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

[Steve King via llvm-dev]

On Wed, Aug 19, 2015 at 10:58 PM, Mehdi Amini <mehdi.amini at apple.com> wrote:
>
> 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.
>

Yes and also the issue of needing more information to make a smart
isIntDivCheap() decision.

In visitSDIV(), checking looks like this when the denominator is a power of 2.

    if (TLI.isIntDivCheap(N->getValueType(0), MinSize))
      return SDValue();

    // Target-specific implementation of sdiv x, pow2.
    if (SDValue Res = BuildSDIVPow2(N))
      return Res;

How about this for isIntDivCheap?
1) pass Function* to allow the target to decide for itself which
attributes matter, and
2) pass a boolean indicating a power-of-2 denominator.

    const Function* Fn = DAG.getMachineFunction().getFunction();
    if (TLI.isIntDivCheap(N->getValueType(0), Fn, true))
      return SDValue();

    // Target-specific implementation of sdiv x, pow2.
    if (SDValue Res = BuildSDIVPow2(N))
      return Res;