Use emitLastGroup that works for single/multi align instructions

[kunalspathak]

In #114569, I was relying on emitAlignLast to update the pred head, if we end up creating an extra IG. However, for arm64, since we emit 4 align instructions and just keep the first of those align instructions with updates, it is not accurate to rely on emitAlignLast because it points to the last of the 4 align instructions. Instead use emitAlignLastGroup which is more accurate and designed for such purposes.

Fixes: #116836

[Copilot]

Pull Request Overview

This PR fixes an issue where the incorrect align instruction pointer (emitAlignLast) was used for arm64, replacing it with emitAlignLastGroup to better handle both single and multi align instructions.

• Updated condition and assignment to use emitAlignLastGroup instead of emitAlignLast
• Addresses the bug reported in #114569

Comments suppressed due to low confidence (2)

src/coreclr/jit/emit.cpp:2869

• Consider adding or updating tests to verify that the use of emitAlignLastGroup properly updates the loop head prediction for both single and multi align scenarios, particularly ensuring correct behavior on arm64.

            emitAlignLastGroup->idaLoopHeadPredIG = emitCurIG;

src/coreclr/jit/emit.cpp:2859

• The condition updated to use emitAlignLastGroup correctly reflects the intended behavior for arm64. Ensure that this change is consistently applied in all align instruction updates if similar logic exists elsewhere in the code.

        if (!currIGWasNonEmpty && (emitAlignLastGroup != nullptr) &&

[kunalspathak]

@dotnet/jit-contrib @AndyAyersMS PTAL

[kunalspathak]

/azp runtime-coreclr superpmi-replay

[azure-pipelines]

Command 'runtime-coreclr' is not supported by Azure Pipelines.

Supported commands

• help:
• Get descriptions, examples and documentation about supported commands
• Example: help "command_name"
• list:
• List all pipelines for this repository using a comment.
• Example: "list"
• run:
• Run all pipelines or specific pipelines for this repository using a comment. Use this command by itself to trigger all related pipelines, or specify specific pipelines to run.
• Example: "run" or "run pipeline_name, pipeline_name, pipeline_name"
• where:
• Report back the Azure DevOps orgs that are related to this repository and org
• Example: "where"

See additional documentation.

[amanasifkhalid]

/azp run runtime-coreclr superpmi-replay

[azure-pipelines]

Azure Pipelines could not run because the pipeline triggers exclude this branch/path.

[dotnet-policy-service]

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

[amanasifkhalid]

Looks like CI is being uncooperative, though superpmi-diffs is clean. @kunalspathak do you know why we started seeing this failure all of a sudden?

[kunalspathak]

@kunalspathak do you know why we started seeing this failure all of a sudden?

This is another case of stressing of block layout where the backedges are spread, enclosing other loops that are alignment candidate.

[kunalspathak]

/azp run runtime-coreclr superpmi-replay

yeah, we cannot trigger this anymore. I even tried to do it manually but didn't see a way to do it.

[kunalspathak]

@amanasifkhalid - anything else needed for this PR?

[amanasifkhalid]

No, LGTM

[AndyAyersMS]

You should update the last line in the comment just above.

Does emitAlignLast need to be reset too?

[AndyAyersMS]

Thought I had submitted this yesterday, sorry for not getting to you sooner.

[kunalspathak]

You should update the last line in the comment just above.

sent out #116869

Does emitAlignLast need to be reset too?

Not needed, emitAlignLast and emitAlignLastGroup should point to the same object (for xarch). It is only different for arm, and we were incorrectly referring to the last align instruction instead of the first one of the 4-align instructions we insert.

runtime/src/coreclr/jit/emit.cpp

Lines 5738 to 5741 in 296780d

	// Since we never align overlapping instructions, it is always guaranteed that
	// the emitAlignLastGroup points to the loop that is in process of getting aligned.
	emitAlignLastGroup->idaLoopHeadPredIG = emitCurIG;