[Wasm RyuJIT] Add Instruction Encoding for Local Var Declarations and Emit Local Declarations #122425

adamperlin · 2025-12-11T00:53:30Z

This PR adds a new instrDesc subtype for encoding Wasm local declarations, which have the form count: u32 type:valtype, where valtype can be encoded as a single byte for number and vector types.

The PR also adds code to emit local count and local declarations as part of the prolog for a function. Some of this may need to change based on what we decide around calling convention and register (locals) allocation.

…al JIT, since the RyuJIT backend is now emitting Wasm

…-arg-initialization

Copilot

Pull request overview

This PR introduces support for WebAssembly local variable declarations in the JIT compiler by adding a new instruction descriptor subtype and emission logic for encoding local declarations in the function prolog.

Key Changes

Adds instWasmValueType enum and mapping functions to convert between JIT types and WASM types
Introduces instrDescLclVarDecl instruction descriptor for encoding local declarations with count and type
Implements local declaration emission in the function prolog via genWasmLocals()

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 12 comments.

Show a summary per file

File	Description
src/coreclr/jit/wasmtypesdef.h	New header defining WASM value types and type conversion functions
src/coreclr/jit/registeropswasm.h	Removes trailing blank line
src/coreclr/jit/instrswasm.h	Adds `local_cnt` and `local_decl` instructions
src/coreclr/jit/emitwasm.h	Declares new emission methods for local declarations
src/coreclr/jit/emitwasm.cpp	Implements local declaration emission and descriptor handling
src/coreclr/jit/emitfmtswasm.h	Adds `IF_LOCAL_CNT` and `IF_LOCAL_DECL` instruction formats
src/coreclr/jit/emit.h	Adds `instrDescLclVarDecl` structure and bitfield flag
src/coreclr/jit/codegenwasm.cpp	Adds `INS_end` instruction to epilog
src/coreclr/jit/codegencommon.cpp	Implements prolog local count and declaration emission
src/coreclr/jit/codegen.h	Declares `genWasmLocals()` method

src/coreclr/jit/emitwasm.cpp

src/coreclr/jit/codegencommon.cpp

src/coreclr/jit/emit.h

src/coreclr/jit/wasmtypesdef.h

src/coreclr/jit/emitfmtswasm.h

src/coreclr/jit/emitwasm.cpp

src/coreclr/jit/wasmtypesdef.h

src/coreclr/jit/emitwasm.cpp

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/coreclr/jit/codegen.h

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/coreclr/jit/codegencommon.cpp

src/coreclr/jit/emit.h

src/coreclr/jit/wasmtypesdef.h

AndyAyersMS · 2025-12-11T20:17:36Z

src/coreclr/jit/codegenwasm.cpp

    // TODO-WASM-CQ: do not emit "return" in case this is the last block

    instGen(INS_return);
+    instGen(INS_end);


Is this end required at the end of methods? We can have multiple epilogs (though we might want to reconsider)... and there's no guarantee even with one epilog that the epilog will be at the end of the instruction stream.

Yeah it is required. If the epilog isn't guaranteed to be the end, where would be a safe place to ensure that the end comes at the actual end of the instruction stream for a method?

Probably at the end of genCodeForBBlist or similar.

genFnEpilog is a BasicBlock* parameter. I am pretty sure you can just check block->IsLast() and switch whether you return return or end based on that. That would address the TODO above as well.

It might not be the actual last block when we have funclets.

That's a good point. This case can be detected by checking if the next block is a funclet entry.

Adding this at the end of genCodeForBBList proved a little bit tricky because of instruction groups. It does look like we track the last used instruction group and could grab that and append after we've finished all method code? Will the last used group be guaranteed to be at the end of the method though? Also, what is the best way to detect if the next block is a funclet? For now I've added an IsLast check to genFnEpilog with a comment that we need to revisit if we're doing funclets.

src/coreclr/jit/codegencommon.cpp

src/coreclr/jit/wasmtypesdef.h

src/coreclr/jit/emit.h

…pilog

src/coreclr/jit/codegenwasm.cpp

src/coreclr/jit/codegenlinear.cpp

src/coreclr/jit/codegencommon.cpp

src/coreclr/jit/codegen.h

src/coreclr/jit/instrswasm.h

src/coreclr/jit/emitwasm.cpp

src/coreclr/jit/registeropswasm.h

…lization

dotnet-policy-service · 2025-12-17T01:32:07Z

Tagging subscribers to 'arch-wasm': @lewing, @pavelsavara
See info in area-owners.md if you want to be subscribed.

Co-authored-by: SingleAccretion <62474226+SingleAccretion@users.noreply.github.com>

…cific debug info field on m_debugInfoSize

AndyAyersMS

Mostly looks good, just one issue with the epilog.

src/coreclr/jit/codegenwasm.cpp

Co-authored-by: Andy Ayers <andya@microsoft.com>

SingleAccretion

Looking pretty good! Just a few comments.

SingleAccretion · 2025-12-17T05:12:42Z

src/coreclr/jit/codegenwasm.cpp

 }

+//------------------------------------------------------------------------
+// genWasmLocals: generate wasm locals for all locals


Suggested change

// genWasmLocals: generate wasm locals for all locals

// genWasmLocals: generate wasm local declarations

SingleAccretion · 2025-12-17T05:14:28Z

src/coreclr/jit/emit.h

        CORINFO_SIG_INFO* idCallSig;     // Used to report native call site signatures to the EE
        BasicBlock*       idTargetBlock; // Target block for branches
+#ifdef TARGET_WASM
+        int lclOffset;


Suggested change

int lclOffset;

int lclOffset; // Base index of the WASM locals being declared

SingleAccretion · 2025-12-17T05:22:33Z

src/coreclr/jit/emitwasm.cpp

+        case IF_LOCAL_DECL:
+        {
+            assert(idIsLclVarDecl());
+            instrDescLclVarDecl* idl      = static_cast<instrDescLclVarDecl*>(const_cast<instrDesc*>(this));


Suggested change

instrDescLclVarDecl* idl = static_cast<instrDescLclVarDecl*>(const_cast<instrDesc*>(this));

const instrDescLclVarDecl* idl = static_cast<const instrDescLclVarDecl*>(this);

Avoid the const_cast?

Or you could use the emitGetLclVarDeclType/... helpers you added here by making them take const instrDesc*.

SingleAccretion · 2025-12-17T05:24:13Z

src/coreclr/jit/emit.h

+    struct instrDescLclVarDecl : instrDesc
+    {
+        instrDescLclVarDecl() = delete;
+        cnsval_ssize_t lclCnt;


This is technically larger than necessary since the local count can't really exceed 32 bits (and cnsval_ssize_t is a signed 64 bit number). Base instrDesc is 16 bytes, this desc is going to be 32 bytes (due to alignment), but if this field is made to be 32 bits, it can fit into 24 bytes.

(Using an unsigned integer might also get rid of some casts)

SingleAccretion · 2025-12-17T05:29:03Z

src/coreclr/jit/emitwasm.cpp

+        {
+            cnsval_ssize_t count   = emitGetLclVarDeclCount(id);
+            WasmValueType  valType = emitGetLclVarDeclType(id);
+            int            offs    = id->idDebugOnlyInfo()->lclOffset;


I would move this down to the if (m_debugInfoSize > 0) branch. Right now it is going to access nominally uninitialized memory with m_debugInfoSize == 0.

SingleAccretion · 2025-12-17T05:29:10Z

src/coreclr/jit/emitwasm.cpp

+            {
+                // No debug info case: just print the count and type of the locals
+                printf(" count=%llu type=%s", (uint64_t)count, WasmValueTypeName(valType));
+                break;


Suggested change

break;

SingleAccretion · 2025-12-17T05:30:05Z

src/coreclr/jit/instrswasm.h

+INST(label,       "label",       0, IF_RAW_ULEB128,   0x00)
+INST(local_cnt,   "local.cnt",   0, IF_RAW_ULEB128, 0x00)
+INST(local_decl,  "local",       0, IF_LOCAL_DECL, 0x00)


Suggested change

INST(label, "label", 0, IF_RAW_ULEB128, 0x00)

INST(local_cnt, "local.cnt", 0, IF_RAW_ULEB128, 0x00)

INST(local_decl, "local", 0, IF_LOCAL_DECL, 0x00)

INST(label, "label", 0, IF_RAW_ULEB128, 0x00)

INST(local_cnt, "local.cnt", 0, IF_RAW_ULEB128, 0x00)

INST(local_decl, "local", 0, IF_LOCAL_DECL, 0x00)

Technically the ambition is to have all these aligned across all the instructions but we can make that happen after the list settles down a bit.

adamperlin added 6 commits December 8, 2025 20:20

Revert wasm method stub method in crossgen to allow invoking the actu…

e93e57d

…al JIT, since the RyuJIT backend is now emitting Wasm

WIP add args as locals to the beginning of method epilog in Wasm codegen

4b991da

Multiple fixes for emitting local declarations for incoming args

5933e67

jit-format

7b8e339

Merge branch 'main' of github.com:dotnet/runtime into adamperlin/wasm…

78b7c70

…-arg-initialization

Generate locals only for non-args

9fbfb86

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adamperlin changed the title ~~Add Instruction Encoding for Local Var Declarations and Emit Local Declarations~~ [Wasm RyuJIT] Add Instruction Encoding for Local Var Declarations and Emit Local Declarations Dec 11, 2025

adamperlin and others added 5 commits December 11, 2025 09:16

Update src/coreclr/jit/emit.h

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Update src/coreclr/jit/emitfmtswasm.h

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Update src/coreclr/jit/codegencommon.cpp

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Update src/coreclr/jit/emitwasm.cpp

c9d3046

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Remove incorrect noway_asserts

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Remove empty ifdef block

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"Over release of MOW - COM" assert in COM wrappers #122447

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adamperlin added 2 commits December 12, 2025 18:45

Add block->IsLast() guard for emitting end instruction in Wasm genFnE…

f3e2f82

…pilog

Emit correct local count for current state of Wasm codegen

0820a24

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Address some more review feedback

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Merge branch 'main' of github.com:dotnet/runtime into wasm-arg-initia…

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	// genWasmLocals: generate wasm locals for all locals
	// genWasmLocals: generate wasm local declarations

	int lclOffset;
	int lclOffset; // Base index of the WASM locals being declared

	instrDescLclVarDecl* idl = static_cast<instrDescLclVarDecl>(const_cast<instrDesc>(this));
	const instrDescLclVarDecl* idl = static_cast<const instrDescLclVarDecl*>(this);

[Wasm RyuJIT] Add Instruction Encoding for Local Var Declarations and Emit Local Declarations #122425

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Pull request overview

Key Changes

Reviewed changes

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