[llvm-dev] [RFC] Improving integer divide optimization (related to D12082)

[Mehdi Amini via llvm-dev]

> On Aug 19, 2015, at 3:48 PM, escha via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 
>> On Aug 19, 2015, at 1:45 PM, Steve King via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> In the targets I know, shifts are
>> cheaper than divides in both speed and size.
> 
> From what I remember, udiv by power of 2 already gets turned into a shift in instcombine; the tricky case is sdiv by power of 2, which takes significantly more than one instruction. The generic implementation is this:
> 
>     // Splat the sign bit into the register
>     SDValue SGN =
>         DAG.getNode(ISD::SRA, DL, VT, N0,
>                     DAG.getConstant(VT.getScalarSizeInBits() - 1, DL,
>                                     getShiftAmountTy(N0.getValueType())));
>     AddToWorklist(SGN.getNode());
> 
>     // Add (N0 < 0) ? abs2 - 1 : 0;
>     SDValue SRL =
>         DAG.getNode(ISD::SRL, DL, VT, SGN,
>                     DAG.getConstant(VT.getScalarSizeInBits() - lg2, DL,
>                                     getShiftAmountTy(SGN.getValueType())));
>     SDValue ADD = DAG.getNode(ISD::ADD, DL, VT, N0, SRL);
>     AddToWorklist(SRL.getNode());
>     AddToWorklist(ADD.getNode());    // Divide by pow2
>     SDValue SRA = DAG.getNode(ISD::SRA, DL, VT, ADD,
>                   DAG.getConstant(lg2, DL,
>                                   getShiftAmountTy(ADD.getValueType())));
> 
> And there’s already a target-specific hook to add a custom implementation (e.g. on PowerPC):
> 
>     // Target-specific implementation of sdiv x, pow2.
>     if (SDValue Res = BuildSDIVPow2(N))
>       return Res;


Isn’t the problem the fact that the patch makes it harder for a target to get the generic code to reach its custom hook?
Now the "cheap pow2 sdiv” is merged with the generic “cheap div” you can’t distinguish anymore.

— 
Mehdi

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