simd-f128 1.3.4

High-performance cross-platform 128-bit arithmetic for SIMD applications.

High-performance, zero-allocation 128-bit floating-point arithmetic powered by hardware SIMD.

simd-f128

Read the Official Documentation: docs/index.md
Try the Live WebAssembly Demo: https://tiw302.github.io/simd-f128/demo/

Verified Compatibility — 11/11 Platforms Passing

Architecture	Platform	Verified Backend
x86_64 (Modern)	Linux / Windows	AVX2 (Vectorized)
x86_64 (Legacy)	Linux / Windows	SSE2 (Vectorized)
ARM64 (Apple)	macOS (M1/M2/M3)	NEON (Vectorized)
ARM64 (Android)	Mobile	NEON (Vectorized)
ARMv7 (Android)	Mobile	Scalar C11
WebAssembly	Chrome / Node.js	WASM-SIMD128
WebAssembly	Universal Web	WASM Scalar
RISC-V64	Linux (QEMU)	Scalar C11
General Desktop	Linux / Windows	Scalar C11 Fallback

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Table of Contents

• Introduction
  • Why simd-f128?
  • Design Philosophy
• Requirements & Toolchains
• Build and Installation
• Library Components
  • simd_f128.h (Core)
  • simd_f128_consts.h
  • simd_f128_io.h
  • simd_f128_math.h
  • simd_f128_utils.h
  • simd_f128.hpp (C++)
• API Reference
• Performance & Benchmarks
• Double-Double Arithmetic
• Examples
• Platform Support & CI Status
• Language Bindings
• Project Structure

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Introduction

simd-f128 is a professional-grade, header-only C library for 128-bit (Double-Double) floating-point arithmetic, featuring automatic hardware SIMD acceleration (AVX2, NEON, WASM-SIMD). It explicitly targets the precision gap between standard 64-bit IEEE 754 doubles and heavyweight arbitrary-precision libraries like GMP.

By delivering 31-32 decimal digits of accuracy with zero heap allocation overhead, simd-f128 is purpose-built for demanding workloads—such as fractal rendering, physical simulations, and orbital mechanics. While the core engine is pure C11, it provides seamless native bindings for C++, Python, WebAssembly, and Rust, allowing developers across multiple ecosystems to easily overcome the limits of standard double precision.

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Why simd-f128?

Ever zoomed into a Mandelbrot set and watched the detail dissolve into grey mush? That's double precision dying — at zoom levels beyond ~10^-14, two distinct coordinates become the same value and the image collapses entirely. The same silent failure happens in long-running simulations, ill-conditioned linear algebra, and anywhere small errors compound over time.

The usual fixes each carry a significant cost:

Option	Precision	Performance	Allocation	Portability
double	~15 digits	Native Hardware	None	Universal
long double	18-19 digits (x87)	Fast	None	Compiler-dependent
__float128 (GCC)	~33 digits	Emulated (Slow)	None	GCC/Clang only
GMP / MPFR	Arbitrary	Very Slow	Heap	Portable
simd-f128	~31 digits	Hardware SIMD (Fast)	None	Universal

__float128 gets close on precision but locks you into GCC/Clang and is noticeably slower due to software emulation. GMP/MPFR are powerful but heap-allocating inside a render loop is a non-starter.

simd-f128 occupies the exact gap: it doubles usable precision with zero allocation, zero dependencies, and no compiler lock-in — proven in practice by mandelbrot-c, which achieves stable deep-zoom rendering at coordinates down to 10^-28, far beyond what standard double can represent.

Performance Benchmarks

Below is a benchmark comparison of basic arithmetic operations running on 10,000,000 iterations (latency mode):

Data Type	Add (ms)	Mul (ms)	Div (ms)	Relative Multiplication Speed
double (64-bit)	9.26	9.23	41.12	1.00x (Baseline)
long double (x87)	20.21	20.33	47.49	0.45x
__float128 (GCC)	139.67	186.94	298.76	0.05x
simd-f128 (SIMD)	97.76	73.32	204.05	0.13x (2.55x faster than GCC)

As shown, simd-f128 is 1.4x to 2.5x faster than GCC's software-emulated __float128, making it the highest-performance choice for 128-bit precision.

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Design Philosophy

The library is built around three constraints that were never relaxed during development:

Zero allocation. Every operation executes entirely in CPU registers. There are no calls to malloc, no temporary buffers, and no GC pressure. This makes simd-f128 suitable for use inside tight render loops, interrupt handlers, and embedded firmware where heap allocation is prohibited.

No configuration required. The correct SIMD backend — AVX2, SSE2, NEON, WASM-SIMD, or scalar — is selected automatically at compile time based on the target architecture. If a specific hardware SIMD instruction set is not detected by the compiler, it seamlessly and safely falls back to a highly portable scalar implementation.

Standard C foundation. The library is built entirely on IEEE 754 double arithmetic and C11 standard library functions. It does not rely on compiler extensions, non-standard intrinsics outside of guarded #ifdef blocks, or platform-specific ABI assumptions. The scalar fallback compiles and produces correct results on any C99-compliant toolchain.

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Limitations & Technical Notes

Double-Double vs IEEE 754 128-bit: Please note that simd-f128 uses Double-Double arithmetic (an unevaluated sum of two standard 64-bit double values) to achieve approximately 31 decimal digits of precision. It is not a strictly compliant IEEE 754 binary128 implementation.

While this approach offers massive performance benefits and is perfect for deeply zooming into fractals (like in mandelbrot-c), it is susceptible to Catastrophic Cancellation in specific scenarios (e.g., subtracting two nearly identical values). If you are building highly sensitive physics simulations or rigorous numerical analysis tools where IEEE 754 edge-case compliance is strictly required, a heavier library like GMP/MPFR or compiler-specific __float128 may be more appropriate.

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Requirements

Component	Requirement
C Standard	C11 or later (C99 compatible for scalar path)
C++ Standard	C++11 or later (for simd_f128.hpp only)
Compiler	GCC 4.9+, Clang 3.5+, MSVC 2019+, Emscripten 3.0+
Math library	-lm required on Linux/UNIX (for fma())

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Verified Toolchains

The following toolchains are tested on every commit via CI. All others fall back to the scalar path and are expected to produce correct results.

Toolchain	Version	Platform	Backend
GCC	11+	Linux x86_64	Scalar, SSE2, AVX2
GCC (aarch64-linux-gnu)	11+	Linux ARM64 (QEMU)	NEON
GCC (arm-linux-gnueabihf)	11+	Linux ARMv7 (QEMU)	Scalar + VFPv4
GCC (riscv64-linux-gnu)	11+	Linux RISC-V64 (QEMU)	Scalar
Clang	14+	macOS Apple Silicon	NEON
Clang	14+	macOS Intel	Scalar, SSE2, AVX2
MSVC	2022	Windows x64	SSE2, AVX2
Emscripten	3.0+	WASM (Node.js/Web)	WASM-SIMD, Scalar

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Build and Installation

simd-f128 can be integrated natively via C/C++ headers, Python, or JavaScript (WebAssembly).

Python (PyPI)

pip install simd-f128

JavaScript / Node.js (NPM)

npm install @tiw302/simd-f128

C/C++ (Header Only)

simd-f128 is header-only. The simplest integration is copying the include/ directory directly into your project, then defining the implementation macro in exactly one translation unit:

#define SIMD_F128_IMPLEMENTATION
#include <simd_f128.h>
#include <simd_f128_io.h>   // optional

All other translation units include the headers without the macro.

For C++ projects, include the convenience wrapper instead:

#define SIMD_F128_IMPLEMENTATION
#include <simd_f128.hpp>   // pulls in all headers automatically

CMake

System Install (Recommended) You can install the library system-wide to easily use find_package in other projects:

cmake -S . -B build
sudo cmake --install build

Then in your project's CMakeLists.txt:

find_package(simd_fp REQUIRED)
target_link_libraries(my_app PRIVATE simd_fp::simd_fp)

Local Build Options

# Scalar backend (default - works everywhere)
cmake -S . -B build
cmake --build build

# AVX2 backend (Intel/AMD Haswell+)
cmake -S . -B build -DSIMD_F128_AVX2=ON
cmake --build build

# WebAssembly + SIMD128 (Chrome 91+, Firefox 89+, Safari 16.4+, Node.js 16+)
emcmake cmake -S . -B build -DSIMD_F128_WASM=ON
cmake --build build

# WebAssembly Scalar (maximum browser compatibility)
emcmake cmake -S . -B build
cmake --build build

# ARMv7 - optional flag for hardware FMA on VFPv4 cores
cmake -S . -B build -DCMAKE_C_FLAGS="-mfpu=neon-vfpv4 -mfloat-abi=hard"
cmake --build build

AArch64 (Apple Silicon, Graviton, Android ARM64) requires no flags - NEON is auto-detected. Run tests after building:

ctest --test-dir build

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Library Components

simd_f128.h (Core)

The central engine of the library. Implements the Double-Double type and all fundamental arithmetic operations. All functions are static inline - no separate compilation unit is needed beyond the SIMD_F128_IMPLEMENTATION guard.

Key properties:

• ~106-bit mantissa - roughly 31-32 decimal digits of precision.
• Zero heap allocation - all operations execute directly in CPU registers, suitable for tight inner loops.
• Automatic SIMD dispatch - selects AVX2/SSE2 (__m128d) on Intel/AMD, NEON (float64x2_t) on ARM64/Apple Silicon, WASM-SIMD (v128_t) on the web, or falls back to scalar C99.
• Branch-free fast paths - minimal branching (restricted to Inf/NaN guards) ensures consistent execution time and avoids pipeline stalls in the hot path.
• Strict IEEE 754 foundation - built on standard double, fully compatible with existing hardware.

#define SIMD_F128_IMPLEMENTATION
#include <simd_f128.h>

int main() {
    simd_f128 a = simd_f128_from_double(1.234567890123456789);
    simd_f128 b = simd_f128_from_double(2.0);

    simd_f128 sum  = simd_f128_add(a, b);
    simd_f128 diff = simd_f128_sub(a, b);
    simd_f128 prod = simd_f128_mul(a, b);
    simd_f128 quot = simd_f128_div(a, b);
    simd_f128 root = simd_f128_sqrt(a);

    return 0;
}

---

simd_f128_consts.h

Pre-computed, high-precision mathematical constants stored as Double-Double pairs. Each constant captures the full ~106-bit mantissa, avoiding the precision loss inherent in standard 64-bit initialisers.

#include <simd_f128.h>
#include <simd_f128_consts.h>

int main() {
    simd_f128 pi     = SIMD_F128_PI;    // 3.14159265358979323846...
    simd_f128 e      = SIMD_F128_E;     // 2.71828182845904523536...
    simd_f128 sqrt_2 = SIMD_F128_SQRT2; // 1.41421356237309504880...
    simd_f128 ln2    = SIMD_F128_LN2;   // 0.69314718055994530941...

    return 0;
}

---

simd_f128_io.h

Handles conversion between the internal Double-Double representation and human-readable decimal strings. Standard printf formatting cannot faithfully render 128-bit values; this header uses an iterative high-precision extraction algorithm to produce up to 32 correct decimal places.

#define SIMD_F128_IMPLEMENTATION
#include <simd_f128.h>
#include <simd_f128_io.h>

int main() {
    // parsing from string maintains the full 31-digit precision
    simd_f128 val = simd_f128_from_string("3.1415926535897932384626433832795");

    // direct console output
    simd_f128_print(val);

    // string conversion for logging or ui
    char buffer[128];
    simd_f128_to_string(buffer, sizeof(buffer), val);

    return 0;
}

---

simd_f128_math.h

Advanced mathematical functions built on top of the core Double-Double primitives. All functions are static inline and require no additional compilation unit.

Algorithms used:

• exp — range reduction to $N=16$ intervals followed by a high-degree Chebyshev minimax polynomial, then exact scaling via ldexp and a 16-entry lookup table. Handles overflow (> 709.78) and underflow explicitly.
• log — seeds from the standard double log(), then refines with 1 iteration of Halley's method, which is mathematically sufficient to recover all 31-32 digits due to cubic convergence.
• pow — computed as exp(exp * log(base)). Supports base-zero inputs and propagates NaN according to IEEE-754.
• sin — range-reduces to quadrant ($[-\pi/4, \pi/4]$) then evaluates a highly-tuned Chebyshev minimax polynomial.
• cos — range-reduces to quadrant ($[-\pi/4, \pi/4]$) then evaluates a highly-tuned Chebyshev minimax polynomial.
• sincos — computes both sine and cosine simultaneously, saving redundant Range Reduction and polynomial evaluation steps.

#define SIMD_F128_IMPLEMENTATION
#include <simd_f128.h>
#include <simd_f128_consts.h>
#include <simd_f128_math.h>

int main() {
    simd_f128 x = SIMD_F128_PI;

    // e^π
    simd_f128 epi = simd_f128_exp(x);

    // ln(e) == 1
    simd_f128 one = simd_f128_log(SIMD_F128_E);

    // 2^10 == 1024
    simd_f128 base = simd_f128_from_double(2.0);
    simd_f128 exp  = simd_f128_from_double(10.0);
    simd_f128 pw   = simd_f128_pow(base, exp);

    // sin(π/6) == 0.5
    simd_f128 half_pi = simd_f128_mul(x, simd_f128_from_double(1.0 / 6.0));
    simd_f128 s       = simd_f128_sin(half_pi);

    // cos(0) == 1
    simd_f128 c = simd_f128_cos(simd_f128_from_double(0.0));

    return 0;
}

Note: sin and cos use a simplified range reduction suitable for moderate arguments. For very large inputs (|x| > ~10^15), consider applying Payne-Hanek argument reduction externally before calling these functions.

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simd_f128_utils.h

Comparison operators and utility functions. All are static inline and work with any SIMD backend.

The foundation is simd_f128_cmp, which compares the hi components first and only falls through to the lo components when hi values are identical — matching the canonical Double-Double ordering rule.

#include <simd_f128.h>
#include <simd_f128_utils.h>

int main() {
    simd_f128 a = simd_f128_from_double(1.0);
    simd_f128 b = simd_f128_from_double(2.0);

    // comparisons
    int lt = simd_f128_lt(a, b);  // 1
    int eq = simd_f128_eq(a, b);  // 0
    int ge = simd_f128_ge(b, a);  // 1

    // utility
    simd_f128 neg = simd_f128_from_double(-3.14);
    simd_f128 abs_val = simd_f128_abs(neg);       // 3.14...
    simd_f128 lo      = simd_f128_min(a, b);      // 1.0
    simd_f128 hi      = simd_f128_max(a, b);      // 2.0

    return 0;
}

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simd_f128.hpp

A modern C++ wrapper that makes simd_f128 feel like a native arithmetic type. Include this single header in C++ projects — it pulls in all other headers automatically.

Features:

• f128::float128 class with full operator overloading (+, -, *, /, +=, -=, *=, /=).
• Full interoperability with std::complex<double> via f128::complex128.
• Seamless integration with the Eigen matrix library via simd_f128_eigen.hpp.
• All six comparison operators (==, !=, <, >, <=, >=).
• Unary negation (-x).
• std::ostream integration (std::cout << val).
• Free functions mirroring <cmath>: f128::exp, f128::log, f128::pow, f128::sin, f128::cos, f128::sqrt, f128::abs.
• Predefined constants: f128::pi, f128::e, f128::sqrt2, f128::ln2.

#define SIMD_F128_IMPLEMENTATION
#include <simd_f128.hpp>
#include <iostream>

int main() {
    f128::float128 a(1.5);
    f128::float128 b(2.5);

    // natural arithmetic
    f128::float128 sum  = a + b;
    f128::float128 prod = a * b;

    // math functions
    f128::float128 root = f128::sqrt(a);
    f128::float128 s    = f128::sin(f128::pi);

    // stream output
    std::cout << "a + b = " << sum  << "\n";
    std::cout << "a * b = " << prod << "\n";
    std::cout << "sqrt(a) = " << root << "\n";

    // comparisons
    if (a < b) {
        std::cout << "a is smaller\n";
    }

    return 0;
}

The float128 class stores a simd_f128 data member publicly, so it can be passed directly to any C API function when needed:

f128::float128 val(3.14);
simd_f128_print(val.data);  // call c api directly

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API Reference

simd_f128.h

Function	Signature	Description
simd_f128_from_double	simd_f128 simd_f128_from_double(double d)	Promote a double to 128-bit. lo is initialised to 0.0.
simd_f128_extract	void simd_f128_extract(simd_f128 x, double* hi, double* lo)	Extract the hi and lo components into separate doubles.
simd_f128_add	simd_f128 simd_f128_add(simd_f128 a, simd_f128 b)	Double-Double addition via Knuth's TwoSum.
simd_f128_sub	simd_f128 simd_f128_sub(simd_f128 a, simd_f128 b)	Double-Double subtraction (negates b, then adds).
simd_f128_mul	simd_f128 simd_f128_mul(simd_f128 a, simd_f128 b)	Double-Double multiplication via Dekker's TwoProd + FMA.
simd_f128_div	simd_f128 simd_f128_div(simd_f128 a, simd_f128 b)	Double-Double division via Newton-Raphson reciprocal refinement.
simd_f128_sqrt	simd_f128 simd_f128_sqrt(simd_f128 x)	Square root via inverse-sqrt Newton-Raphson + residual correction.

simd_f128_consts.h

Constant	Value (first 32 digits)
SIMD_F128_PI	3.14159265358979323846264338327950...
SIMD_F128_E	2.71828182845904523536028747135266...
SIMD_F128_SQRT2	1.41421356237309504880168872420969...
SIMD_F128_LN2	0.69314718055994530941723212145817...

simd_f128_io.h

Function	Signature	Description
simd_f128_print	void simd_f128_print(simd_f128 x)	Print the value to stdout followed by a newline.
simd_f128_to_string	void simd_f128_to_string(char* buf, size_t buf_size, simd_f128 x)	Write up to 32 decimal digits into buf. buf must be at least 64 bytes. Handles nan, inf, and negative values.

simd_f128_math.h

Function	Signature	Description
simd_f128_exp	simd_f128 simd_f128_exp(simd_f128 x)	e^x. Returns +Inf for x > 709.78, 0 for x < -745.
simd_f128_log	simd_f128 simd_f128_log(simd_f128 x)	Natural logarithm. Returns NaN for x ≤ 0.
simd_f128_pow	simd_f128 simd_f128_pow(simd_f128 base, simd_f128 exp)	base^exp. Correctly handles base zero, infinity, and NaN according to IEEE-754.
simd_f128_sin	simd_f128 simd_f128_sin(simd_f128 x)	Sine (radians). Best accuracy for moderate arguments.
simd_f128_cos	simd_f128 simd_f128_cos(simd_f128 x)	Cosine (radians). Best accuracy for moderate arguments.
simd_f128_sincos	void simd_f128_sincos(simd_f128 x, simd_f128* s, simd_f128* c)	Computes sine and cosine simultaneously.

simd_f128_utils.h

Function	Signature	Description
simd_f128_cmp	int simd_f128_cmp(simd_f128 a, simd_f128 b)	Returns -1 if a < b, 1 if a > b, 0 if equal.
simd_f128_eq	int simd_f128_eq(simd_f128 a, simd_f128 b)	1 if a == b.
simd_f128_gt	int simd_f128_gt(simd_f128 a, simd_f128 b)	1 if a > b.
simd_f128_lt	int simd_f128_lt(simd_f128 a, simd_f128 b)	1 if a < b.
simd_f128_ge	int simd_f128_ge(simd_f128 a, simd_f128 b)	1 if a >= b.
simd_f128_le	int simd_f128_le(simd_f128 a, simd_f128 b)	1 if a <= b.
simd_f128_abs	simd_f128 simd_f128_abs(simd_f128 x)	Absolute value. Correctly handles -0.0 in the lo component.
simd_f128_min	simd_f128 simd_f128_min(simd_f128 a, simd_f128 b)	Returns the lesser of a and b.
simd_f128_max	simd_f128 simd_f128_max(simd_f128 a, simd_f128 b)	Returns the greater of a and b.

simd_f128.hpp (C++ only)

Symbol	Kind	Description
f128::float128	Class	C++ wrapper around simd_f128.
f128::float128(double)	Constructor	Construct from a double.
f128::float128(simd_f128)	Constructor	Construct from a raw simd_f128.
float128::extract(hi, lo)	Method	Extract hi and lo components.
+, -, *, /	Operators	Arithmetic operators.
+=, -=, *=, /=	Operators	Compound assignment operators.
==, !=, <, >, <=, >=	Operators	Comparison operators.
operator-()	Unary	Negation.
float128::to_string()	Method	Returns std::string with 32-digit representation.
operator<<	Stream	std::ostream integration.
f128::exp, f128::log, f128::pow	Free functions	Transcendental math.
f128::sin, f128::cos, f128::sqrt, f128::abs	Free functions	Trigonometric and utility math.
f128::pi, f128::e, f128::sqrt2, f128::ln2	Constants	High-precision constants as float128.

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Precision Demonstration & Test Results

The core advantage of simd-f128 is preserving small values that standard 64-bit doubles silently discard. All operations execute strictly within SIMD registers without heap allocation.

Here is an actual test run and precision comparison from the Extreme Performance build:

~/Public/simd-f128 master* ⇡
❯ ctest --test-dir build -C Release
Test project /simd-f128/build
    Start 1: arithmetic_test
1/2 Test #1: arithmetic_test ..................   Passed    0.00 sec
    Start 2: arithmetic_test_cpp
2/2 Test #2: arithmetic_test_cpp ..............   Passed    0.00 sec

100% tests passed, 0 tests failed out of 2

~/Public/simd-f128 master* ⇡
❯ ./build/example_precision
--- precision comparison: double vs simd-f128 ---

[double]  1.0 + 1e-17 = 1.00000000000000000000
          precision lost: yes

[simd-f128] 1.0 + 1e-17 = 1.00000000000000001000000000000000
          precision lost: no


~/Public/simd-f128 master* ⇡
❯ ./build/example_mandelbrot
--- mandelbrot core loop (128-bit precision) ---

did not escape after 500 iterations (point is inside the Mandelbrot set)

final |z| components:
  zx = -0.78124578860038387003505655582563
  zy = 0.35443468442007221298624089031401

Performance & Benchmarks

Because simd-f128 operations are purely CPU-register bound, they are extremely fast.

1. Comparative Speed vs __float128

While raw nanoseconds are interesting, a direct comparison against __float128 demonstrates the massive advantage of hardware SIMD over software emulation. The test simulates loop-carried dependency latency (e.g., a = a + b) simulating tight inner-loops in numerical algorithms. Tests run for 10,000,000 operations.

Data Type	Add (ms)	Mul (ms)	Div (ms)
double (64-bit)	9.24	9.23	41.83
long double (x87)	20.70	20.66	48.49
__float128 (GCC)	153.37	193.23	325.37
simd-f128 (AVX2)	99.44	74.46	207.98

View raw console output from bench_compare

$ ./build/benchmarks/bench_compare

simd-f128 Manual Benchmark Comparison
Iterations: 10000000 operations per test (latency mode)

| Data Type          | Add (ms) | Mul (ms) | Div (ms) |
|--------------------|----------|----------|----------|
|--------------------|----------|----------|----------|
| double (64-bit)    |     9.24 |     9.23 |    41.83 |
| long double (x87)  |    20.70 |    20.66 |    48.49 |
| __float128 (GCC)   |   153.37 |   193.23 |   325.37 |
| simd-f128 (SIMD)   |    99.44 |    74.46 |   207.98 |

Analysis: simd-f128 on AVX2 decisively outperforms GCC's software-emulated __float128. Specifically, multiplication is 2.59x faster, addition is 1.54x faster, and division is 1.56x faster. This is achieved through the aggressive use of Hardware FMA (Fused Multiply-Add), which rapidly resolves Dekker's split algorithms natively in silicon without relying on slower branching software emulation.

2. WebAssembly (In-Browser) Benchmarks

The library ships with dual WebAssembly modules to maximise both performance and compatibility. The benchmarks below reflect 1,000,000 continuous simd_f128_mul operations running entirely inside the V8 JavaScript engine (Chrome).

Module Type	Time (ms)	Notes
WASM-SIMD128	~295 ms	Native 128-bit SIMD processing inside the browser.
WASM-Scalar	~481 ms	Fallback for older browsers without SIMD support.
Native JS Number	~1.5 ms	Native 64-bit precision (loss of 15 digits of precision).

Takeaway: WASM-SIMD128 achieves a ~1.6x speedup over scalar WASM inside the browser. While native JS Number is incredibly fast due to JIT compilation of single hardware instructions, it completely fails to preserve precision past 15 digits. simd-f128 enables software running in the browser to maintain 32-digit precision with highly acceptable latency for real-time visualization and mathematical processing.

3. Raw Speed (Google Benchmark)

A single simd_f128_mul completes in ~10 nanoseconds, and advanced math functions run in the ~170-490ns range.

Run on (12 X 3266.69 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x6)
  L1 Instruction 32 KiB (x6)
  L2 Unified 512 KiB (x6)
  L3 Unified 16384 KiB (x1)
-----------------------------------------------------------
Benchmark                 Time             CPU   Iterations
-----------------------------------------------------------
BM_SimdF128_Add        11.7 ns         11.7 ns     60057911
BM_SimdF128_Mul        10.1 ns         10.1 ns     69579904
BM_SimdF128_Div        2.87 ns         2.86 ns    244206304
BM_SimdF128_Sqrt       6.05 ns         6.04 ns    115940003
BM_SimdF128_Exp         192 ns          192 ns      3646032
BM_SimdF128_Log         240 ns          240 ns      2920704
BM_SimdF128_Sin         192 ns          192 ns      3645663
BM_SimdF128_Cos         200 ns          199 ns      3510110
BM_SimdF128_Atan        402 ns          401 ns      1743733
BM_SimdF128_Pow         492 ns          491 ns      1426559

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Double-Double Arithmetic

simd-f128 represents a value $x$ as the unevaluated sum of two IEEE 754 doubles:

$$x = x_{hi} + x_{lo}, \quad |x_{lo}| \leq \frac{1}{2} , \text{ulp}(x_{hi})$$

This non-overlapping constraint provides ~106 bits of mantissa — approximately double the precision of a single double.

Implementation basis:

• Addition: TwoSum (Knuth) — An error-free transformation (EFT) for addition that captures the exact rounding residual.
• Multiplication: TwoProd (Dekker) — Exploits hardware FMA (Fused Multiply-Add) where available. On platforms lacking FMA, it seamlessly falls back to Veltkamp's Split to divide 53-bit mantissas into 26-bit halves, calculating the exact error product natively without precision loss.
• Division: Newton-Raphson Iteration — Approximates the reciprocal $1/b_{hi}$ and refines it quadratically. Includes rigorous guards against NaN propagation during division-by-zero scenarios.
• Square Root: Newton-Raphson with Residual Correction — Uses the hardware sqrt instruction to generate a perfect 53-bit initial guess, followed by a Newton-Raphson iteration with residual correction to accurately recover the full ~106-bit mantissa.
• Normalisation — Every arithmetic operation rigidly re-establishes the non-overlapping property before returning.

No memory allocation is required. The entire number lives in two registers.

Known limitations:

• Numerical range is identical to IEEE 754 double (~1.8 × 10^308). The library extends mantissa precision only; exponent range is unchanged.
• NaN and Infinity propagate through standard double rules.
• sin and cos use simplified range reduction. For large arguments (|x| ≫ 2π), apply Payne-Hanek reduction externally before calling.
• pow does not support negative bases; use simd_f128_mul + simd_f128_exp for integer powers of negative numbers.
• On ARMv7, FMA requires VFPv4 hardware (Cortex-A7, A15, A17, A53+) and the -mfpu=neon-vfpv4 flag.

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Examples

Three runnable examples are provided under examples/.

basic_arithmetic.c — entry point for new users. Loads SIMD_F128_PI and SIMD_F128_E from simd_f128_consts.h, performs addition, subtraction, and multiplication, then prints all three results at full 32-digit precision.

precision_demo.c — demonstrates the core motivation for using this library. Adds 1e-17 to 1.0 using both a standard double and a simd_f128, then prints both results side by side. The double result silently loses the small value; the simd_f128 result preserves it in the lo component.

mandelbrot_core.c — a realistic use case. Runs the Mandelbrot iteration z = z^2 + c at a deep-zoom coordinate that exceeds 64-bit precision, with the correct escape condition (|z|^2 > 4). Reports whether the point escapes and prints the final zx/zy values at full precision.

Quick example — circle area at 32-digit precision:

#include <stdio.h>

#define SIMD_F128_IMPLEMENTATION
#include <simd_f128.h>
#include <simd_f128_consts.h>
#include <simd_f128_io.h>

int main() {
    simd_f128 r    = simd_f128_from_double(10.0);
    simd_f128 r2   = simd_f128_mul(r, r);
    simd_f128 area = simd_f128_mul(SIMD_F128_PI, r2);

    // output: 314.15926535897932384626433832795028
    printf("Circle Area: ");
    simd_f128_print(area);

    return 0;
}

Same example using the C++ wrapper:

#define SIMD_F128_IMPLEMENTATION
#include <simd_f128.hpp>
#include <iostream>

int main() {
    f128::float128 r(10.0);
    f128::float128 area = f128::pi * r * r;

    // output: 314.15926535897932384626433832795028
    std::cout << "Circle Area: " << area << "\n";

    return 0;
}

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Platform Support & CI Status

Every commit is tested across all backends via GitHub Actions. The table below maps each workflow to the platforms and backends it covers.

Workflow	Platform	Backend	Runner
	Linux x86_64	Scalar, AVX2	ubuntu-latest
	Linux ARM64, ARMv7, RISC-V64	NEON, Scalar	ubuntu-latest + QEMU
	Apple Silicon (M1/M2/M3)	NEON	macos-latest
	macOS Intel	Scalar, AVX2	macos-13
	Windows x64 (MSVC)	Scalar	windows-latest
	WebAssembly (Node.js)	WASM-SIMD, Scalar	ubuntu-latest + Emscripten
	Android ARM64, Android ARMv7	NEON, Scalar	ubuntu-latest + QEMU

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Language Bindings

simd-f128 is designed to provide 128-bit precision not just to C/C++, but to higher-level ecosystems.

Python

Using pybind11, the library is exposed as a native CPython extension, bringing 31-digit precision directly into Python scripts.

import simd_f128 as f128

a = f128.from_string("3.14159265358979323846")
b = f128.from_double(2.0)
print((a * b).to_string())

JavaScript / WebAssembly

Compiled via Emscripten, the JS bindings automatically select between WASM-SIMD128 and WASM-Scalar depending on the user's browser support, providing 31-digit precision directly in the browser or Node.js.

Rust

A fully memory-safe Rust wrapper (via cc and bindgen), exposing the C functions safely through idiomatic Rust structs and operator overloads.

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Project Structure

.
├── assets/images/        # Logo and documentation media
├── benchmarks/           # Performance benchmarks (Google Benchmark & Native)
├── examples/             # Runnable usage examples
│   ├── basic_arithmetic.c
│   ├── precision_demo.c
│   └── mandelbrot_core.c
├── tests/                # Arithmetic unit tests (C and C++)
├── .github/workflows/    # CI pipelines (linux, macos, windows, wasm, mobile)
├── include/              # Core library and headers
│   ├── simd_f128.h           # Double-Double arithmetic engine
│   ├── simd_f128_consts.h    # High-precision mathematical constants
│   ├── simd_f128_io.h        # String conversion and console output
│   ├── simd_f128_math.h      # Advanced mathematical functions (exp, log, sin, cos, pow)
│   ├── simd_f128_utils.h     # Comparison and utility functions (cmp, abs, min, max)
│   └── simd_f128.hpp         # Modern C++ wrapper with operator overloading
├── js/                   # JavaScript bindings and WebAssembly module
├── python/               # Python bindings (pybind11)
├── rust/                 # Rust bindings (FFI via cc)
├── CMakeLists.txt        # Cross-platform build configuration
└── LICENSE               # MIT License

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Used By

Project	Description
mandelbrot-c	Deep-zoom Mandelbrot renderer in C, using simd-f128 for 128-bit precision coordinates

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Development Methodology & AI Assistance

Building a high-performance, header-only Double-Double (128-bit) floating-point library from scratch involves handling incredibly complex edge cases—from vectorized SIMD alignment to IEEE 754 catastrophic cancellation and precision loss bounds.

To achieve this level of stability and performance within a short timeframe, this project was architected and rigorously verified in collaboration with Advanced Agentic AI. AI was specifically utilized to:

• Stress-test the arithmetic core and transcendental functions (such as sin, exp, log, pow) against extreme floating-point edge cases (including subnormals, underflow/overflow thresholds, and NaN propagation).
• Assist in optimizing cross-platform SIMD intrinsics (AVX2, NEON, WASM-SIMD128) and ensuring strict adherence to zero-heap-allocation constraints.
• Automate the generation of robust cross-platform CI/CD pipelines and verification suites (covering C, C++, Rust, Python, and WebAssembly).

However, human agency remains at the core of this project. Every single line of code generated or suggested was manually inspected, audited, and strictly verified. The core architecture, mathematical algorithms, and memory constraints were meticulously human-planned. This hybrid approach—combining human architectural vision with AI-driven debugging and verification—allowed us to push the boundaries of performance and reliability in a modern C library without compromising mathematical rigor or code ownership.

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Author's Note

I'm just a kid building projects as a hobby. Thank you for showing interest in my little library! It really means a lot to me. :)

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Contributing

I am still a learner in the field of numerical computing and low-level C programming. If you spot a precision bug, an incorrect algorithm, or an edge case I have missed — especially around FMA behaviour, normalisation stability, or platform-specific SIMD quirks — I would be genuinely grateful for the feedback. Every correction and suggestion is a lesson I would not have found on my own.

If you would like to help:

1. Open an issue to discuss bugs, inaccuracies, or potential improvements.
2. To contribute code, please fork the repository and open a pull request with a clear description of what was changed and why.
3. If you have expertise in Double-Double arithmetic or compiler-level float optimisation, architectural feedback is especially welcome.

Thank you for taking the time to read this far, and for helping make this project more correct.

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License

This project is licensed under the MIT License - see the LICENSE file for details.