[CodeGen] EVT::changeVectorElementType can't handle conversion from simple to extended vector types

[RKSimon]

As discussed here: #171459 (comment)

llvm-project/llvm/include/llvm/CodeGen/ValueTypes.h

Lines 100 to 109 in 578a26a

	/// Return a VT for a vector type whose attributes match ourselves
	/// with the exception of the element type that is chosen by the caller.
	EVT changeVectorElementType(EVT EltVT) const {
	if (isSimple()) {
	assert(EltVT.isSimple() &&
	"Can't change simple vector VT to have extended element VT");
	return getSimpleVT().changeVectorElementType(EltVT.getSimpleVT());
	}
	return changeExtendedVectorElementType(EltVT);
	}

The problem is if the original vector was simple (e.g. v512i32) - but you change it to one that isn't (e.g. v512i64) - MVT::changeVectorElementType is called and will assert internally:

llvm-project/llvm/include/llvm/CodeGenTypes/MachineValueType.h

Lines 211 to 218 in 578a26a

	/// Return a VT for a vector type whose attributes match ourselves
	/// with the exception of the element type that is chosen by the caller.
	MVT changeVectorElementType(MVT EltVT) const {
	MVT VecTy = MVT::getVectorVT(EltVT, getVectorElementCount());
	assert(VecTy.SimpleTy != MVT::INVALID_SIMPLE_VALUE_TYPE &&
	"Simple vector VT not representable by simple integer vector VT!");
	return VecTy;
	}

Ideally we'd do something similar to EVT::getVectorVT, check for MVT::INVALID_SIMPLE_VALUE_TYPE and fallback to changeExtendedVectorElementType if necessary.

This would then allow quite a few EVT::getVectorVT uses in DAGCombine etc. to be simplified to call EVT::changeElementType or EVT::changeVectorElementType directly without fear of an assert

[llvmbot]

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[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Simon Pilgrim (RKSimon)

Details

As discussed here: https://github.com//pull/171459#discussion_r2606039181

llvm-project/llvm/include/llvm/CodeGen/ValueTypes.h

Lines 100 to 109 in 578a26a

	/// Return a VT for a vector type whose attributes match ourselves
	/// with the exception of the element type that is chosen by the caller.
	EVT changeVectorElementType(EVT EltVT) const {
	if (isSimple()) {
	assert(EltVT.isSimple() &&
	"Can't change simple vector VT to have extended element VT");
	return getSimpleVT().changeVectorElementType(EltVT.getSimpleVT());
	}
	return changeExtendedVectorElementType(EltVT);
	}

The problem is if the original vector was simple (e.g. v512i32) - but you change it to one that isn't (e.g. v512i64) - MVT::changeVectorElementType is called and will assert internally:

llvm-project/llvm/include/llvm/CodeGenTypes/MachineValueType.h

Lines 211 to 218 in 578a26a

	/// Return a VT for a vector type whose attributes match ourselves
	/// with the exception of the element type that is chosen by the caller.
	MVT changeVectorElementType(MVT EltVT) const {
	MVT VecTy = MVT::getVectorVT(EltVT, getVectorElementCount());
	assert(VecTy.SimpleTy != MVT::INVALID_SIMPLE_VALUE_TYPE &&
	"Simple vector VT not representable by simple integer vector VT!");
	return VecTy;
	}

Ideally we'd do something similar to EVT::getVectorVT, check for MVT::INVALID_SIMPLE_VALUE_TYPE and fallback to changeExtendedVectorElementType if necessary.

This would then allow quite a few EVT::getVectorVT uses in DAGCombine etc. to be simplified to call EVT::changeElementType or EVT::changeVectorElementType directly without fear of an assert

[Islam-Imad]

Hi @RKSimon :)
Could you assign this issue to me, please :) ?