[llvm-dev] [RFC] Improving integer divide optimization (related to D12082)
Mehdi Amini via llvm-dev
llvm-dev at lists.llvm.org
Wed Aug 19 22:58:25 PDT 2015
> 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,
> // Add (N0 < 0) ? abs2 - 1 : 0;
> SDValue SRL =
> DAG.getNode(ISD::SRL, DL, VT, SGN,
> DAG.getConstant(VT.getScalarSizeInBits() - lg2, DL,
> SDValue ADD = DAG.getNode(ISD::ADD, DL, VT, N0, SRL);
> AddToWorklist(ADD.getNode()); // Divide by pow2
> SDValue SRA = DAG.getNode(ISD::SRA, DL, VT, ADD,
> DAG.getConstant(lg2, DL,
> 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.
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the llvm-dev