https://github.com/adbancroft/avr-fast-shift

Optimized 32-bit shifting for avr-gcc
https://github.com/adbancroft/avr-fast-shift

avr gcc performance

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Optimized 32-bit shifting for avr-gcc

• Host: GitHub
• URL: https://github.com/adbancroft/avr-fast-shift
• Owner: adbancroft
• License: lgpl-2.1
• Created: 2024-08-27T02:12:21.000Z (4 months ago)
• Default Branch: master
• Last Pushed: 2024-10-11T17:28:59.000Z (3 months ago)
• Last Synced: 2024-10-16T06:34:19.776Z (3 months ago)
• Topics: avr, gcc, performance
• Language: C++
• Homepage:
• Size: 75.2 KB
• Stars: 0
• Watchers: 2
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: readme.md
  • License: LICENSE

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# avr-fast-shift: optimized 32-bit shifting for avr-gcc

As of AVR-GCC 14.2.0, the code produced for unsigned 32-bit shifts with a *compile time shift distance* is very poor when using -O2 or -O3. E.g.

    rpmDelta = (toothDeltaV << 10) / (6 * toothDeltaT);

The functions provided here implement optimised left and right shifting of `uint32_t` up to 31 places.

On a physical AtMega2560, up to: 

* 35% increase in right shift performance

* 22% increase in left shift performance

See timing unit tests.

## Using the library

### Installation

The library is available in both the [Arduino Library](https://www.arduino.cc/reference/en/libraries/avr-fast-shift/) and [PlatformIO Library](https://registry.platformio.org/libraries/adbancroft/avr-fast-shift) registries. 

The library can also be cloned & included locally or included directly from GitHub (if your tooling supports it). 

### Code

1. `#include `

2. Replace all 32-bit shift operations that use a compile time constant shift distance with the equivalent shift function. I.e.

    * `a >> b` -> `rshift(a)`

    * `a << b` -> `lshift(a)`

    * E.g.

    * `rpmDelta = (toothDeltaV << 10) / (6 * toothDeltaT);` -> `rpmDelta = lshift<10U>(toothDeltaV) / (6 * toothDeltaT);`