[LLVMdev] Suggestion About Adding Target Dependent Decision in LSR Please

[Yin Ma]

Hi Andy,

 

Actually, if we just add hooks that preserves the existing behavior, 

It is not difficult. For example, 

 

For case one, we can define one function like

  virtual const SCEV* getTargetPreferredWinnerReg(const SCEV*& ScaledReg,

           SmallVector<const SCEV *, 4>& BaseRegs, GlobalValue*& BaseGV)
const;

 

In NarrowSearchSpaceByPickingWinnerRegs, we can preserves the winner

reg from target and winner reg from the original algorithm if this function 

returns NULL, it is just like before 

 

For case two, we can define a general cost from TTI function, like

  virtual int getLSRFormulaCost(const unsigned NumRegs,

                            const unsigned AddRecCost, const unsigned
NumIVMuls,

                            const unsigned NumBaseAdds, const unsigned
ImmCost,

                            const unsigned SetupCost) const;

Then we do something like 

  int thisCost = TTI->getLSRFormulaCost(NumRegs, AddRecCost, NumIVMuls,

                                           NumBaseAdds, ImmCost, SetupCost);

  if (thisCost >= 0) {

    int otherCost = TTI->getLSRFormulaCost(Other.NumRegs, Other.AddRecCost,

                                            Other.NumIVMuls,
Other.NumBaseAdds,

                                            Other.ImmCost, Other.SetupCost);

    if (otherCost >= 0)

      return thisCost < otherCost;

  }

In bool Cost::operator<(const Cost &Other) const 

 

We could have more decision from target backend.

 

However, from the problem I am dealing with, which has a lot of branches in
multiple level

Loop nests. LSR is still not able to perform the best because 

1.       LSR is not control flow sensitive. It treats all USE equally, which
doesn't care which 

USE is on critical path and which USE is on a branch. Without these kind of
information,

We cannot predict AddRec precisely because we only can assume all USEs can
be post

Increment or all not.

2.       The most occurred winner regs pruning may not be the best approach.
Because target

may prefer certain regs than others, even some registers do occur more.
Specially, 

register with small computation is more likely to occur in formulas.
However, register

with small computation may not always be a best choice if the content in
register are

loop invariant.

 

Therefore,  We may need a systemic agreement or plan to address the existing
LSR problems. I

would like to ask if any party has any improvement plan about LSR? So we can
come together

to have an unified solution to handle all known problem in one round?

 

Thanks,

 

Yin 

 

 

From: Andrew Trick [mailto:atrick at apple.com] 
Sent: Thursday, March 14, 2013 9:42 AM
To: Yin Ma
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Suggestion About Adding Target Dependent Decision in
LSR Please

 

 

On Mar 13, 2013, at 4:37 PM, Yin Ma <yinma at codeaurora.org> wrote:





Hi All,

 

In the target I am working, we comes cross a situation that the loop
strength reduction

could deliver a better result but currently not, because

1.       the algorithm narrows search space by winner registers without
considering

the target preferred format. (NarrowSearchSpaceByPickingWinnerRegs)

2.       Cost comparison solely favors the number register without
considering other

Impacts.

 

For the case one,

NarrowSearchSpaceByPickingWinnerRegs filters by most occurred registers.

ld(basereg, immediate) is a target preferred addressing mode. However, it
may

be deleted because basereg is very likely not to be the most occurred
register

because the less opportunity in a combination.

 

For the case two, by observing the cost comparison equation

bool Cost::operator<(const Cost &Other) const {

  if (NumRegs != Other.NumRegs)                            return NumRegs <
Other.NumRegs;

  if (AddRecCost != Other.AddRecCost)                  return AddRecCost <
Other.AddRecCost;

  if (NumIVMuls != Other.NumIVMuls)                   return NumIVMuls <
Other.NumIVMuls;

  if (NumBaseAdds != Other.NumBaseAdds)       return NumBaseAdds <
Other.NumBaseAdds;

  if (ImmCost != Other.ImmCost)                               return ImmCost
< Other.ImmCost;

  if (SetupCost != Other.SetupCost)                         return SetupCost
< Other.SetupCost;

  return false;

}

 

If we have a case to compare

Cost at 5 regs, with addrec cost 1, plus 15 base adds, plus 1 imm cost, plus
4 setup cost.

Cost at 4 regs, with addrec cost 1, plus 28 base adds, plus 1 imm cost, plus
2 setup cost.

The current mode will select 4 regs case even there are 14 more base adds.
And base

Adds matters in our targets.

 

So I think the current LSR should be pushing more decision into target
dependent backend.

Like calling new functions in TargetTransformInfo. At least, in narrow
search space and cost

comparison phase, or more in cost rating phase. LSR can be tightly cooped
with the target

attributes in order to get the most beneficial result.

 

Yes. LSR decisions are tightly coupled with the target architecture and
potentially the subtarget microarcthitecture. As you figured out, the way to
handle it is to communicate more information to LSR through TTI. It's easy
to do this to solve individual benchmarks on your target. It's hard to know
if you have a general solution that works across targets. But if you can add
hooks in a way that preserves existing behavior on other targets it
shouldn't be a problem. We want to design general hooks, but leave it up to
someone doing the benchmarking to tune them for a particular target.

 

-Andy

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