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WebRandomX — PoC no-SIMD build

Quick start

Prerequisites: emcc, cmake, make

make -f Makefile.noscimd

Or manually:

mkdir build-noscimd && cd build-noscimd
emcmake cmake -DARCH=native -DRANDOMX_NO_SIMD=ON ..
make

This produces a WebAssembly binary without SIMD128, bulk-memory, sign-extension, or non-trapping float-to-int instructions. The output is functionally equivalent (bit-identical hashes) to the standard SIMD build.

Test: SIMD vs no-SIMD comparison

To verify functional equivalence and measure performance:

./test_simd_comparison.sh [--nonces N]   # default: 100

The script builds both variants (with tests and benchmarks enabled) and runs:

  1. WASM feature audit — counts WASM extension instructions via wasm-objdump
  2. Functional correctness — runs the test suite (known hash vectors) on both builds
  3. Hash equivalence — compares test outputs between SIMD and no-SIMD
  4. Benchmark — measures ms/hash on both builds and computes the slowdown factor

Why a no-SIMD build?

The problem: WASM extensions limit portability

Modern WebAssembly toolchains (Emscripten >= 3.x) emit binaries that use non-baseline WASM extensions enabled by default:

Extension Emscripten flag Instructions emitted
SIMD128 -msimd128 v128.*, i32x4.*, f64x2.*, etc.
Bulk memory (default ON) memory.copy, memory.fill
Sign extension (default ON) i32.extend8_s, i64.extend32_s, etc.
Non-trapping float-to-int (default ON) i32.trunc_sat_*, i64.trunc_sat_*

WebRandomX explicitly uses SIMD128 for its Argon2 memory-hard function (argon2_simd.c, blamka-round-simd.h) and AES emulation (intrin_wasm.h). The other three extensions are injected implicitly by the compiler backend.

These extensions are not supported in several environments:

  • Dynamic analysis frameworks
  • Lightweight/IoT runtimes
  • Older browsers and embedded WebView

This PoC explores whether WebRandomX can be compiled using only baseline WASM instructions, making it portable to the widest possible range of environments.

Functional equivalence guarantee

The no-SIMD build replaces SIMD intrinsics with semantically identical scalar operations. The 128-bit v128_t type is replaced by a union of scalar fields (uint64_t u64[2], uint32_t u32[4], double f64[2]), and each SIMD intrinsic is replaced by equivalent element-wise operations.

This is verified empirically: given identical inputs, both builds produce bit-identical hash outputs across the entire RandomX test vector suite.

Implementation details

Build system (CMakeLists.txt)

A CMake option RANDOMX_NO_SIMD (default OFF) controls the build variant:

option(RANDOMX_NO_SIMD "Build without WASM SIMD128 instructions" OFF)

if(NOT RANDOMX_NO_SIMD)
  set_source_files_properties(${randomx_sources} COMPILE_FLAGS -msimd128)
else()
  add_definitions(-DRANDOMX_NO_SIMD)
  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -mno-bulk-memory -mno-sign-ext -mno-nontrapping-fptoint")
  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -mno-bulk-memory -mno-sign-ext -mno-nontrapping-fptoint")
endif()