JIT: Handle FIELD_LIST for tailcall arg/parameter interference checks

[jakobbotsch]

Since tailcalls place the arguments in the incoming parameter area it corrupts the value of existing parameters to the function. To handle this there is logic in place to create defensive copies of the parameters if we detect that we will need those later in the tailcalling sequence. However, this logic was missing handling when the argument was a FIELD_LIST:

• When an argument was a FIELD_LIST we did not take into account that we needed to look for parameter uses starting from its operands, and not just from the FIELD_LIST itself. Fix this by introducing a FirstOperand function and use it here.

• When the value of a PUTARG_STK was a contained FIELD_LIST we did not take into account that its codegen may invalidate the values of its own operands. Detect this case and introduce temps for it.

Fix #122138

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[Copilot]

Pull request overview

This PR fixes a bug in the JIT compiler's tailcall handling where FIELD_LIST nodes were not properly accounted for during argument/parameter interference checks. When a tailcall places arguments in the incoming parameter area, it can corrupt existing parameters, so the JIT must create defensive copies when needed. The fix introduces a FirstOperand helper function to find the earliest operand of a node (handling contained nodes recursively), and updates the interference detection logic to properly handle FIELD_LIST cases.

Key Changes

• Added FirstOperand helper function to recursively find the earliest evaluated operand of a node, including contained operands
• Updated LowerFastTailCall to use FirstOperand instead of gtGetOp1() to properly handle FIELD_LIST nodes
• Enhanced interference detection to check for self-interfering non-atomic copies when the source is contained

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/jit/lower.h	Declares the new FirstOperand helper function
src/coreclr/jit/lower.cpp	Implements FirstOperand and updates tailcall interference logic to use it; adds debugging output and improves comments
src/tests/JIT/Regression/Regression_ro_1.csproj	Adds the new regression test to the project file
src/tests/JIT/Regression/JitBlue/Runtime_122138/Runtime_122138.cs	Regression test that verifies correct handling of nullable parameters in tailcall scenarios

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @EgorBo

Diffs. Some minor regressions where we now introduce some more defensive copying.

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @AndyAyersMS

Minor diffs

(Edit: oh, already tagged above)

[jakobbotsch]

/backport to release/10.0

[github-actions]

Started backporting to release/10.0 (link to workflow run)

[github-actions]

@jakobbotsch backporting to release/10.0 failed, the patch most likely resulted in conflicts. Please backport manually!

git am output

$ git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patch

Applying: JIT: Handle `FIELD_LIST` for tailcall arg/parameter interference checks
Using index info to reconstruct a base tree...
M	src/coreclr/jit/lower.cpp
M	src/coreclr/jit/lower.h
A	src/tests/JIT/Regression/Regression_ro_1.csproj
Falling back to patching base and 3-way merge...
Auto-merging src/coreclr/jit/lower.cpp
Auto-merging src/coreclr/jit/lower.h
CONFLICT (modify/delete): src/tests/JIT/Regression/Regression_ro_1.csproj deleted in HEAD and modified in JIT: Handle `FIELD_LIST` for tailcall arg/parameter interference checks.  Version JIT: Handle `FIELD_LIST` for tailcall arg/parameter interference checks of src/tests/JIT/Regression/Regression_ro_1.csproj left in tree.
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
Patch failed at 0001 JIT: Handle `FIELD_LIST` for tailcall arg/parameter interference checks
Error: The process '/usr/bin/git' failed with exit code 128

Link to workflow output