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Introduction
Compiler Command-line Options
-Aopt
--allow_fpreg_for_nonfpdata, --no_allow_fpreg_for_
--allow_null_this, --no_allow_null_this
--alternative_tokens, --no_alternative_tokens
--anachronisms, --no_anachronisms
--apcs=qualifer...qualifier
--arm
--arm_only
--asm
--asm_dir=directory_name
--autoinline, --no_autoinline
--bigend
--bitband
--brief_diagnostics, --no_brief_diagnostics
--bss_threshold=num
-c
-C
--c90
--c99
--code_gen, --no_code_gen
--compatible=name
--compile_all_input, --no_compile_all_input
--conditionalize, --no_conditionalize
--cpp
--cpu=list
--cpu=name
--create_pch=filename
-Dname[(parm-list)][=def]
--data_reorder, --no_data_reorder
--debug, --no_debug
--debug_macros, --no_debug_macros
--default_extension=ext
--dep_name, --no_dep_name
--depend=filename
--depend_dir=directory_name
--depend_format=string
--depend_single_line, --no_depend_single_line
--depend_system_headers, --no_depend_system_header
--depend_target=target
--diag_error=tag[,tag,...]
--diag_remark=tag[,tag,...]
--diag_style={arm|ide}
--diag_suppress=tag[,tag,...]
--diag_suppress=optimizations
--diag_warning=tag[,tag,...]
--diag_warning=optimizations
--dollar, --no_dollar
--dwarf2
--dwarf3
-E
--echo
--emit_frame_directives, --no_emit_frame_directive
--enum_is_int
--errors=filename
--exceptions, --no_exceptions
--exceptions_unwind, --no_exceptions_unwind
--extended_initializers, --no_extended_initializer
--feedback=filename
--force_new_nothrow, --no_force_new_nothrow
--forceinline
--fp16_format=format
--fpmode=model
--fpu=list
--fpu=name
--friend_injection, --no_friend_injection
-g
--global_reg=reg_name[,reg_name,...]
--guiding_decls, --no_guiding_decls
--help
-Idir[,dir,...]
--ignore_missing_headers
--implicit_include, --no_implicit_include
--implicit_include_searches, --no_implicit_include
--implicit_key_function, --no_implicit_key_functio
--implicit_typename, --no_implicit_typename
--info=totals
--inline, --no_inline
--interface_enums_are_32_bit
--interleave
-Jdir[,dir,...]
--kandr_include
-Lopt
--library_interface=lib
--library_type=lib
--licretry
--link_all_input, --no_link_all_input
--list
--list_dir=directory_name
--list_macros
--littleend
--locale=lang_country
--long_long
--loose_implicit_cast
--lower_ropi, --no_lower_ropi
--lower_rwpi, --no_lower_rwpi
--ltcg
-M
--md
--message_locale=lang_country[.codepage]
--min_array_alignment=opt
--mm
--multibyte_chars, --no_multibyte_chars
--multifile, --no_multifile
--multiply_latency=cycles
--nonstd_qualifier_deduction, --no_nonstd_qualifie
-o filename
-Onum
--old_specializations, --no_old_specializations
--old_style_preprocessing
--omf_browse
-Ospace
-Otime
--output_dir=directory_name
-P
--parse_templates, --no_parse_templates
--pch
--pch_dir=dir
--pch_messages, --no_pch_messages
--pch_verbose, --no_pch_verbose
--pending_instantiations=n
--phony_targets
--pointer_alignment=num
--preinclude=filename
--preprocess_assembly
--preprocessed
--protect_stack, --no_protect_stack
--reassociate_saturation, --no_reassociate_saturat
--reduce_paths, --no_reduce_paths
--remarks
--remove_unneeded_entities, --no_remove_unneeded_e
--restrict, --no_restrict
--retain=option
--rtti, --no_rtti
--rtti_data, --no_rtti_data
-S
--show_cmdline
--signed_bitfields, --unsigned_bitfields
--signed_chars, --unsigned_chars
--split_ldm
--split_sections
--strict, --no_strict
--strict_warnings
--sys_include
--thumb
--trigraphs, --no_trigraphs
--type_traits_helpers, --no_type_traits_helpers
-Uname
--unaligned_access, --no_unaligned_access
--use_frame_pointer
--use_pch=filename
--using_std, --no_using_std
--version_number
--vfe, --no_vfe
--via=filename
--vla, --no_vla
--vsn
-W
--wchar, --no_wchar
--wchar16
--wchar32
--whole_program
--wrap_diagnostics, --no_wrap_diagnostics
Language Extensions
Compiler-specific Features
C and C++ Implementation Details
Semihosting
ARMv6 SIMD Instruction Intrinsics
Via File Syntax
Standard C Implementation Definition
Standard C++ Implementation Definition
C and C++ Compiler Implementation Limits
Compiler Reference Guide
--unaligned_access, --no_unaligned_access
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| Home > Compiler Command-line Options > --unaligned_access, --no_unaligned_access | |||
These options enable and disable unaligned accesses to data on ARM architecture-based processors.
DefaultThe default is --unaligned_access on ARM-architecture
based processors that support unaligned accesses to data. This includes:
all ARMv6 architecture-based processors
ARMv7-R and ARMv7-M 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
ARMv6-M architecture-based processors.
--unaligned_accessUse
--unaligned_accesson 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
Abit, bit 1, of CP15 register 1 in your initialization code.Set the
Ubit, bit 22, of CP15 register 1 in your initialization code.The initial value of the
Ubit is determined by the UBITINIT input to the core. The MMU must be on, and the memory marked as normal memory.
The libraries include special versions of certain library functions designed to exploit unaligned accesses. When unaligned access support is enabled, the compilation tools use these library functions to take advantage of unaligned accesses.
--no_unaligned_accessUse
--no_unaligned_accessto 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
Abit, bit 1, of CP15 register 1 in your initialization code.Set the
Ubit, bit 22, of CP15 register 1 in your initialization code.The initial value of the
Ubit 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 = 1in CP15 register 1 is not supported by the compiler, or more generally, by the ARM compiler toolset.The 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 have to specify
--no_unaligned_accesson both the compiler command line and the assembler command line when compiling a mixture of C and C++ source files and assembly language source files.
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.
Assembler command line syntax in the Assembler Reference.
| Copyright © 2007-2008, 2011-2012 ARM. All rights reserved. | ARM DUI 0376D |
| Non-Confidential | ID062912 |
 PDF version | |
| Home > Compiler Command-line Options > --unaligned_access, --no_unaligned_access |
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