math_bits

Header-only C++20 library for multiplying integers by a constant floating-point factor using integer bit-shifting — no FPU, no runtime division, compile-time unit tests.

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Features

• No floating-point at runtime — all FPU operations happen at compile time. The generated code is pure integer arithmetic.
• Compile-time parameter generation — multiplier, bit-shift count, and integer scale factor are all derived at compile time from the floating-point input.
• Overflow safe — the maximum multiplication factor is computed at compile time to guarantee no overflow for the given input range.
• Configurable accuracy — max_error (in the options traits class) sets the allowed deviation from the true floating-point result. Defaults to ±1 LSB.
• Compile-time unit tests — a static_assert runs a full test suite at compile time. A broken instantiation will not compile.
• Header-only — single .h file, no dependencies beyond the C++ standard library.
• Always-inlined hot path — mult() is unconditionally [[gnu::always_inline]], so the integer multiply-and-shift fuses into the caller with no extra flag.

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Requirements

• C++20 or later (std::bit_width is used for compile-time bit counting)
• Any C++20 compiler (GCC, Clang, MSVC)
• No hardware FPU required — designed for Cortex-M0/M0+ and other FPU-less targets

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Usage

Basic

#include "math_bits.h"

// Multiply uint16_t values by 0.75, input range [0, 1000], default options
using scale75 = mult_bitshift<0.75, (uint16_t)1000, uint16_t, uint32_t>;

uint16_t result = scale75::mult(800);  // result ≈ 600

Operator overload

scale75 scaler;
uint16_t result = scaler * 800;  // same as scale75::mult(800)

Customizing options

The optional flags (max_error, deep_test, clamp_input) live in a traits-class struct. Derive from mult_bitshift_options and override only what you want — everything else stays at its default.

struct fast_safe : mult_bitshift_options {
    static constexpr bool deep_test   = false;   // skip deep compile-time sweep
    static constexpr bool clamp_input = true;    // clamp inputs > max_input_value
};
using scale75_safe = mult_bitshift<0.75, (uint16_t)1000, uint16_t, uint32_t, fast_safe>;

uint16_t result = scale75_safe::mult(2000); // returns mult(1000), not garbage

Option structs compose — derive from another option struct to extend it:

struct fast : mult_bitshift_options {
    static constexpr bool deep_test = false;
};
struct fast_with_clamp : fast {
    static constexpr bool clamp_input = true;
};

Backwards-compatible legacy form

The previous positional-argument signature is preserved as mult_bitshift_legacy. Existing call sites can keep working by renaming mult_bitshift → mult_bitshift_legacy:

// Same configuration as scale75_safe above, in the old positional form.
// The force_inlining parameter (position 6) is accepted for source
// compatibility but has no effect — mult() is now unconditionally
// always_inline.
using scale75_safe_legacy =
    mult_bitshift_legacy<0.75, (uint16_t)1000, uint16_t, uint32_t,
                         /*max_error*/1, /*force_inlining (unused)*/false,
                         /*deep_test*/false, /*clamp_input*/true>;

New code should prefer the traits-class form — the legacy form is kept only to avoid breaking existing instantiations. Note that the legacy form preserves the pre-refactor default deep_test=true, while the traits-class form's default is false; set it explicitly if the difference matters.

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Template Parameters

mult_bitshift takes five template parameters: two required values, two type parameters with defaults, and a traits-class type carrying the optional flags.

Parameter	Default	Description
multvalue	—	Floating-point multiplier (float, double, or long double)
max_input_value	—	Maximum input value the multiplier must handle without overflow
io_type	uint32_t	Input and output integer type. Must be unsigned.
calc_type	uint32_t	Internal calculation type. Must be unsigned and at least as wide as io_type.
Options	mult_bitshift_options	Traits-class type carrying the optional flags below. Derive from mult_bitshift_options and override only the members you want.

mult_bitshift_options members

All members are static constexpr. Override only the ones you want by deriving a new struct:

Member	Type	Default	Description
max_error	uint64_t	1	Maximum allowed deviation from the true floating-point result (in LSB). Generalized to uint64_t so the struct doesn't depend on io_type; the class casts back to io_type internally. Must fit in io_type.
deep_test	bool	false	Default false runs a quick 100-sample smoke test at compile time — fast to build. Set true for the full sweep (up to 65535 inputs) when you want maximum assurance and can absorb the compile-time cost.
clamp_input	bool	false	If true, clamp inputs above max_input_value to max_input_value before multiplying — guarantees output stays within the max_input_value * mult_factor envelope. Adds ~5 instructions on the hot path. When false, the clamp disappears entirely (zero cost).

Legacy positional form: mult_bitshift_legacy

For backwards compatibility, the previous positional signature is preserved as a separate alias. The legacy form's defaults match the pre-refactor mult_bitshift defaults — notably deep_test=true (full 65535-sample sweep). The new traits-class form's deep_test default was deliberately changed to false for faster compiles; if you want the deep sweep, either use the legacy form or override deep_test=true in your options struct.

Position	Parameter	Default
1	multvalue	—
2	max_input_value	—
3	io_type	uint32_t
4	calc_type	uint32_t
5	max_error	1
6	force_inlining (no effect — kept for source compatibility)	false
7	deep_test	true
8	clamp_input	false

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API Reference

Function	Description
mult(input)	Multiply input by the configured factor. Static — no instance needed.
operator*(val)	Instance operator overload — calls mult(val).
operator*(val, rhs)	Friend operator overload — val * scaler.

Compile-time constants

Constant	Description
mult_factor	The original floating-point multiplier
max_input_int	The configured maximum input value
bitShifts	Number of bits shifted in the integer multiplication
mult_factor_int	The integer scale factor derived from mult_factor
max_output_int	Precomputed mult(max_input_int) — the largest value mult() will ever return
max_error	The configured max_error from Options, cast to io_type
max_deviation	Same as max_error — kept for backwards compatibility
deep_test	The configured deep_test flag from Options
clamp_input	The configured clamp_input flag from Options
options	The Options traits-class type itself, exposed for inspection

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Design Notes

Why bit-shifting instead of floating-point? On Cortex-M0/M0+ there is no FPU. A floating-point multiply compiles to a software library call — slow, non-deterministic, and unsuitable for ISRs. By computing the scale factor at compile time and using a single integer multiply + shift at runtime, the hot path becomes 2–3 instructions with deterministic latency.

Why compile-time unit tests? The test suite verifies that every value in a representative sample of the input range produces a result within max_error of the true floating-point result. If the chosen max_error is too tight for the given multiplier and types, the build fails with a clear message — no separate test binary required. By default (deep_test=false) the sweep runs 100 samples — fast to compile and adequate for catching gross errors. Set deep_test=true for the full sweep (up to 65535 samples) when you want maximum assurance.

Why waste one extra type parameter for calc_type? The intermediate product input * mult_factor_int can overflow io_type. Using a wider calc_type (e.g. uint32_t when io_type is uint16_t) keeps the intermediate value safe and shifts back down to io_type at the end.

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Performance (STM32G051, Cortex-M0+, -Os)

Verified by inspecting arm-none-eabi-g++ output for representative configurations:

• mult() with calc_type ≤ uint32_t: hot path is muls + lsrs — one integer multiply, one bit-shift. A 1–2 instruction movs/lsls constant-load preamble brings the total to ~4 instructions on Cortex-M0+ (the constant load is an M0+ immediate-encoding limitation, not a library limitation).
• mult() with calc_type = uint64_t: the multiply is widened, so the compiler emits a call to the integer runtime helper __aeabi_lmul — still no FPU, still deterministic, but no longer a single instruction. Prefer calc_type=uint32_t on Cortex-M0+ when your inputs and multiplier allow it.
• No FPU instructions in either case — zero soft-float library calls at runtime.
• Compile-time overhead: parameter generation and unit test run entirely at compile time — zero runtime cost.
• clamp_input=true is fully inlined into the caller (~9 instructions on the common path) thanks to the unconditional [[gnu::always_inline]] on mult(). The clamp path uses an early return with a precomputed max_output_int.
• [[gnu::flatten]] on user code is the strongest way to force transitive inlining at a specific call site — useful when calling mult() from a hot loop where you want every nested call (e.g. the operator overloads, or chained scalers) inlined alongside mult() itself.

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License

Copyright (c) 2026 Erik Nørskov / PxQ Technologies — https://pxq.dk

This software is dual-licensed:

1. Open Source — GNU General Public License v3.0 (GPLv3): Free to use, modify, and distribute under the terms of the GNU General Public License version 3, as published by the Free Software Foundation. Note that GPLv3 is strong copyleft — derivative works and products that incorporate this software must also be released under GPLv3.

2. Commercial License: For use in proprietary or closed-source products that cannot or do not wish to comply with the GPLv3, a commercial license is available from PxQ Technologies — either as a written agreement, or via direct delivery by Erik Nørskov as part of a paid engagement (in which case the license is granted for that specific project scope only).

Each commercial license covers only the version of the software actually delivered into the licensee's project by the licensor. Later versions become covered only when likewise delivered as part of a paid engagement or written agreement, or when the licensee obtains a separate paid license for that later version. The licensee may not substitute or upgrade the software to any later version on their own initiative without such a license.

Contact: https://pxq.dk