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Broaden wasm vector-cast fencing and add deterministic lossless_cast regressions#9038

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copilot/fix-fuzz-error-correctness-lossless-cast
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Broaden wasm vector-cast fencing and add deterministic lossless_cast regressions#9038
Copilot wants to merge 5 commits intomainfrom
copilot/fix-fuzz-error-correctness-lossless-cast

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Copilot AI commented Mar 15, 2026 •
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correctness/lossless_cast was intermittently tripping an LLVM WebAssembly lowering assertion (cast<LoadSDNode>) under wasm-32-wasmrt-wasm_simd128. The failures were seed-dependent and appeared after lossless_cast() rewrote expressions into forms that the existing wasm workaround no longer recognized as load-derived.

  • WebAssembly codegen

    • Broaden the existing optimization_fence() workaround for LLVM's vector-extend/load bug.
    • Apply it to all widening vector integer casts in CodeGen_WebAssembly, not just casts whose input is trivially recognized as a direct load.
    • This keeps load-derived expressions safe even after lossless_cast() rewrites and vectorization obscure the original load shape.
    if (op->type.is_int_or_uint() &&
    op->value.type().is_int_or_uint() &&
    op->type.bits() > op->value.type().bits()) {
    llvm::Value *v = codegen(op->value);
    v = optimization_fence(v);
    value = builder->CreateIntCast(v, llvm_type_of(op->type),
                                   op->value.type().is_int());
    return;
}

  • Regression coverage

    • Add deterministic correctness/lossless_cast coverage for the reported failing seeds:
      • 1926104395
      • 3082698823
    • Run these before the randomized fuzz loop so the known crash modes stay directly reproducible.

  • Intent of the change

    • Preserve the existing wasm workaround, but make it robust to IR reshaping done by lossless_cast().
    • Keep the fix localized to wasm integer-widening vector casts rather than changing lossless_cast() behavior globally.
Original prompt

This section details on the original issue you should resolve

<issue_title>Fuzz error: correctness/lossless_cast</issue_title>
<issue_description>```
HL_JIT_TARGET=wasm-32-wasmrt-wasm_simd128
HL_TARGET=wasm-32-wasmrt-wasm_simd128
#CTEST_RESOURCE_GROUP_COUNT=
/Users/halidenightly/build_bot/worker/halide-testbranch-main-llvm_main-x86-64-osx-cmake/halide-build/test/correctness/correctness_lossless_cast
Assertion failed: (isa(Val) && "cast() argument of incompatible type!"), function cast, file Casting.h, line 560.
Fuzz testing with root seed 1769885050


From https://buildbot.halide-lang.org/master/#/builders/260/builds/378</issue_description>

<agent_instructions>The first step is to reproduce the error.</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
<comment_new><author>@mcourteaux</author><body>
@alexreinking Any chance one of those LLMs can take a shot at this?</body></comment_new>
<comment_new><author>@alexreinking</author><body>
I would need to reproduce it locally first, and so far I have not been able. I could take the buildbot down and try it there, though.</body></comment_new>
<comment_new><author>@alexreinking</author><body>
There's a new failure here:

1926104395
Assertion failed: (isa(Val) && "cast() argument of incompatible type!"), function cast, file Casting.h, line 560.


https://buildbot.halide-lang.org/master/#/builders/400/builds/192</body></comment_new>
<comment_new><author>@alexreinking</author><body>
Another one:

3082698823
correctness_lossless_cast: /__w/halide-llvm/halide-llvm/src_cache/baac39c0d2a46a1ff6d810168bd6277008fbd0bf/llvm/include/llvm/Support/Casting.h:560: decltype(auto) llvm::cast(const From&) [with To = LoadSDNode; From = SDValue]: Assertion `isa(Val) && "cast() argument of incompatible type!"' failed.

</comments>

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@alexreinking
Broaden wasm cast fence for lossless_cast regression
7b7fb8c 
Co-authored-by: alexreinking <169273+alexreinking@users.noreply.github.com>
Copilot AI changed the title [WIP] [ISSUE] Fix fuzz error in correctness lossless cast Broaden wasm vector-cast fencing and add deterministic lossless_cast regressions Mar 15, 2026
Copilot AI requested a review from alexreinking March 15, 2026 15:23
@alexreinking
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alexreinking commented Mar 15, 2026

@copilot - Please extract the failing expressions so that the regression tests do not depend on the RNG, even deterministically.

@alexreinking
Extract lossless_cast regression expressions
d9fac99 
Co-authored-by: alexreinking <169273+alexreinking@users.noreply.github.com>
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Copilot AI commented Mar 15, 2026

@copilot - Please extract the failing expressions so that the regression tests do not depend on the RNG, even deterministically.

Addressed in d9fac99. The regression cases now build the two reported failing Expr trees directly and compile them explicitly, so they no longer depend on RNG state or buffer initialization.

@alexreinking
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alexreinking commented Mar 15, 2026

@copilot - I'm glad to see the regression tests fail at this stage. Let's try to fix the bugs that are causing those failing tests

@alexreinking
Fix extracted lossless_cast regression harness
611b60c 
Co-authored-by: alexreinking <169273+alexreinking@users.noreply.github.com>
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Copilot AI commented Mar 15, 2026

@copilot - I'm glad to see the regression tests fail at this stage. Let's try to fix the bugs that are causing those failing tests

Fixed in 611b60c. The extracted regressions were failing early in the test harness because I was still applying UB screening to the compile-only cases; that check is gone now, so the explicit expressions once again exercise lossless_cast() plus compilation instead of bailing out before codegen.

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abadams commented Mar 15, 2026

I do not believe this approach is viable. See the comment I made on the individual commit.

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Fuzz error: correctness/lossless_cast

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