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2.1.117. --[no_]unaligned_access

This option enables or disables unaligned accesses to data on ARM architecture-based processors.

Default

The default is --unaligned_access on ARM-architecture based processors that support unaligned accesses to data. This includes:

  • all ARMv6 architecture-based processors

  • ARMv7-A and ARMv7-R architecture-based processors.

The default is --no_unaligned_access on ARM-architecture based processors that do not support unaligned accesses to data. This includes:

  • all pre-ARMv6 architecture-based processors

  • ARMv7-M architecture-based processors.

Usage

--unaligned_access

Use --unaligned_access on processors that support unaligned accesses to data, for example --cpu=ARM1136J-S, to speed up accesses to packed structures.

To enable unaligned support, you must:

  • Clear the A bit, bit 1, of CP15 register 1 in your initialization code.

  • Set the U bit, bit 22, of CP15 register 1 in your initialization code.

    The initial value of the U bit is determined by the UBITINIT input to the core.

The RVCT libraries include special versions of certain library functions designed to exploit unaligned accesses. When unaligned access support is enabled, the RVCT tools use these library functions to take advantage of unaligned accesses.

--no_unaligned_access

Use --no_unaligned_access to disable the generation of unaligned word and halfword accesses on ARMv6 processors.

To enable modulo four-byte alignment checking on an ARMv6 target without unaligned accesses, you must:

  • Set the A bit, bit 1, of CP15 register 1 in your initialization code.

  • Set the U bit, bit 22, of CP15 register 1 in your initialization code.

    The initial value of the U bit is determined by the UBITINIT input to the core.

Note

Unaligned doubleword accesses, for example unaligned accesses to long long integers, are not supported by ARM processor cores. Doubleword accesses must be either eight-byte or four-byte aligned.

The compiler does not provide support for modulo eight-byte alignment checking. That is, the configuration U = 0, A = 1 in CP15 register 1 is not supported by the compiler, or more generally, by the RVCT toolset.

The RVCT libraries include special versions of certain library functions designed to exploit unaligned accesses. To prevent these enhanced library functions being used when unaligned access support is disabled, you need to specify --no_unaligned_access on both the compiler command line and the assembler command line when compiling a mixture of C and C++ source files and asssembly language source files.

Restrictions

Code compiled for processors supporting unaligned accesses to data can run correctly only if the choice of alignment support in software matches the choice of alignment support on the processor core.

See also

Copyright © 2007, 2010 ARM Limited. All rights reserved.ARM DUI 0348A
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