JIT: Refine assert for RMW check in crc32 and hw intrinsic codegen

[jakobbotsch]

The crc32 HW intrinsic is a 2 operand node that produces a register; however, the underlying instruction is an RMW instruction that destructively modifies op1. For that reason the codegen needs to issue a move of op1 into the target register before crc32 is issued.

There is an assert that this mov does not overwrite op2 in the process. When LSRA builds uses for the operands it needs to mark op2 as delay freed to ensure op2 and the target do not end up in the same register. However, due to optimizations LSRA may skip this marking when op1 is a last use. Thus we can end up hitting the assert.

The most logical solution seems to be to actually ensure that op2 is always delay freed, such that it never shares the register with the target. However, the handling for other RMW nodes (like GT_SUB) does not have similar logic to this; instead it relies on the situation to not happen except for one particular case where it is ok: when op1 and op2 are actually the same local. This PR enhances the assert to match the checks that happen for other RMW nodes.

Also enhance a similar assert for arm64 HW intrinsics.

Fix #108601
Fix #117612

[jakobbotsch]

This matches the logic in genCodeForBinary from here:

runtime/src/coreclr/jit/codegenxarch.cpp

Lines 1150 to 1167 in e32148a

	#ifdef DEBUG
	unsigned lclNum1 = (unsigned)-1;
	unsigned lclNum2 = (unsigned)-2;
	GenTree* op1Skip = op1->gtSkipReloadOrCopy();
	GenTree* op2Skip = op2->gtSkipReloadOrCopy();
	if (op1Skip->OperIsLocalRead())
	{
	lclNum1 = op1Skip->AsLclVarCommon()->GetLclNum();
	}
	if (op2Skip->OperIsLocalRead())
	{
	lclNum2 = op2Skip->AsLclVarCommon()->GetLclNum();
	}
	assert(GenTree::OperIsCommutative(oper) || (lclNum1 == lclNum2));
	#endif

To be honest I do not see the constraint properly encoded in LSRA for the common RMW nodes or for crc32, which is a bit scary. In other words, if we have IR like

/- op1 LCL_VAR V01 last use
/- op2 LCL_VAR V02
GT_SUB

then I think op2 will have a use created that is not marked delay freed. That means the following register assignment is perfectly allowable:

/- op1 LCL_VAR V01 last use rdx
/- op2 LCL_VAR V02 rcx
GT_SUB rcx

however, this assignment would hit the assert in genCodeForBinary. The only reason this does not happen seems to be due to preferencing making it so that LSRA ends up not picking this assignment.

[Copilot]

Pull Request Overview

This PR refines an assertion in the CRC32 hardware intrinsic code generation to handle edge cases where LSRA optimizations may cause operand registers to overlap with the target register. The fix aligns the CRC32 assert with the handling used for other RMW (read-modify-write) operations.

Key changes:

• Enhanced the assertion in CRC32 codegen to allow the case where both operands are the same local variable
• Extracted and reused common logic for checking if two trees represent the same local variable
• Simplified existing duplicate assertion logic in binary operation codegen

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/jit/hwintrinsiccodegenxarch.cpp	Enhanced CRC32 assertion to include same-local-variable check
src/coreclr/jit/codegenxarch.cpp	Added genIsSameLocalVar helper function and simplified binary operation assertion
src/coreclr/jit/codegen.h	Added declaration for the new genIsSameLocalVar helper function

[jakobbotsch]

@tannergooding can you please take another look at this?

Also cc @dotnet/jit-contrib and PTAL @amanasifkhalid

[jakobbotsch]

/ba-g Out of space infra failure