[Wasm RyuJit] throw helper / null check preliminaries

[AndyAyersMS]

On Wasm null checks must be explicit and exceptions raised via helper call.

Set up some of the mechanism we'll need for this:

• Add null check special code kind
• Track Wasm ACD entries by handler region only (instead of by try or handler). The code address of the helper cannot be used in Wasm to infer EH region containment; we will use use the virtual IP for that. So we need at most one throw helper (per kind) in each funclet and in the main method region.
• Ensure throw helper blocks have Wasm labels that are always on the stack in their regions by putting the throw helpers at the end of the region RPO and pretending there is a branch from the region entry.
• Add plausible codegen for GT_NULLCHECK

[AndyAyersMS]

FYI @dotnet/jit-contrib (still WIP, need to see how much of this is testable)

See #123021 (comment) for some context.

[AndyAyersMS]

I don't have the code to make SCK_NULLCHECK demands in place yet. Seeing as nullchecks get generated in many places it seems less than ideal to track all these sites all down and add code to each one. I wonder if we can just do this in lower or similar.

[SingleAccretion]

I don't have the code to make SCK_NULLCHECK demands in place yet. Seeing as nullchecks get generated in many places it seems less than ideal to track all these sites all down and add code to each one. I wonder if we can just do this in lower or similar.

+1. In fact I don't see the point of this two-phase setup that exists. Why not add all the blocks late (in stack setter or lower) and remove the handling from morph?

[AndyAyersMS]

+1. In fact I don't see the point of this two-phase setup that exists.

I think this is just a long-standing practice that we've never reexamined. Perhaps it is time.

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[AndyAyersMS]

The inline throw helpers require a bit of coordination since we have to emit a block/end wrapper around the whole thing (so we need to know in advance if we're using throw helpers or not).

So I have been working up the divide by zero checks to see how to best generalize the throw helper expansions we'll need. There (whether we use inline or common throw helpers) we need to dup an operand, but Wasm has no dup, so this requires saving to a temp local and then loading back, something like:

... push dividend on the stack
... push divisor on stack
block
tee.local $temp
br_if 0
call ThrowHelper
unreachable
end
load.local $temp
... divide

Any thoughts on how to best go about having a pool of temps we can use?

For the (MinInt/-1) case we need access to the dividend as well. So we might also consider adding these checks explicitly in lower.

[SingleAccretion]

Any thoughts on how to best go about having a pool of temps we can use?

I was thinking we'd reuse the internal register mechanism for that. It's ifdef-ed out currently though, so that will need to be fixed.

[AndyAyersMS]

@SingleAccretion is this last commit what you had in mind for the unchecked offset?

I may push for merging this more or less in its current form and then revisit once #123044 is there to hook up the calls.

Still todo, likely again as separate PRs:

• figure out the upstream throw helper "demand" aspect. Going to look at revising this to just happen in some prior phase (lower? stacklevelsetter?) rather than what we do now.
• mechanism for spilling operands we need to duplicate (say for divide by zero checks)
• refactoring to allow the throw helper codegen to generalize across multiple cases (possibly similar to arm64's functor-based approach).

[SingleAccretion]

is this last commit what you had in mind for the unchecked offset?

Yes. Though I think the comparison should be unsigned?

[AndyAyersMS]

is this last commit what you had in mind for the unchecked offset?

Yes. Though I think the comparison should be unsigned?

Yep, changed this.

[AndyAyersMS]

@dotnet/jit-contrib think this is worth getting in as is, we will need to revisit once some supporting pieces are in place.

[Copilot]

Pull request overview

This PR establishes infrastructure for WebAssembly (Wasm) RyuJIT to handle explicit null checks and exception throwing via helper calls. Unlike other platforms where null checks can be implicit via OS page fault mechanisms, Wasm requires explicit null checks with exceptions raised through helper calls.

Changes:

• Added SCK_NULL_CHECK special code kind for null reference exceptions on Wasm
• Modified ACD (Add Code Descriptor) tracking to use handler regions only on Wasm (instead of try or handler regions)
• Ensured throw helper blocks have proper Wasm labels by positioning them at region end and treating them as successors of region entries
• Implemented codegen for GT_NULLCHECK nodes with both throw helper block and inline variants

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/coreclr/vm/jitinterface.h	Updated MAX_UNCHECKED_OFFSET_FOR_NULL_OBJECT to 1023 for Wasm (smaller than other platforms)
src/coreclr/tools/Common/JitInterface/CorInfoImpl.cs	Added Wasm-specific logic for maxUncheckedOffsetForNullObject in AOT compiler
src/coreclr/jit/gentree.h	Added SCK_NULL_CHECK enum value for null check special code kind
src/coreclr/jit/flowgraph.cpp	Added SCK_NULL_CHECK handling throughout exception infrastructure, modified bbThrowIndex for Wasm ACD tracking, fixed typos in comments
src/coreclr/jit/stacklevelsetter.cpp	Added GT_NULLCHECK case to register throw helper blocks for Wasm
src/coreclr/jit/fgwasm.h	Modified successor enumeration to treat ACD blocks as successors of region entries for proper Wasm control flow
src/coreclr/jit/compiler.cpp	Added cross-replay support for Wasm null object offset
src/coreclr/jit/codegenwasm.cpp	Implemented null check codegen with comparison against max unchecked offset; added placeholder for divide-by-zero checks
src/coreclr/jit/codegen.h	Added inst_JMP overload with isTempLabel parameter for Wasm

[AndyAyersMS]

@dotnet/jit-contrib PTAL. We are going to use 1024 for the max unchecked offset for now and will decrease it to 0 later on.

[AndyAyersMS]

I have a bunch of fixes sitting on top of this, so it would be good if we can get this reviewed. Or else I can add those fixes here...

[AndyAyersMS]

Merging up to retrigger CI

[AndyAyersMS]

I pushed some of the follow-on fixes here. This covers most of the flow related stuff seen replaying the x86 crossgen collection.

[kg]

There are a couple bits around ACDs I don't completely understand but everything looks good.

[AndyAyersMS]

/ba-g ongoing issues with arm64