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Compiler Command-line Options

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Compiler-specific Features

Keywords and operators

__pure

__svc_indirect_r7

__value_in_regs

__declspec attributes

__declspec(dllexport)

__declspec(dllimport)

__declspec(noinline)

__declspec(noreturn)

__declspec(nothrow)

__declspec(notshared)

__declspec(thread)

Function attributes

__attribute__((alias))

__attribute__((always_inline))

__attribute__((const))

__attribute__((deprecated))

__attribute__((malloc))

__attribute__((noinline))

__attribute__((noreturn))

__attribute__((pure))

__attribute__((section))

__attribute__((unused))

__attribute__((used))

__attribute__((weak))

Type attributes

__attribute__((aligned))

__attribute((packed))

__attribute__((transparent_union))

Variable attributes

__attribute__((alias))

__attribute__((at(address)))

__attribute__((aligned))

__attribute__((deprecated))

__attribute__((packed))

__attribute__((section))

__attribute__((transparent_union))

__attribute__((unused))

__attribute__((used))

__attribute__((weak))

__attribute__((zero_init))

#pragma [no_]anon_unions

#pragma arm section [section_sort_list]

#pragma diag_default tag[,tag,...]

#pragma diag_error tag[,tag,...]

#pragma diag_remark tag[,tag,...]

#pragma diag_suppress tag[,tag,...]

#pragma diag_warning tag[, tag, ...]

#pragma [no_]exceptions_unwind

#pragma hdrstop

#pragma import symbol_name

#pragma [no_]inline

#pragma [no_]softfp_linkage

#pragma unroll [(n)]

#pragma unroll_completely

Instruction intrinsics

__builtin_constant_p

__builtin_expect

__builtin_frame_address

__builtin_return_address

__builtin_popcount

__memory_changed

__return_address

__schedule_barrier

ARMv6 SIMD intrinsics

ETSI basic operations

C55x intrinsics

Named register variables

Compiler predefines

Predefined macros

C and C++ Implementation Details

Via File Syntax

Standard C Implementation Definition

Standard C++ Implementation Definition

C and C++ Compiler Implementation Limits

__pure

RealView^® Compilation Tools for μVision Compiler Reference Guide

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Home > Compiler-specific Features > Keywords and operators > __pure

4.1.12. `__pure`

The __pure keyword asserts that a function declaration is pure.

A function is pure only if:

the result depends exclusively on the values of its arguments
the function has no side effects.

__pure is a function qualifier. It affects the type of a function.

Note

This keyword has the function attribute equivalent __attribute__((const)).

Default

By default, functions are assumed to be impure.

Usage

Pure functions are candidates for common subexpression elimination.

Restrictions

A function that is declared as pure can have no side effects. For example, pure functions:

cannot call impure functions
cannot use global variables or dereference pointers, because the compiler assumes that the function does not access memory, except stack memory
must return the same value each time when called twice with the same parameters.

Example

int factr(int n) __pure
{
    int f = 1;

    while (n > 0)
        f *= n--;
    return f;}

See also

__attribute__((const))
__pure in the Compiler User Guide
Placing ARM function qualifiers in the Compiler User Guide.

Copyright © 2007 ARM Limited. All rights reserved.	ARM DUI 0376A
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