Emit line number information for native AOT/WASM by MichalStrehovsky · Pull Request #135740 · dotnet/runtime

MichalStrehovsky · 2026-10-14T02:41:50Z

wasmtime with WASMTIME_BACKTRACE_DETAILS=1 can now print the backtrace including line numbers. Locals/parameters are not emitted because RyuJIT seems to be lacking that info.

d:\git\runtime2>wasmtime artifacts\bin\repro\x64\Debug\reproZero.wasm
Hi
Hi
Error: failed to run main module `artifacts\bin\repro\x64\Debug\reproZero.wasm`

Caused by:
    0: failed to invoke command default
    1: error while executing at wasm backtrace:
    0:    0x86d - abort
                    at wasisdk://v34.0/build/sysroot/wasi-libc-wasm32-wasip1-build-prefix/src/wasi-libc-wasm32-wasip1-build-build/wasisdk://v34.0/src/wasi-libc/libc-bottom-half/sources/abort.c:5:3
    1:    0x43b - reproZero_Program__DoAbort
                    at D:\git\runtime2\src\coreclr\tools\aot\ILCompiler\reproZero\zerosharp.cs:164:0
    2:    0x3d5 - reproZero_Program__Main
                    at D:\git\runtime2\src\coreclr\tools\aot\ILCompiler\reproZero\zerosharp.cs:156:0
    3:    0x349 - reproZero_Program__NativeMain
                    at D:\git\runtime2\src\coreclr\tools\aot\ILCompiler\reproZero\zerosharp.cs:141:0
    4:    0x773 - reproZero.wasm!__main_void
    5:    0x2d7 - _start
                    at wasisdk://v34.0/build/sysroot/wasi-libc-wasm32-wasip1-lto-build-prefix/src/wasi-libc-wasm32-wasip1-lto-build-build/wasisdk://v34.0/src/wasi-libc/libc-bottom-half/crt/crt1-command.c:55:11
    2: wasm trap: wasm `unreachable` instruction executed

wasmtime with `WASMTIME_BACKTRACE_DETAILS=1` can now print the backtrace including line numbers: ``` d:\git\runtime2>wasmtime artifacts\bin\repro\x64\Debug\reproZero.wasm Hi Hi Error: failed to run main module `artifacts\bin\repro\x64\Debug\reproZero.wasm` Caused by: 0: failed to invoke command default 1: error while executing at wasm backtrace: 0: 0x86d - abort at wasisdk://v34.0/build/sysroot/wasi-libc-wasm32-wasip1-build-prefix/src/wasi-libc-wasm32-wasip1-build-build/wasisdk://v34.0/src/wasi-libc/libc-bottom-half/sources/abort.c:5:3 1: 0x43b - reproZero_Program__DoAbort at D:\git\runtime2\src\coreclr\tools\aot\ILCompiler\reproZero\zerosharp.cs:164:0 2: 0x3d5 - reproZero_Program__Main at D:\git\runtime2\src\coreclr\tools\aot\ILCompiler\reproZero\zerosharp.cs:156:0 3: 0x349 - reproZero_Program__NativeMain at D:\git\runtime2\src\coreclr\tools\aot\ILCompiler\reproZero\zerosharp.cs:141:0 4: 0x773 - reproZero.wasm!__main_void 5: 0x2d7 - _start at wasisdk://v34.0/build/sysroot/wasi-libc-wasm32-wasip1-lto-build-prefix/src/wasi-libc-wasm32-wasip1-lto-build-build/wasisdk://v34.0/src/wasi-libc/libc-bottom-half/crt/crt1-command.c:55:11 2: wasm trap: wasm `unreachable` instruction executed ```

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🟡 Changes recommended

The Wasm writer silently ignores the requested --gdwarf-5 option.

1 open finding

Wasm writer ignores --gdwarf-5 and always emits DWARF v4 · New

What changed in this PR

Adds DWARF line information to NativeAOT WebAssembly output for source-level Wasmtime backtraces.

Changes:

Emits DWARF custom sections and Wasm relocations.
Maps methods and funclets to source locations.
Adds Wasm-specific DWARF handling.

File	Description
`WasmRelocatableObjectWriter.cs`	Emits and relocates DWARF sections.
`WasmObjectWriter.cs`	Creates debug custom sections.
`WasmSection.cs`	Exposes section prefix sizes.
`MethodCodeNode.cs`	Restarts mappings for funclets.
`DwarfLineSequenceWriter.cs`	Completes line sequences correctly.
`DwarfInfo.cs`	Omits unsupported Wasm frame expressions.
`DwarfBuilder.cs`	Adds Wasm architecture support.

🧠 Review effort: Balanced

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adamperlin · 2026-10-14T16:42:15Z

@MichalStrehovsky are the line numbers here correct? We had been concerned that IL -> Native offset mappings in the JIT were incorrect on Wasm.

MichalStrehovsky · 2026-10-14T20:43:05Z

@MichalStrehovsky are the line numbers here correct? We had been concerned that IL -> Native offset mappings in the JIT were incorrect on Wasm.

They looked good for the stack I pasted in top-post.

Re-enables RyuJIT variable debug info for ReadyToRun WebAssembly code, which was disabled before wasm locals could be encoded, and defines the encoding that readers consume. - **Register fields** hold `localIndex + WASM_LOCAL_REGNUM_BASE` (3); values 0–2 keep their meanings (`REGNUM_PC`, `REGNUM_COUNT`, `REGNUM_AMBIENT_SP`). - **Stack records** use base 2 (`REG_FPBASE` = `REG_SPBASE` = `REGNUM_AMBIENT_SP` on wasm), so they are slots relative to the logical frame, which readers reconstruct from the stack walk. - **Scopes** open in the first emitted block: wasm is AOT-only, so `getVars` always reports method-wide scopes, and wasm block order doesn't follow IL order. - The hidden portable-entrypoint argument is excluded from source variables, and R2RDump decodes wasm registers. Both constants are fixed for DebugInfo contract `c1` and documented in `DebugInfo.md`. Changing either needs a new contract version and, because R2R images persist the encoding, a ReadyToRun format version change. Browser and WASI publishes strip debug info by default (#134690), so these records reach an image only with `PublishReadyToRunStripDebugInfo=false`; generated code is unchanged. The cDAC consumer is #133890. <details><summary>Test coverage and follow-ups</summary> `WasmWebcilModule` checks invariants rather than exact codegen: register records name declared locals with the expected wasm types, stack records use base 2 without overlapping ranges, recorded frame slots are actually accessed (including from a finally funclet), and GC locals' ranges cover the safepoint. Removing the `+3` bias makes it fail. Follow-ups: NativeAOT DWARF (#135740) can describe locals with `DW_OP_WASM_location` (local index = register − 3); frame-slot variables additionally need the engine local holding the frame pointer, which this format doesn't record. #134808 plans R2R DWARF on top of these records. </details> > [!NOTE] > This pull request description was updated with the assistance of GitHub Copilot. --------- Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Co-authored-by: Jakob Botsch Nielsen <Jakob.botsch.nielsen@gmail.com> Copilot-Session: 51b56d7e-45e5-464d-8c2b-78c744680ff8

MichalStrehovsky requested review from adamperlin and jtschuster October 14, 2026 02:41

MichalStrehovsky added arch-wasm WebAssembly architecture area-NativeAOT-coreclr labels Oct 14, 2026

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MichalStrehovsky requested a balanced review from Copilot October 14, 2026 02:43

Copilot started reviewing on behalf of MichalStrehovsky October 14, 2026 02:44

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Comment thread src/coreclr/tools/Common/Compiler/ObjectWriter/WasmRelocatableObjectWriter.cs Resolved

Update DwarfBuilder initialization to use options

772d2e1

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jtschuster approved these changes Oct 14, 2026

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MichalStrehovsky merged commit 81ce95c into dotnet:main Oct 14, 2026
100 of 102 checks passed

MichalStrehovsky deleted the linenumber branch October 14, 2026 21:21

lewing mentioned this pull request Oct 17, 2026

[wasm] Preserve R2R variable debug information #133086

Merged

dotnet-maestro Bot mentioned this pull request Oct 19, 2026

[main] Source code updates from dotnet/runtime dotnet/dotnet#10269

Merged

dotnet-milestone-bot Bot added this to the 12.0-preview1 milestone Oct 19, 2026

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Emit line number information for native AOT/WASM - #135740

Emit line number information for native AOT/WASM#135740
MichalStrehovsky merged 2 commits into
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