JIT: Expand simple stfld addresses early

[jakobbotsch]

Alternative to #125126

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[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]

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;

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.