[llvm] r273545 - [IfConversion] Bugfix: Don't use undef flag while adding use operands.

[Peter Collingbourne via llvm-commits]

Hi Jonas,

I reverted this in r273707 as it caused PR28295.

I should have a reproducer for you soon.

Thanks,
Peter

On Thu, Jun 23, 2016 at 1:13 AM, Jonas Paulsson via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> Author: jonpa
> Date: Thu Jun 23 03:13:20 2016
> New Revision: 273545
>
> URL: http://llvm.org/viewvc/llvm-project?rev=273545&view=rev
> Log:
> [IfConversion] Bugfix: Don't use undef flag while adding use operands.
>
> IfConversion used to always add the undef flag when adding a use operand
> on a newly predicated instruction. This would be an operand for the
> register
> being conditionally redefined. Due to the undef flag, the liveness of this
> register prior to the predicated instruction would get lost.
>
> This patch changes this so that such use operands are added only when the
> register is live, without the undef flag.
>
> Reviewed by Quentin Colombet.
> http://reviews.llvm.org/D209077
>
> Modified:
>     llvm/trunk/lib/CodeGen/IfConversion.cpp
>
> Modified: llvm/trunk/lib/CodeGen/IfConversion.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/IfConversion.cpp?rev=273545&r1=273544&r2=273545&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/IfConversion.cpp (original)
> +++ llvm/trunk/lib/CodeGen/IfConversion.cpp Thu Jun 23 03:13:20 2016
> @@ -1046,8 +1046,19 @@ void IfConverter::RemoveExtraEdges(BBInf
>  }
>
>  /// Behaves like LiveRegUnits::StepForward() but also adds implicit uses
> to all
> -/// values defined in MI which are not live/used by MI.
> +/// values defined in MI which are also live/used by MI.
>  static void UpdatePredRedefs(MachineInstr &MI, LivePhysRegs &Redefs) {
> +  const TargetRegisterInfo *TRI = MI.getParent()->getParent()
> +    ->getSubtarget().getRegisterInfo();
> +
> +  // Before stepping forward past MI, remember which regs were live
> +  // before MI. This is needed to set the Undef flag only when reg is
> +  // dead.
> +  SparseSet<unsigned> LiveBeforeMI;
> +  LiveBeforeMI.setUniverse(TRI->getNumRegs());
> +  for (auto &Reg : Redefs)
> +    LiveBeforeMI.insert(Reg);
> +
>    SmallVector<std::pair<unsigned, const MachineOperand*>, 4> Clobbers;
>    Redefs.stepForward(MI, Clobbers);
>
> @@ -1061,7 +1072,8 @@ static void UpdatePredRedefs(MachineInst
>      if (Op.isRegMask()) {
>        // First handle regmasks.  They clobber any entries in the mask
> which
>        // means that we need a def for those registers.
> -      MIB.addReg(Reg.first, RegState::Implicit | RegState::Undef);
> +      if (LiveBeforeMI.count(Reg.first))
> +        MIB.addReg(Reg.first, RegState::Implicit);
>
>        // We also need to add an implicit def of this register for the
> later
>        // use to read from.
> @@ -1078,7 +1090,8 @@ static void UpdatePredRedefs(MachineInst
>        if (Redefs.contains(Op.getReg()))
>          Op.setIsDead(false);
>      }
> -    MIB.addReg(Reg.first, RegState::Implicit | RegState::Undef);
> +    if (LiveBeforeMI.count(Reg.first))
> +      MIB.addReg(Reg.first, RegState::Implicit);
>    }
>  }
>
>
>
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-- 
-- 
Peter
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