[LLVMdev] Any plan to add MIN/MAX isd node?

[Evan Cheng]

On Oct 30, 2012, at 1:39 PM, Yin Ma <yinma at codeaurora.org> wrote:

> Hi Duncan,
> 
>     Yes, exactly. However, we need define Opcode MIN/MAX
> Into ISDOpcodes.h. Do you like to add those two definitions
> Into the tree?

No, it's only desirable to have opcodes if we intend to add generic lowering and optimizations for them. Since min / max can be represented with select, there is no need. Please use target specific opcodes to handle them.

Evan

> 
> Thanks,
> 
>                   Yin 
> 
> -----Original Message-----
> From: Duncan Sands [mailto:duncan.sands at gmail.com] On Behalf Of Duncan Sands
> Sent: Tuesday, October 30, 2012 12:10 PM
> To: Yin Ma
> Cc: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Any plan to add MIN/MAX isd node?
> 
> Hi Yin,
> 
>>     To use select, usually, there is a compare before select.
>> Presence of comparison will disable some opportunities to optimize 
>> some code. Select and Compare is not associative neither.
> 
> at the IR level LLVM already has pattern matching helpers for identifying
> min/max idioms, here is part of a transform using this, from
> InstructionSimplify.cpp:
> 
>   // Signed variants on "max(a,b)>=a -> true".
>   if (match(LHS, m_SMax(m_Value(A), m_Value(B))) && (A == RHS || B == RHS))
> {
>     if (A != RHS) std::swap(A, B); // smax(A, B) pred A.
>     EqP = CmpInst::ICMP_SGE; // "A == smax(A, B)" iff "A sge B".
>     // We analyze this as smax(A, B) pred A.
>     P = Pred;
>   }
> 
> At the codegen level, the same could be done.  Also, if a target has a
> max/min instruction it can transform compare+select to max/min, in fact I'm
> pretty sure some targets do this already.
> 
> Ciao, Duncan.
> 
>> 
>> Thanks,
>> 
>>                        Yin
>> 
>> -----Original Message-----
>> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] 
>> On Behalf Of Duncan Sands
>> Sent: Tuesday, October 30, 2012 12:38 AM
>> To: llvmdev at cs.uiuc.edu
>> Subject: Re: [LLVMdev] Any plan to add MIN/MAX isd node?
>> 
>> Hi Yin,
>> 
>>>        Do you have any plan to add MIN/MAX in ISD node?
>>> 
>>> Atomic operations have Min/Max operator already. This should
>>> 
>>> be easy to add Min/Max node in ISD list. Because after
>>> 
>>> converting control flow into Min/Max operator, it will
>>> 
>>> result in better optimization because associative attribute
>>> 
>>> and can be moved around.
>>> 
>>>       Could you give some opinions on this issue?
>> 
>> what are the advantages over using "select" (or one of its codegen 
>> variants)?
>> 
>> Ciao, Duncan.
>> 
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> 
> 
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