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d0a774c8d1
optimize memset and memclr for ARM This commit optimizes those routines by rewriting them in assembly and performing the memory copying in 32-bit chunks, rather than in 8-bit chunks as it was done before this commit. This assembly implementation is compatible with the ARMv6 and ARMv7 architectures. This change results in a reduction of runtime of about 40-70% in all cases that matter (the compiler will never use these intrinsics for sizes smaller than 4 bytes). See data below: | Bytes | HEAD | this PR | diff | | ----- | ---- | ------- | ---------- | | 0 | 6 | 14 | +133.3333% | | 1 | 10 | 13 | +30% | | 2 | 14 | 13 | -7.1429% | | 3 | 18 | 13 | -27.77% | | 4 | 24 | 21 | -12.5% | | 16 | 70 | 36 | -48.5714% | | 64 | 263 | 97 | -63.1179% | | 256 | 1031 | 337 | -67.3133% | | 1024 | 4103 | 1297 | -68.389% | All times are in clock cycles. The measurements were done on a Cortex-M3 processor running at 8 MHz using the technique described [here]. [here]: http://blog.japaric.io/rtfm-overhead --- For relevance all pure Rust programs for Cortex-M microcontrollers use memclr to zero the .bss during startup so this change results in a quicker boot time. Some questions / comments: - ~~the original code (it had a bug) comes from this [repo] and it's licensed under the ICS license. I have preserved the copyright and license text in the source code. IANAL, is that OK?~~ no longer applies. The intrinsics are written in Rust now. - ~~I don't know whether this ARM implementation works for ARMv4 or ARMv5. @FenrirWolf and @Uvekilledkenny may want to take look at it first.~~ no longer applies. The intrinsics are written in Rust now. - ~~No idea whether this implementation works on processors that have no thumb instruction set. The current implementation uses 16-bit thumb instructions.~~ no longer applies. The intrinsics are written in Rust now. - ~~The loop code can be rewritten in less instructions but using 32-bit thumb instructions. That 32-bit version would only work on ARMv7 though. I have yet to check whether that makes any difference in the runtime of the intrinsic.~~ no longer applies. The intrinsics are written in Rust now. - ~~I'll look into memcpy4 next.~~ done [repo]: https://github.com/bobbl/libaeabi-cortexm0