Remove or guard usage of `wasm_f64x2_relaxed_madd` to support builds without `-mrelaxed-simd`

[jjyuhub]

The current build system uses the wasm_f64x2_relaxed_madd intrinsic, which requires the -mrelaxed-simd compiler flag. This leads to build failures in environments where relaxed SIMD is not supported or enabled, including common CI environments such as ubuntu-latest on GitHub Actions.

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Details

• The following compiler error occurs when building without -mrelaxed-simd:

  error: always_inline function 'wasm_f64x2_relaxed_madd' requires target feature 'relaxed-simd', but would be inlined into function 'mul_residue_fma' that is compiled without support for 'relaxed-simd'
  

• Additionally, wat2wasm fails to parse binaries generated with relaxed SIMD instructions, even with the --enable-all flag:

  parse exception: invalid code after SIMD prefix: 263
  

• Relaxed SIMD is not yet supported by all WebAssembly engines or toolchains. Relying on it by default limits portability and breaks GitHub Actions and other automated build environments.

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What Works

• Replacing wasm_f64x2_relaxed_madd(wasm_f64x2_neg(c), b, a) with the portable alternative:

  wasm_f64x2_add(wasm_f64x2_mul(a, b), wasm_f64x2_neg(c))

  allows the build to complete without needing -mrelaxed-simd.

• All tests (randomx_single.js, randomx_threaded.js, superscalarhash.js) pass with this substitution.

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Proposed Solution

Introduce a preprocessor guard or build-time toggle to control usage of relaxed SIMD:

Option 1: Use preprocessor fallback in C code

#ifdef __wasm_relaxed_simd__
  return wasm_f64x2_relaxed_madd(wasm_f64x2_neg(c), b, a);
#else
  return wasm_f64x2_add(wasm_f64x2_mul(a, b), wasm_f64x2_neg(c));
#endif

Option 2: Add USE_RELAXED_SIMD flag to Makefile

ifeq ($(USE_RELAXED_SIMD),1)
  WASM_CFLAGS += -mrelaxed-simd
endif

Then conditionally define the relaxed version or fallback in the C source.

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Benefits

• Improves compatibility across compilers and runtimes.
• Allows GitHub Actions and other CI systems to build the project without patching source code.
• Maintains support for relaxed SIMD for advanced/local use.

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Temporary CI workaround example:

# Remove the SIMD flag
sed -i 's/-mrelaxed-simd//g' Makefile

# Replace the relaxed intrinsic with standard instructions
sed -i 's/wasm_f64x2_relaxed_madd(wasm_f64x2_neg(c), b, a)/wasm_f64x2_add(wasm_f64x2_mul(a, b), wasm_f64x2_neg(c))/g' src/jit/stubs/semifloat.c

[l1mey112]

Hey, sorry I haven't been checking this. Just been swamped with work. I see that you've forked the project, let me know how that goes and if you have any questions.