ATTRIBUTE(3) Library Functions Manual ATTRIBUTE(3)
attribute -- non-standard GCC attribute extensions
The GNU Compiler Collection (GCC) provides many extensions to the
standard C language. Among these are the so-called attributes. In
NetBSD all attributes are provided in a restricted namespace. The
described macros should be preferred instead of using the GCC's
__attribute__ extension directly.
The gcc(1) compiler knows that certain functions such as abort(3)
and exit(3) can never return any value. When such a function is
equipped with __dead, certain optimizations are possible. Obviously
a __dead function can never have return type other than void.
A __pure function is defined to be one that has no effects except
the return value, which is assumed to depend only on the function
parameters and/or global variables. Any access to parameters and/or
global variables must also be read-only. A function that depends on
volatile memory, or other comparable system resource that can change
between two consecutive calls, can never be __pure. Many math(3)
functions satisfy the definition of a __pure function, at least in
theory. Other examples include strlen(3) and strcmp(3).
A ``const function'' is a stricter variant of ``pure functions''.
In addition to the restrictions of pure functions, a function
declared with __constfunc can never access global variables nor take
pointers as parameters. The return value of these functions must
depend only on the passed-by-value parameters. Note also that a
function that calls non-const functions can not be __constfunc. The
canonical example of a const function would be abs(3). As with pure
functions, certain micro-optimizations are possible for functions
declared with __constfunc.
GCC is known for aggressive function inlining. Sometimes it is
known that inlining is undesirable or that a function will perform
incorrectly when inlined. The __noinline macro expands to a
function attribute that prevents GCC for inlining the function,
irrespective whether the function was declared with the inline
keyword. The attribute takes precedence over all other compiler
options related to inlining.
In most GCC versions the common -Wall flag enables warnings produced
by functions that are defined but unused. Marking an unused
function with the __unused macro inhibits these warnings.
The __used macro expands to an attribute that informs GCC that a
static variable or function is to be always retained in the object
file even if it is unreferenced.
The __packed macro expands to an attribute that forces a variable or
structure field to have the smallest possible alignment, potentially
disregarding architecture specific alignment requirements. The
smallest possible alignment is effectively one byte for variables
and one bit for fields. If specified on a struct or union, all
variables therein are also packed. The __packed macro is often
useful when dealing with data that is in a particular static format
on the disk, wire, or memory.
The __aligned() macro expands to an attribute that specifies the
minimum alignment in bytes for a variable, structure field, or
function. In other words, the specified object should have an
alignment of at least x bytes, as opposed to the minimum alignment
requirements dictated by the architecture and the ABI. Possible use
o Mixing assembly and C code.
o Dealing with hardware that may impose alignment
requirements greater than the architecture itself.
o Using instructions that may impose special alignment
requirements. Typical example would be alignment of
frequently used objects along processor cache lines.
Note that when used with functions, structures, or structure
members, __aligned() can only be used to increase the alignment. If
the macro is however used as part of a typedef, the alignment can
both increase and decrease. Otherwise it is only possible to
decrease the alignment for variables and fields by using the
__packed macro. The effectiveness of __aligned() is largely
dependent on the linker. The __alignof__(3) operator can be used to
examine the alignment.
The __section() macro expands to an attribute that specifies a
particular section to which a variable or function should be placed.
Normally the compiler places the generated objects to sections such
as ``data'' or ``text''. By using __section(), it is possible to
override this behavior, perhaps in order to place some variables
into particular sections specific to unique hardware.
The __read_mostly macro uses __section() to place a variable or
function into the ``.data.read_mostly'' section of the (kernel)
elf(5). The use of __read_mostly allows infrequently modified data
to be grouped together; it is expected that the cachelines of rarely
and frequently modified data structures are this way separated.
Candidates for __read_mostly include variables that are initialized
once, read very often, and seldom written to.
The __cacheline_aligned macro behaves like __read_mostly, but the
used section is ``.data.cacheline_aligned'' instead. It also uses
__aligned() to set the minimum alignment into a predefined coherency
unit. This should ensure that frequently used data structures are
aligned on cacheline boundaries. Both __cacheline_aligned and
__read_mostly are only available for the kernel.
A branch is generally defined to be a conditional execution of a
program depending on whether a certain flow control mechanism is
altered. Typical example would be a ``if-then-else'' sequence used
in high-level languages or a jump instruction used in machine-level
code. A branch prediction would then be defined as an attempt to
guess whether a conditional branch will be taken.
The macros __predict_true() and __predict_false() annotate the
likelihood of whether a branch will evaluate to true or false. The
rationale is to improve instruction pipelining. Semantically
__predict_true expects that the integral expression exp equals 1.
The __predict_false expands to an attribute that instructs the
compiler to predict that a given branch will be likely false. As
programmers are notoriously bad at predicting the likely behavior of
their code, profiling and empirical evidence should precede the use
of __predict_false and __predict_true.
gcc(1), __builtin_object_size(3), cdefs(3), c(7)
It goes without saying that portable applications should steer clear from
non-standard extensions specific to any given compiler. Even when
portability is not a concern, use these macros sparsely and wisely.
NetBSD 6.1.5 December 19, 2010 NetBSD 6.1.5