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[libc] Improve memcpy for ARM Cortex-M supporting unaligned accesses. (#144872)

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This implementation has been compiled with the [pigweed toolchain](https://pigweed.dev/toolchain.html) and tested on:
 - Raspberry Pi Pico 2 with the following options\
`--target=armv8m.main-none-eabi`
`-march=armv8m.main+fp+dsp`
`-mcpu=cortex-m33` 
 - Raspberry Pi Pico with the following options\
`--target=armv6m-none-eabi`
`-march=armv6m`
`-mcpu=cortex-m0+` 

They both compile down to a little bit more than 200 bytes and are between 2 and 10 times faster than byte per byte copies.

For best performance the following options can be set in the `libc/config/baremetal/arm/config.json`
```
{
  "codegen": {
    "LIBC_CONF_KEEP_FRAME_POINTER": {
      "value": false
    }
  },
  "general": {
    "LIBC_ADD_NULL_CHECKS": {
      "value": false
    }
  }
}
```
This commit is contained in:
Guillaume Chatelet
2025-06-26 08:18:02 +00:00
committed by GitHub
parent 0e268791c5
commit 7289b6789e
6 changed files with 226 additions and 2 deletions
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utils/bazel/llvm-project-overlay/libc/BUILD.bazel
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@@ -4218,6 +4218,7 @@ libc_support_library(
"src/string/memory_utils/aarch64/inline_memcpy.h",
"src/string/memory_utils/aarch64/inline_memmove.h",
"src/string/memory_utils/aarch64/inline_memset.h",
"src/string/memory_utils/arm/inline_memcpy.h",
"src/string/memory_utils/generic/aligned_access.h",
"src/string/memory_utils/generic/byte_per_byte.h",
"src/string/memory_utils/inline_bcmp.h",