aspects should be considered while
> defining a new CC. Actually in this case the new CC does not really
> required to define how parameters should be passed or any special rule for
> return value etc , it just required to set callee saved registers to be
> none. So what are the minimal things required to define such a CC?
>
> Other alternative that I have thought was to add new attribute for
> function and use it like following in
> TargetFrameLowering::determineCalleeSaves()
>
>  // In Naked functions we aren't going to save any registers.
>   if (MF.getFunction()->hasFnAttribute(Attribute::Naked))
>     return;
>
> Any suggestions / thoughts are welcomed !
>
>
> My humble opinion is that you should avoid hacks as they will likely break
> as LLVM changes.  If the GHC calling convention or the naked function
> attribute guarantee that you will always get the behavior that you want on
> all architectures, then go ahead and use them; just make sure to add a
> clear and conspicuous comment explaining why are you using them as it is
> not obvious.
>
> If the GHC calling convention or the naked attribute does not guarantee to
> give you what you need, I'd add an attribute or a calling convention.
>
Actually I am not really in favor of using this kind of hack because using
GHC CC or naked attribute have there other rules (in addition to no callee
saved registers) and that are not really wanted. So it is desired to add
new CC or attribute to achieve the required effect.

Sincerely,
Vivek
>
> That said, I'm not familiar enough with the code generator or these
> attributes/calling conventions to tell you what you should do.  You'll
need
> input from others more familiar with them.
>
> My two cents,
>
> John Criswell
>
>
> Sincerely,
> Vivek
>
>
> _______________________________________________
> LLVM Developers mailing listllvm-dev at
lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
> --
> John Criswell
> Assistant Professor
> Department of Computer Science, University of
Rochesterhttp://www.cs.rochester.edu/u/criswell
>
>
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I just discussed this with vivek on IRC (and I think we agreed on this):

Let me first state the motivation clearly to ease later discussions:
As far as the motivation for this change goes: Changing the calling convention
allows us to choose whether a register is saved by the callee or the caller.
Usually it is best to have a mix of both as too many caller saved registers
leads to unnecessary save/restores when the called function turns out to only
touch a fraction of the registers (as is typically for smaller leaf-functions of
the call graph). While too many callee saved registers may lead to unnecessary
saves/restores of registers even though the calling function didn't have a
live value in the register anyway. With IPRA the first problem is mitigated
since we propagate the actually clobbered set of registers up the callgraph
instead of relying on conventions, so it is best to aim for more caller saved
registers (though we should check for code size increases and store/restore code
being potentially less good than the tuned sequences generated during
FrameLowering).

To the disucssion at hand:
- Introducing a new calling convention at the IR level is the wrong approach:
The calling convention is mostly a contract when calling and being called across
translation unit boundaries. The details about how this contract is fulfilled
are part of CodeGen IMO but do not need to be visible at the IR level.
- The only thing we want to influence here is which registers are saved by the
callee. Changing TargetFrameLowering::determineCalleeSaves() is a good place to
achieve this without affecting unrelated things like parameter and return value
handling which would be part of the calling convention.
- We could experiment with dynamically changing the number of caller saved
registers in the future. I could imagine heuristics like functions called from
many places using some callee saved registers in order to avoid code size
increases because of extra spills/restores at all the call sites. We can hardly
create new calling conventions for these combinations of marking registers as
callee/caller saved.

- Matthias
> On Jun 20, 2016, at 7:39 AM, vivek pandya via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Dear Community,
> 
> To improve current interprocedural register allocation (IPRA) , we have
planned to set callee saved registers to none for local functions, currently I
am doing it in following way:
> 
> if (F->hasLocalLinkage()  && !F->hasAddressTaken()) {
>    DEBUG(dbgs() << "Function has LocalLinkage \n");
> F->setCallingConv(CallingConv::GHC); 
>  }
> 
> but we think threre should be clean and properway to do this perhaps like:
> 
> if (F->hasLocalLinkage() && !F->hasAddressTaken()) {
>     DEBUG(dbgs() << "Function has LocalLinkage \n");
>     F->setCallingConv(CallingConv::NO_Callee_Saved);
>   }
> 
> So I would like to know any better suggestions and if it is better to add a
new CC for this purpose then what aspects should be considered while defining a
new CC. Actually in this case the new CC does not really required to define how
parameters should be passed or any special rule for return value etc , it just
required to set callee saved registers to be none. So what are the minimal
things required to define such a CC?
> 
> Other alternative that I have thought was to add new attribute for function
and use it like following in TargetFrameLowering::determineCalleeSaves()
> 
>  // In Naked functions we aren't going to save any registers.
>   if (MF.getFunction()->hasFnAttribute(Attribute::Naked))
>     return;
> 
> Any suggestions / thoughts are welcomed !
> 
> Sincerely,
> Vivek
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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> On Jun 20, 2016, at 11:12 AM, John Criswell via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> On 6/20/16 9:39 AM, vivek pandya via llvm-dev wrote:
>> Dear Community,
>> 
>> To improve current interprocedural register allocation (IPRA) , we have
planned to set callee saved registers to none for local functions, currently I
am doing it in following way:
>> 
>> if (F->hasLocalLinkage()  && !F->hasAddressTaken()) {
> 
> As an aside, you might want to analyze how many functions have both local
linkage and are not address taken.  I recall that many functions returned false
for hasAddressTaken() because some direct calls casted the function to a
different function type before calling it.  Such functions are still not address
taken, but the simple hasAddressTaken() method can't determine it.
Looks like hasAddressTaken could be updated to handle these simple case maybe?
> 
> If you see that happening, you can simply scan through a function's
def-use chains and see if any "indirect calls" are really direct calls
that cast the function pointer.  I believe SAFECode has some code somewhere that
does this if you need it.
> 
> 
>>    DEBUG(dbgs() << "Function has LocalLinkage \n");
>> F->setCallingConv(CallingConv::GHC); 
>>  }
>> 
>> but we think threre should be clean and properway to do this perhaps
like:
>> 
>> if (F->hasLocalLinkage() && !F->hasAddressTaken()) {
>>     DEBUG(dbgs() << "Function has LocalLinkage \n");
>>     F->setCallingConv(CallingConv::NO_Callee_Saved);
>>   }
>> 
>> So I would like to know any better suggestions and if it is better to
add a new CC for this purpose then what aspects should be considered while
defining a new CC. Actually in this case the new CC does not really required to
define how parameters should be passed or any special rule for return value etc
, it just required to set callee saved registers to be none. So what are the
minimal things required to define such a CC?
>> 
>> Other alternative that I have thought was to add new attribute for
function and use it like following in
TargetFrameLowering::determineCalleeSaves()
>> 
>>  // In Naked functions we aren't going to save any registers.
>>   if (MF.getFunction()->hasFnAttribute(Attribute::Naked))
>>     return;
>> 
>> Any suggestions / thoughts are welcomed !
> 
> My humble opinion is that you should avoid hacks as they will likely break
as LLVM changes.
In-tree code with appropriate tests is unlikely to break. 
But we should avoid hacks anyway because they make LLVM harder to change in
general.


— 
Mehdi

> f the GHC calling convention or the naked function attribute guarantee that
you will always get the behavior that you want on all architectures, then go
ahead and use them; just make sure to add a clear and conspicuous comment
explaining why are you using them as it is not obvious.
> 
> If the GHC calling convention or the naked attribute does not guarantee to
give you what you need, I'd add an attribute or a calling convention.
> 
> That said, I'm not familiar enough with the code generator or these
attributes/calling conventions to tell you what you should do.  You'll need
input from others more familiar with them.
> 
> My two cents,
> 
> John Criswell
> 
>> 
>> Sincerely,
>> Vivek
>> 
>> 
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
> 
> 
> -- 
> John Criswell
> Assistant Professor
> Department of Computer Science, University of Rochester
> http://www.cs.rochester.edu/u/criswell
<http://www.cs.rochester.edu/u/criswell>_______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
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On Tue, Jun 21, 2016 at 12:31 AM, Matthias Braun <matze at braunis.de>
wrote:
> I just discussed this with vivek on IRC (and I think we agreed on this):
>
> Let me first state the motivation clearly to ease later discussions:
> As far as the motivation for this change goes: Changing the calling
> convention allows us to choose whether a register is saved by the callee or
> the caller. Usually it is best to have a mix of both as too many caller
> saved registers leads to unnecessary save/restores when the called function
> turns out to only touch a fraction of the registers (as is typically for
> smaller leaf-functions of the call graph). While too many callee saved
> registers may lead to unnecessary saves/restores of registers even though
> the calling function didn't have a live value in the register anyway.
With
> IPRA the first problem is mitigated since we propagate the actually
> clobbered set of registers up the callgraph instead of relying on
> conventions, so it is best to aim for more caller saved registers (though
> we should check for code size increases and store/restore code being
> potentially less good than the tuned sequences generated during
> FrameLowering).
>
> To the disucssion at hand:
> - Introducing a new calling convention at the IR level is the wrong
> approach: The calling convention is mostly a contract when calling and
> being called across translation unit boundaries. The details about how this
> contract is fulfilled are part of CodeGen IMO but do not need to be visible
> at the IR level.
> - The only thing we want to influence here is which registers are saved by
> the callee. Changing TargetFrameLowering::determineCalleeSaves() is a good
> place to achieve this without affecting unrelated things like parameter and
> return value handling which would be part of the calling convention.
>
Hello Matthias,

As per our discussion, the above trick will make sure that there is no
callee saved registers and also we have thought that
RegUsageInfoCalculator.cpp is having regmask that will make caller to save
restore registers if both callee and caller is using any common register
but this would require following change in RegUsageInfoCalculator.cpp :

if (!F->hasLocalLinkage() || F->hasAddressTaken()) {
      const uint32_t *CallPreservedMask         TRI->getCallPreservedMask(MF,
MF.getFunction()->getCallingConv());
      // Set callee saved register as preserved.
      for (unsigned i = 0; i < RegMaskSize; ++i)
        RegMask[i] = RegMask[i] | CallPreservedMask[i];
  }

because RegUsageInfoCalculator.cpp marks register as preserved if MF's CC
preserves it. But While optimizing for callee saved register we need to
skip above code so that register save/restore code is adder around call
site.

Apart from that my hunch is that IPO inlining of static function also
creates problem for this ( I have this feeling because some test case from
test-suite fails when using -O > 0 ). I am still working on this.

Please share your thoughts on this.

Sincerely,
- Vivek

> - We could experiment with dynamically changing the number of caller saved
> registers in the future. I could imagine heuristics like functions called
> from many places using some callee saved registers in order to avoid code
> size increases because of extra spills/restores at all the call sites. We
> can hardly create new calling conventions for these combinations of marking
> registers as callee/caller saved.
>
> - Matthias
>
> On Jun 20, 2016, at 7:39 AM, vivek pandya via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Dear Community,
>
> To improve current interprocedural register allocation (IPRA) , we have
> planned to set callee saved registers to none for local functions,
> currently I am doing it in following way:
>
> if (F->hasLocalLinkage()  && !F->hasAddressTaken()) {
>    DEBUG(dbgs() << "Function has LocalLinkage \n");
> F->setCallingConv(CallingConv::GHC);
>  }
>
> but we think threre should be clean and properway to do this perhaps like:
>
> if (F->hasLocalLinkage() && !F->hasAddressTaken()) {
>     DEBUG(dbgs() << "Function has LocalLinkage \n");
>     F->setCallingConv(CallingConv::NO_Callee_Saved);
>   }
>
> So I would like to know any better suggestions and if it is better to add
> a new CC for this purpose then what aspects should be considered while
> defining a new CC. Actually in this case the new CC does not really
> required to define how parameters should be passed or any special rule for
> return value etc , it just required to set callee saved registers to be
> none. So what are the minimal things required to define such a CC?
>
> Other alternative that I have thought was to add new attribute for
> function and use it like following in
> TargetFrameLowering::determineCalleeSaves()
>
>  // In Naked functions we aren't going to save any registers.
>   if (MF.getFunction()->hasFnAttribute(Attribute::Naked))
>     return;
>
> Any suggestions / thoughts are welcomed !
>
> Sincerely,
> Vivek
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
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