[llvm-commits] [llvm] r108109 - /llvm/trunk/lib/CodeGen/MachineLICM.cpp

[Evan Cheng]

On Jul 11, 2010, at 5:00 PM, Chris Lattner <sabre at nondot.org> wrote:

> Author: lattner
> Date: Sun Jul 11 19:00:35 2010
> New Revision: 108109
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=108109&view=rev
> Log:
> change machinelicm to use MachineInstr::isSafeToMove.  No
> intended functionality change.
> 
> The avoidance of hoistiing implicitdef seems wrong though.

It's not. We want to avoid creating implicitdef with no trivial live intervals. The trivial ones will be converted to undef machine operands which do not take up registers. 

Evan

> 
> Modified:
>    llvm/trunk/lib/CodeGen/MachineLICM.cpp
> 
> Modified: llvm/trunk/lib/CodeGen/MachineLICM.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/MachineLICM.cpp?rev=108109&r1=108108&r2=108109&view=diff
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/MachineLICM.cpp (original)
> +++ llvm/trunk/lib/CodeGen/MachineLICM.cpp Sun Jul 11 19:00:35 2010
> @@ -127,8 +127,8 @@
>     void AddToLiveIns(unsigned Reg);
> 
>     /// IsLICMCandidate - Returns true if the instruction may be a suitable
> -    /// candidate for LICM. e.g. If the instruction is a call, then it's obviously
> -    /// not safe to hoist it.
> +    /// candidate for LICM. e.g. If the instruction is a call, then it's
> +    /// obviously not safe to hoist it.
>     bool IsLICMCandidate(MachineInstr &I);
> 
>     /// IsLoopInvariantInst - Returns true if the instruction is loop
> @@ -497,26 +497,16 @@
> /// candidate for LICM. e.g. If the instruction is a call, then it's obviously
> /// not safe to hoist it.
> bool MachineLICM::IsLICMCandidate(MachineInstr &I) {
> +  // It is not profitable to hoist implicitdefs.  FIXME: Why not?  what if they
> +  // are an argument to some other otherwise-hoistable instruction?
>   if (I.isImplicitDef())
>     return false;
> -
> -  const TargetInstrDesc &TID = I.getDesc();
> 
> -  // Ignore stuff that we obviously can't hoist.
> -  if (TID.mayStore() || TID.isCall() || TID.isTerminator() ||
> -      TID.hasUnmodeledSideEffects())
> +  // Check if it's safe to move the instruction.
> +  bool DontMoveAcrossStore = true;
> +  if (!I.isSafeToMove(TII, AA, DontMoveAcrossStore))
>     return false;
> -
> -  if (TID.mayLoad()) {
> -    // Okay, this instruction does a load. As a refinement, we allow the target
> -    // to decide whether the loaded value is actually a constant. If so, we can
> -    // actually use it as a load.
> -    if (!I.isInvariantLoad(AA))
> -      // FIXME: we should be able to hoist loads with no other side effects if
> -      // there are no other instructions which can change memory in this loop.
> -      // This is a trivial form of alias analysis.
> -      return false;
> -  }
> +  
>   return true;
> }
> 
> 
> 
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