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Arm Compiler armclang Reference Guide : -munaligned-access, -mno-unaligned-access

-munaligned-access, -mno-unaligned-access

Enables or disables unaligned accesses to data on Arm® processors.

The compiler defines the __ARM_FEATURE_UNALIGNED macro when -munaligned-access is enabled.

The libraries include special versions of certain library functions designed to exploit unaligned accesses. When unaligned access support is enabled, using -munaligned-access, the compilation tools use these library functions to take advantage of unaligned accesses. When unaligned access support is disabled, using -mno-unaligned-access, these special versions are not used.

Default

-munaligned-access is the default for architectures that support unaligned accesses to data. This applies to all architectures supported by Arm Compiler 6, except Armv6‑M, and Armv8‑M without the Main Extension.

Usage

-munaligned-access

Use this option on processors that support unaligned accesses to data, to speed up accesses to packed structures.

Note

Compiling with this option generates an error for the following architectures:

  • Armv6‑M.
  • Armv8‑M without the Main Extension.
-mno-unaligned-access

If unaligned access is disabled, any unaligned data that is wider than 8-bit is accessed one byte at a time. For example, fields wider than 8-bit, in packed data structures, are always accessed one byte at a time even if they are aligned.

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