JIT: Expand simple stfld addresses early

[jakobbotsch]

Alternative to #125126

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

This PR updates CoreCLR JIT import/morph logic to preserve IL evaluation order for stfld when the field address involves a large offset (which may require an explicit null check), and adds a regression test to cover the scenario.

Changes:

• Add a new JIT regression test (Runtime_125124) validating that RHS evaluation is not reordered past a null check for large-offset stfld.
• Teach the importer to expand “simple” instance field store addresses early (when the offset is not “big”), avoiding the need for an explicit null check in those cases.
• Add impCanReorderWithNullCheck helper and adjust String.get_Length intrinsic import by removing a redundant GTF_EXCEPT annotation.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/tests/JIT/Regression/Regression_ro_1.csproj	Adds the new regression test source file to the test project.
src/tests/JIT/Regression/JitBlue/Runtime_125124/Runtime_125124.cs	New regression test covering stfld RHS vs null-check ordering for large offsets.
src/coreclr/jit/lclmorph.cpp	Updates local address recognition to account for GT_LCL_ADDR offsets during struct field promotion/address morphing.
src/coreclr/jit/importercalls.cpp	Removes redundant manual GTF_EXCEPT marking for String.get_Length (array-length node already annotates exceptions).
src/coreclr/jit/importer.cpp	Adds reorderability helper and implements early expansion of small-offset stfld addresses with updated spilling logic.
src/coreclr/jit/compiler.h	Declares the new impCanReorderWithNullCheck helper.

[Copilot]

impCanReorderWithNullCheck only rejects GTF_PERSISTENT_SIDE_EFFECTS (ASG/CALL), but trees with GTF_ORDER_SIDEEFF (e.g., GT_CATCH_ARG) also cannot be legally reordered. Consider also checking GTF_ORDER_SIDEEFF here (or using a broader side-effect mask) so stfld spilling decisions remain correct in handlers and other ordering-sensitive cases.

Suggested change

	if ((tree->gtFlags & GTF_PERSISTENT_SIDE_EFFECTS) != 0)
	if ((tree->gtFlags & (GTF_PERSISTENT_SIDE_EFFECTS | GTF_ORDER_SIDEEFF)) != 0)

[Copilot]

The Return Value comment reads awkwardly: "...whether a null check threw or before the specified node." Consider rewording to clearly express the intended ordering (e.g., "threw before or after" / "was performed before or after").

Suggested change

	// True if it would be unobservable whether a null check threw or before the
	// specified node.
	// True if it would be unobservable whether a null check threw before or after
	// the specified node.

[Copilot]

lvFieldAccessed is only set when obj->IsLclVarAddr() (i.e., GT_LCL_ADDR with offset 0). If obj is a GT_LCL_ADDR with a non-zero offset (address of an interior field of a struct local), this is still a field access that can inform struct promotion decisions. Consider setting lvFieldAccessed for any GT_LCL_ADDR (or otherwise ensuring interior-address cases are covered) to avoid inconsistent promotion behavior.

Suggested change

	if (obj->IsLclVarAddr())
	if (obj->OperIs(GT_LCL_ADDR))

[Copilot]

The test relies on the initial value of the static flag s_barCalled. To make the test self-contained (and resilient to potential re-runs within the same process), consider explicitly resetting s_barCalled to false at the start of TestEntryPoint before executing the scenario.

Suggested change

	{
	{
	s_barCalled = false;