[AMDGPU] Fix -amdgpu-inline-arg-alloca-cost

[rampitec]

Summary

Before D94153 this threshold was in a pre-scaled units.
After D94153 inlining threshold multiplier is not applied
to this portion of the threshold anymore. Restore the
threshold by applying the multiplier.

[arsenm]

llvm/lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp

1192

Should the just adjust for the scale then?

[rampitec]

llvm/lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp

1192

I thought about this, but whenever we will adjust the scale the next time we will have to visit it again.

[dfukalov]

Do you have any test for the fix?

llvm/lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp

1192

Nit: It seems instead of this modification you can just swap two lines

1582:  Threshold *= TTI.getInliningThresholdMultiplier();
1583:  Threshold += TTI.adjustInliningThreshold(&Call);

in InlineCost.cpp so we'll stay with just one place of * getInliningThresholdMultiplier().

[rampitec]

In D98362#2619375, @dfukalov wrote:

Do you have any test for the fix?

These tests tend to be either unreliable or huge. We cannot measure performance with lit tests.

llvm/lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp

1192

That would change behavior for all targets.

[arsenm]

llvm/lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp

1192

I thought the point of the multiplier was to just amplify the expense of calls. I don't understand scaling up the cost here

[rampitec]

llvm/lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp

1192

It's more like a uniform target cost multiplier which shall be applied to everything. But probably Daniil is correct and I have to swap two lines in the inliner instead, until noone uses it as is.

[rampitec]

Swapped order of operations in the InlineCost itself instead. This is still unused by any other target so it is possible to fix it there, which seems to be a more correct way.

[rampitec]

Added test.

[dfukalov]

Thanks!

llvm/lib/Target/AMDGPU/AMDGPUTargetTransformInfo.h

public:
   unsigned getVectorSplitCost() { return 0; }
 
   unsigned getShuffleCost(TTI::ShuffleKind Kind, VectorType *Tp, int Index,
                           VectorType *SubTp);
 
   bool areInlineCompatible(const Function *Caller,
                            const Function *Callee) const;
 
-  unsigned getInliningThresholdMultiplier() { return 11; }
+  unsigned getInliningThresholdMultiplier() const { return 11; }
   unsigned adjustInliningThreshold(const CallBase *CB) const;
 
   int getInlinerVectorBonusPercent() { return 0; }
 
   int getArithmeticReductionCost(
       unsigned Opcode,
       VectorType *Ty,
       bool IsPairwise,

llvm/lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp

if (const AllocaInst *AI = dyn_cast<AllocaInst>(PtrArg)) {
       // to get rid of the scratch anyway, bail out.
       if (AllocaSize > ArgAllocaCutoff) {
         AllocaSize = 0;
         break;
       }
     }
   }
   if (AllocaSize)
-    return ArgAllocaCost;
+    return ArgAllocaCost * getInliningThresholdMultiplier();
   return 0;
 }
 
 void GCNTTIImpl::getUnrollingPreferences(Loop *L, ScalarEvolution &SE,
                                          TTI::UnrollingPreferences &UP) {
   CommonTTI.getUnrollingPreferences(L, SE, UP);
 }