[LLVMdev] Any plan to add MIN/MAX isd node?
Duncan Sands
baldrick at free.fr
Tue Oct 30 12:10:15 PDT 2012
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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