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RealView Compiler User's Guide
Preface Introduction Getting started with the ARM Compiler Compiler Features Coding Practices Optimizing code
Optimizing for size versus speed Optimization levels and the debug view Selecting the target device Optimizing loops Using volatile Code metrics Measuring code and data sizes Measuring stack use Reducing debug information in objects and librarie Functions Minimizing parameter passing overhead __value_in_regs __pure Placing ARM function qualifiers Inlining How the compiler decides to inline When is it practical for the compiler to inline? Managing inlining Inlining functions across multiple files Debugging data and the --[no]_inline keyword Marking functions as static Setting breakpoints on inlined functions in ROM im Aligning data About data alignment The __packed qualifier and unaligned accesses to d __packed structures versus individually __packed f Using floating-point arithmetic Support for floating-point operations VFP architectures The --fpu option Floating-point linkage The --fpmode option Using the --fpu option VFP support Trapping and identifying division-by-zero errors Integer division (Software) Floating point division Support for ARM architecture v6 Instruction generation Alignment support Endian support Diagnostic Messages Using the Inline and Embedded Assemblers SemihostingAbout data alignment
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| Home > Coding Practices > Aligning data > About data alignment | |||
All accesses to data in memory can be classified into the following categories:
Natural alignment, for example, on a word boundary at 0x1000. The ARM compiler normally aligns variables and pads structures so that these items are accessed efficiently using
LDRandSTRinstructions.Known but non-natural alignment, for example, a word at address 0x1001. This type of alignment commonly occurs when structures are packed to remove unnecessary padding. In C and C++, the
__packedqualifier is used to signify that a structure is packed.Unknown alignment, for example, a word at an arbitrary address. This type of alignment commonly occurs when defining a pointer that can point to a word at any address. In C and C++, the
__packedqualifier is used to signify that a pointer that can access a word on a non-natural alignment boundary.
See The __packed qualifier and unaligned accesses to data for more information about the __packed qualifier, packed structures, and unaligned pointers.
| Copyright © 2007 ARM Limited. All rights reserved. | ARM DUI 0375A |
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