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EgorBo wants to merge 5 commits into from
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Improve IsKnownConstant #84002

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b2d407f
Improve IsKnownConstant
EgorBo Mar 28, 2023
f4708cd
Update lower.cpp
EgorBo Mar 28, 2023
d4bb2b3
Update src/coreclr/jit/lower.cpp
EgorBo Mar 28, 2023
a8f20d9
Merge branch 'main' of github.com:dotnet/runtime into improve-isknown…
EgorBo Mar 28, 2023
e473e6d
less conservative VN impl
EgorBo Mar 28, 2023
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5 changes: 5 additions & 0 deletions src/coreclr/jit/gentree.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5145,6 +5145,11 @@ unsigned Compiler::gtSetEvalOrder(GenTree* tree)
costSz = 12;
break;

case NI_System_Runtime_CompilerServices_RuntimeHelpers_IsKnownConstant:
costEx = 1;
costSz = 1;
break;

case NI_System_Math_Abs:
costEx = 5;
costSz = 15;
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25 changes: 22 additions & 3 deletions src/coreclr/jit/lower.cpp
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Expand Up @@ -583,11 +583,9 @@ GenTree* Lowering::LowerNode(GenTree* node)
ContainCheckLclHeap(node->AsOp());
break;

#ifdef TARGET_XARCH
case GT_INTRINSIC:
ContainCheckIntrinsic(node->AsOp());
LowerIntrinsic(node->AsIntrinsic());
break;
#endif // TARGET_XARCH

#ifdef FEATURE_HW_INTRINSICS
case GT_HWINTRINSIC:
Expand Down Expand Up @@ -7786,6 +7784,27 @@ void Lowering::TransformUnusedIndirection(GenTreeIndir* ind, Compiler* comp, Bas
}
}

//------------------------------------------------------------------------
// LowerIntrinsic: a common logic to lower INTRINSIC.
//
// Arguments:
// node - the INTRINSIC node we are lowering.
//
void Lowering::LowerIntrinsic(GenTreeIntrinsic* node)
{
if (node->gtIntrinsicName == NI_System_Runtime_CompilerServices_RuntimeHelpers_IsKnownConstant)
{
// IsKnownConstant is expected to be folded in Importer/Morph/VN+ConstantProp
// This path is just in case if VN+ConstantProp didn't fold them for some reason (or e.g. they were disabled)
Comment on lines +7797 to +7798
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@SingleAccretion SingleAccretion Mar 28, 2023

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This is "not enough"...

Consider a case where VN assigns this 1 (i. e. the operand is constant). Then CSE CSEs this as a def with some other unrelated uses of the value 1. Then lowering comes along, and we get 0 instead of 1.

Essentially, each subsequent phase can only further "refine" the value of this intrinsic. The tricky part here is of course that by the time we get to lowering, VNs are no longer valid. So this needs a separate "lower IsKnownConstant" phase, that would use the still-valid VNs. Bit yucky.

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@EgorBo EgorBo Mar 28, 2023

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I agree, but the current code is correct, isn't it? it justs conservatively assumes that input is not a constant so we end up in the non-constant path.

I agree that it'd be nice to have a separate phase for such transforms somewhere after assert prop with still valid VNs. Maybe I'll add one eventuallly (and move unrollings there e.g.)

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@SingleAccretion SingleAccretion Mar 28, 2023

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but the current code is correct, isn't it?

No - consider the CSE example. We cannot have this assigned 1 in VN and then lower it to zero.

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@EgorBo EgorBo Mar 28, 2023

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but the current code is correct, isn't it?

No - consider the CSE example. We cannot have this assigned 1 in VN and then lower it to zero.

Ah, I see what you mean. I'll implement it in a separate phase if we find a real use case for it

node->gtGetOp1()->SetUnusedValue();
node->BashToConst(0);
return;
}
#ifdef TARGET_XARCH
ContainCheckIntrinsic(node->AsOp());
#endif
}

//------------------------------------------------------------------------
// LowerBlockStoreCommon: a common logic to lower STORE_OBJ/BLK/DYN_BLK.
//
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1 change: 1 addition & 0 deletions src/coreclr/jit/lower.h
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Expand Up @@ -312,6 +312,7 @@ class Lowering final : public Phase
bool LowerUnsignedDivOrMod(GenTreeOp* divMod);
GenTree* LowerConstIntDivOrMod(GenTree* node);
GenTree* LowerSignedDivOrMod(GenTree* node);
void LowerIntrinsic(GenTreeIntrinsic* node);
void LowerBlockStore(GenTreeBlk* blkNode);
void LowerBlockStoreCommon(GenTreeBlk* blkNode);
void ContainBlockStoreAddress(GenTreeBlk* blkNode, unsigned size, GenTree* addr, GenTree* addrParent);
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22 changes: 4 additions & 18 deletions src/coreclr/jit/morph.cpp
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Expand Up @@ -10010,8 +10010,8 @@ GenTree* Compiler::fgMorphSmpOp(GenTree* tree, MorphAddrContext* mac, bool* optA
if (tree->AsIntrinsic()->gtIntrinsicName ==
NI_System_Runtime_CompilerServices_RuntimeHelpers_IsKnownConstant)
{
// Should be expanded by the time it reaches CSE phase
assert(!optValnumCSE_phase);
// Should be expanded in importer for MinOpts
assert(opts.OptimizationEnabled());

JITDUMP("\nExpanding RuntimeHelpers.IsKnownConstant to ");
if (op1->OperIsConst() || gtIsTypeof(op1))
Expand All @@ -10023,22 +10023,8 @@ GenTree* Compiler::fgMorphSmpOp(GenTree* tree, MorphAddrContext* mac, bool* optA
}
else
{
GenTree* op1SideEffects = nullptr;
gtExtractSideEffList(op1, &op1SideEffects, GTF_ALL_EFFECT);
if (op1SideEffects != nullptr)
{
DEBUG_DESTROY_NODE(tree);
// Keep side-effects of op1
tree = gtNewOperNode(GT_COMMA, TYP_INT, op1SideEffects, gtNewIconNode(0));
JITDUMP("false with side effects:\n")
DISPTREE(tree);
}
else
{
JITDUMP("false\n");
DEBUG_DESTROY_NODE(tree, op1);
tree = gtNewIconNode(0);
}
// Leave it for VN/ConstantProp (or Lowering)
return tree;
}
INDEBUG(tree->gtDebugFlags |= GTF_DEBUG_NODE_MORPHED);
return tree;
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6 changes: 6 additions & 0 deletions src/coreclr/jit/valuenum.cpp
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Expand Up @@ -11047,6 +11047,12 @@ void Compiler::fgValueNumberIntrinsic(GenTree* tree)
intrinsic->gtVNPair = vnStore->VNPWithExc(newVNP, excSet);
}
}
else if (intrinsic->gtIntrinsicName == NI_System_Runtime_CompilerServices_RuntimeHelpers_IsKnownConstant)
{
ValueNum isCnsVNL = vnStore->VNForIntCon(vnStore->IsVNConstant(arg0VNP.GetLiberal()) ? 1 : 0);
ValueNum isCnsVNC = vnStore->VNForIntCon(vnStore->IsVNConstant(arg0VNP.GetConservative()) ? 1 : 0);
intrinsic->gtVNPair = vnStore->VNPWithExc(ValueNumPair(isCnsVNL, isCnsVNC), arg0VNPx);
}
else
{
assert(intrinsic->gtIntrinsicName == NI_System_Object_GetType);
Expand Down