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<title>Using the GNU Compiler Collection (GCC): Code Gen Options</title>
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<a name="Code-Gen-Options"></a>
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<div class="header">
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<p>
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Next: <a href="Developer-Options.html#Developer-Options" accesskey="n" rel="next">Developer Options</a>, Previous: <a href="Directory-Options.html#Directory-Options" accesskey="p" rel="prev">Directory Options</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Options-for-Code-Generation-Conventions"></a>
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<h3 class="section">3.16 Options for Code Generation Conventions</h3>
|
<a name="index-code-generation-conventions"></a>
|
<a name="index-options_002c-code-generation"></a>
|
<a name="index-run_002dtime-options"></a>
|
|
<p>These machine-independent options control the interface conventions
|
used in code generation.
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</p>
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<p>Most of them have both positive and negative forms; the negative form
|
of <samp>-ffoo</samp> is <samp>-fno-foo</samp>. In the table below, only
|
one of the forms is listed—the one that is not the default. You
|
can figure out the other form by either removing ‘<samp>no-</samp>’ or adding
|
it.
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</p>
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<dl compact="compact">
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<dt><code>-fstack-reuse=<var>reuse-level</var></code></dt>
|
<dd><a name="index-fstack_005freuse"></a>
|
<p>This option controls stack space reuse for user declared local/auto variables
|
and compiler generated temporaries. <var>reuse_level</var> can be ‘<samp>all</samp>’,
|
‘<samp>named_vars</samp>’, or ‘<samp>none</samp>’. ‘<samp>all</samp>’ enables stack reuse for all
|
local variables and temporaries, ‘<samp>named_vars</samp>’ enables the reuse only for
|
user defined local variables with names, and ‘<samp>none</samp>’ disables stack reuse
|
completely. The default value is ‘<samp>all</samp>’. The option is needed when the
|
program extends the lifetime of a scoped local variable or a compiler generated
|
temporary beyond the end point defined by the language. When a lifetime of
|
a variable ends, and if the variable lives in memory, the optimizing compiler
|
has the freedom to reuse its stack space with other temporaries or scoped
|
local variables whose live range does not overlap with it. Legacy code extending
|
local lifetime is likely to break with the stack reuse optimization.
|
</p>
|
<p>For example,
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample"> int *p;
|
{
|
int local1;
|
|
p = &local1;
|
local1 = 10;
|
....
|
}
|
{
|
int local2;
|
local2 = 20;
|
...
|
}
|
|
if (*p == 10) // out of scope use of local1
|
{
|
|
}
|
</pre></div>
|
|
<p>Another example:
|
</p><div class="smallexample">
|
<pre class="smallexample">
|
struct A
|
{
|
A(int k) : i(k), j(k) { }
|
int i;
|
int j;
|
};
|
|
A *ap;
|
|
void foo(const A& ar)
|
{
|
ap = &ar;
|
}
|
|
void bar()
|
{
|
foo(A(10)); // temp object's lifetime ends when foo returns
|
|
{
|
A a(20);
|
....
|
}
|
ap->i+= 10; // ap references out of scope temp whose space
|
// is reused with a. What is the value of ap->i?
|
}
|
|
</pre></div>
|
|
<p>The lifetime of a compiler generated temporary is well defined by the C++
|
standard. When a lifetime of a temporary ends, and if the temporary lives
|
in memory, the optimizing compiler has the freedom to reuse its stack
|
space with other temporaries or scoped local variables whose live range
|
does not overlap with it. However some of the legacy code relies on
|
the behavior of older compilers in which temporaries’ stack space is
|
not reused, the aggressive stack reuse can lead to runtime errors. This
|
option is used to control the temporary stack reuse optimization.
|
</p>
|
</dd>
|
<dt><code>-ftrapv</code></dt>
|
<dd><a name="index-ftrapv"></a>
|
<p>This option generates traps for signed overflow on addition, subtraction,
|
multiplication operations.
|
The options <samp>-ftrapv</samp> and <samp>-fwrapv</samp> override each other, so using
|
<samp>-ftrapv</samp> <samp>-fwrapv</samp> on the command-line results in
|
<samp>-fwrapv</samp> being effective. Note that only active options override, so
|
using <samp>-ftrapv</samp> <samp>-fwrapv</samp> <samp>-fno-wrapv</samp> on the command-line
|
results in <samp>-ftrapv</samp> being effective.
|
</p>
|
</dd>
|
<dt><code>-fwrapv</code></dt>
|
<dd><a name="index-fwrapv"></a>
|
<p>This option instructs the compiler to assume that signed arithmetic
|
overflow of addition, subtraction and multiplication wraps around
|
using twos-complement representation. This flag enables some optimizations
|
and disables others. This option is enabled by default for the Java
|
front end, as required by the Java language specification.
|
The options <samp>-ftrapv</samp> and <samp>-fwrapv</samp> override each other, so using
|
<samp>-ftrapv</samp> <samp>-fwrapv</samp> on the command-line results in
|
<samp>-fwrapv</samp> being effective. Note that only active options override, so
|
using <samp>-ftrapv</samp> <samp>-fwrapv</samp> <samp>-fno-wrapv</samp> on the command-line
|
results in <samp>-ftrapv</samp> being effective.
|
</p>
|
</dd>
|
<dt><code>-fexceptions</code></dt>
|
<dd><a name="index-fexceptions"></a>
|
<p>Enable exception handling. Generates extra code needed to propagate
|
exceptions. For some targets, this implies GCC generates frame
|
unwind information for all functions, which can produce significant data
|
size overhead, although it does not affect execution. If you do not
|
specify this option, GCC enables it by default for languages like
|
C++ that normally require exception handling, and disables it for
|
languages like C that do not normally require it. However, you may need
|
to enable this option when compiling C code that needs to interoperate
|
properly with exception handlers written in C++. You may also wish to
|
disable this option if you are compiling older C++ programs that don’t
|
use exception handling.
|
</p>
|
</dd>
|
<dt><code>-fnon-call-exceptions</code></dt>
|
<dd><a name="index-fnon_002dcall_002dexceptions"></a>
|
<p>Generate code that allows trapping instructions to throw exceptions.
|
Note that this requires platform-specific runtime support that does
|
not exist everywhere. Moreover, it only allows <em>trapping</em>
|
instructions to throw exceptions, i.e. memory references or floating-point
|
instructions. It does not allow exceptions to be thrown from
|
arbitrary signal handlers such as <code>SIGALRM</code>.
|
</p>
|
</dd>
|
<dt><code>-fdelete-dead-exceptions</code></dt>
|
<dd><a name="index-fdelete_002ddead_002dexceptions"></a>
|
<p>Consider that instructions that may throw exceptions but don’t otherwise
|
contribute to the execution of the program can be optimized away.
|
This option is enabled by default for the Ada front end, as permitted by
|
the Ada language specification.
|
Optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels.
|
</p>
|
</dd>
|
<dt><code>-funwind-tables</code></dt>
|
<dd><a name="index-funwind_002dtables"></a>
|
<p>Similar to <samp>-fexceptions</samp>, except that it just generates any needed
|
static data, but does not affect the generated code in any other way.
|
You normally do not need to enable this option; instead, a language processor
|
that needs this handling enables it on your behalf.
|
</p>
|
</dd>
|
<dt><code>-fasynchronous-unwind-tables</code></dt>
|
<dd><a name="index-fasynchronous_002dunwind_002dtables"></a>
|
<p>Generate unwind table in DWARF format, if supported by target machine. The
|
table is exact at each instruction boundary, so it can be used for stack
|
unwinding from asynchronous events (such as debugger or garbage collector).
|
</p>
|
</dd>
|
<dt><code>-fno-gnu-unique</code></dt>
|
<dd><a name="index-fno_002dgnu_002dunique"></a>
|
<p>On systems with recent GNU assembler and C library, the C++ compiler
|
uses the <code>STB_GNU_UNIQUE</code> binding to make sure that definitions
|
of template static data members and static local variables in inline
|
functions are unique even in the presence of <code>RTLD_LOCAL</code>; this
|
is necessary to avoid problems with a library used by two different
|
<code>RTLD_LOCAL</code> plugins depending on a definition in one of them and
|
therefore disagreeing with the other one about the binding of the
|
symbol. But this causes <code>dlclose</code> to be ignored for affected
|
DSOs; if your program relies on reinitialization of a DSO via
|
<code>dlclose</code> and <code>dlopen</code>, you can use
|
<samp>-fno-gnu-unique</samp>.
|
</p>
|
</dd>
|
<dt><code>-fpcc-struct-return</code></dt>
|
<dd><a name="index-fpcc_002dstruct_002dreturn"></a>
|
<p>Return “short” <code>struct</code> and <code>union</code> values in memory like
|
longer ones, rather than in registers. This convention is less
|
efficient, but it has the advantage of allowing intercallability between
|
GCC-compiled files and files compiled with other compilers, particularly
|
the Portable C Compiler (pcc).
|
</p>
|
<p>The precise convention for returning structures in memory depends
|
on the target configuration macros.
|
</p>
|
<p>Short structures and unions are those whose size and alignment match
|
that of some integer type.
|
</p>
|
<p><strong>Warning:</strong> code compiled with the <samp>-fpcc-struct-return</samp>
|
switch is not binary compatible with code compiled with the
|
<samp>-freg-struct-return</samp> switch.
|
Use it to conform to a non-default application binary interface.
|
</p>
|
</dd>
|
<dt><code>-freg-struct-return</code></dt>
|
<dd><a name="index-freg_002dstruct_002dreturn"></a>
|
<p>Return <code>struct</code> and <code>union</code> values in registers when possible.
|
This is more efficient for small structures than
|
<samp>-fpcc-struct-return</samp>.
|
</p>
|
<p>If you specify neither <samp>-fpcc-struct-return</samp> nor
|
<samp>-freg-struct-return</samp>, GCC defaults to whichever convention is
|
standard for the target. If there is no standard convention, GCC
|
defaults to <samp>-fpcc-struct-return</samp>, except on targets where GCC is
|
the principal compiler. In those cases, we can choose the standard, and
|
we chose the more efficient register return alternative.
|
</p>
|
<p><strong>Warning:</strong> code compiled with the <samp>-freg-struct-return</samp>
|
switch is not binary compatible with code compiled with the
|
<samp>-fpcc-struct-return</samp> switch.
|
Use it to conform to a non-default application binary interface.
|
</p>
|
</dd>
|
<dt><code>-fshort-enums</code></dt>
|
<dd><a name="index-fshort_002denums"></a>
|
<p>Allocate to an <code>enum</code> type only as many bytes as it needs for the
|
declared range of possible values. Specifically, the <code>enum</code> type
|
is equivalent to the smallest integer type that has enough room.
|
</p>
|
<p><strong>Warning:</strong> the <samp>-fshort-enums</samp> switch causes GCC to generate
|
code that is not binary compatible with code generated without that switch.
|
Use it to conform to a non-default application binary interface.
|
</p>
|
</dd>
|
<dt><code>-fshort-wchar</code></dt>
|
<dd><a name="index-fshort_002dwchar"></a>
|
<p>Override the underlying type for <code>wchar_t</code> to be <code>short
|
unsigned int</code> instead of the default for the target. This option is
|
useful for building programs to run under WINE.
|
</p>
|
<p><strong>Warning:</strong> the <samp>-fshort-wchar</samp> switch causes GCC to generate
|
code that is not binary compatible with code generated without that switch.
|
Use it to conform to a non-default application binary interface.
|
</p>
|
</dd>
|
<dt><code>-fno-common</code></dt>
|
<dd><a name="index-fno_002dcommon"></a>
|
<p>In C code, controls the placement of uninitialized global variables.
|
Unix C compilers have traditionally permitted multiple definitions of
|
such variables in different compilation units by placing the variables
|
in a common block.
|
This is the behavior specified by <samp>-fcommon</samp>, and is the default
|
for GCC on most targets.
|
On the other hand, this behavior is not required by ISO C, and on some
|
targets may carry a speed or code size penalty on variable references.
|
The <samp>-fno-common</samp> option specifies that the compiler should place
|
uninitialized global variables in the data section of the object file,
|
rather than generating them as common blocks.
|
This has the effect that if the same variable is declared
|
(without <code>extern</code>) in two different compilations,
|
you get a multiple-definition error when you link them.
|
In this case, you must compile with <samp>-fcommon</samp> instead.
|
Compiling with <samp>-fno-common</samp> is useful on targets for which
|
it provides better performance, or if you wish to verify that the
|
program will work on other systems that always treat uninitialized
|
variable declarations this way.
|
</p>
|
</dd>
|
<dt><code>-fno-ident</code></dt>
|
<dd><a name="index-fno_002dident"></a>
|
<p>Ignore the <code>#ident</code> directive.
|
</p>
|
</dd>
|
<dt><code>-finhibit-size-directive</code></dt>
|
<dd><a name="index-finhibit_002dsize_002ddirective"></a>
|
<p>Don’t output a <code>.size</code> assembler directive, or anything else that
|
would cause trouble if the function is split in the middle, and the
|
two halves are placed at locations far apart in memory. This option is
|
used when compiling <samp>crtstuff.c</samp>; you should not need to use it
|
for anything else.
|
</p>
|
</dd>
|
<dt><code>-fverbose-asm</code></dt>
|
<dd><a name="index-fverbose_002dasm"></a>
|
<p>Put extra commentary information in the generated assembly code to
|
make it more readable. This option is generally only of use to those
|
who actually need to read the generated assembly code (perhaps while
|
debugging the compiler itself).
|
</p>
|
<p><samp>-fno-verbose-asm</samp>, the default, causes the
|
extra information to be omitted and is useful when comparing two assembler
|
files.
|
</p>
|
</dd>
|
<dt><code>-frecord-gcc-switches</code></dt>
|
<dd><a name="index-frecord_002dgcc_002dswitches"></a>
|
<p>This switch causes the command line used to invoke the
|
compiler to be recorded into the object file that is being created.
|
This switch is only implemented on some targets and the exact format
|
of the recording is target and binary file format dependent, but it
|
usually takes the form of a section containing ASCII text. This
|
switch is related to the <samp>-fverbose-asm</samp> switch, but that
|
switch only records information in the assembler output file as
|
comments, so it never reaches the object file.
|
See also <samp>-grecord-gcc-switches</samp> for another
|
way of storing compiler options into the object file.
|
</p>
|
</dd>
|
<dt><code>-fpic</code></dt>
|
<dd><a name="index-fpic"></a>
|
<a name="index-global-offset-table"></a>
|
<a name="index-PIC"></a>
|
<p>Generate position-independent code (PIC) suitable for use in a shared
|
library, if supported for the target machine. Such code accesses all
|
constant addresses through a global offset table (GOT). The dynamic
|
loader resolves the GOT entries when the program starts (the dynamic
|
loader is not part of GCC; it is part of the operating system). If
|
the GOT size for the linked executable exceeds a machine-specific
|
maximum size, you get an error message from the linker indicating that
|
<samp>-fpic</samp> does not work; in that case, recompile with <samp>-fPIC</samp>
|
instead. (These maximums are 8k on the SPARC, 28k on AArch64 and 32k
|
on the m68k and RS/6000. The x86 has no such limit.)
|
</p>
|
<p>Position-independent code requires special support, and therefore works
|
only on certain machines. For the x86, GCC supports PIC for System V
|
but not for the Sun 386i. Code generated for the IBM RS/6000 is always
|
position-independent.
|
</p>
|
<p>When this flag is set, the macros <code>__pic__</code> and <code>__PIC__</code>
|
are defined to 1.
|
</p>
|
</dd>
|
<dt><code>-fPIC</code></dt>
|
<dd><a name="index-fPIC"></a>
|
<p>If supported for the target machine, emit position-independent code,
|
suitable for dynamic linking and avoiding any limit on the size of the
|
global offset table. This option makes a difference on AArch64, m68k,
|
PowerPC and SPARC.
|
</p>
|
<p>Position-independent code requires special support, and therefore works
|
only on certain machines.
|
</p>
|
<p>When this flag is set, the macros <code>__pic__</code> and <code>__PIC__</code>
|
are defined to 2.
|
</p>
|
</dd>
|
<dt><code>-fpie</code></dt>
|
<dt><code>-fPIE</code></dt>
|
<dd><a name="index-fpie"></a>
|
<a name="index-fPIE"></a>
|
<p>These options are similar to <samp>-fpic</samp> and <samp>-fPIC</samp>, but
|
generated position independent code can be only linked into executables.
|
Usually these options are used when <samp>-pie</samp> GCC option is
|
used during linking.
|
</p>
|
<p><samp>-fpie</samp> and <samp>-fPIE</samp> both define the macros
|
<code>__pie__</code> and <code>__PIE__</code>. The macros have the value 1
|
for <samp>-fpie</samp> and 2 for <samp>-fPIE</samp>.
|
</p>
|
</dd>
|
<dt><code>-fno-plt</code></dt>
|
<dd><a name="index-fno_002dplt"></a>
|
<p>Do not use the PLT for external function calls in position-independent code.
|
Instead, load the callee address at call sites from the GOT and branch to it.
|
This leads to more efficient code by eliminating PLT stubs and exposing
|
GOT loads to optimizations. On architectures such as 32-bit x86 where
|
PLT stubs expect the GOT pointer in a specific register, this gives more
|
register allocation freedom to the compiler.
|
Lazy binding requires use of the PLT;
|
with <samp>-fno-plt</samp> all external symbols are resolved at load time.
|
</p>
|
<p>Alternatively, the function attribute <code>noplt</code> can be used to avoid calls
|
through the PLT for specific external functions.
|
</p>
|
<p>In position-dependent code, a few targets also convert calls to
|
functions that are marked to not use the PLT to use the GOT instead.
|
</p>
|
</dd>
|
<dt><code>-fno-jump-tables</code></dt>
|
<dd><a name="index-fno_002djump_002dtables"></a>
|
<p>Do not use jump tables for switch statements even where it would be
|
more efficient than other code generation strategies. This option is
|
of use in conjunction with <samp>-fpic</samp> or <samp>-fPIC</samp> for
|
building code that forms part of a dynamic linker and cannot
|
reference the address of a jump table. On some targets, jump tables
|
do not require a GOT and this option is not needed.
|
</p>
|
</dd>
|
<dt><code>-ffixed-<var>reg</var></code></dt>
|
<dd><a name="index-ffixed"></a>
|
<p>Treat the register named <var>reg</var> as a fixed register; generated code
|
should never refer to it (except perhaps as a stack pointer, frame
|
pointer or in some other fixed role).
|
</p>
|
<p><var>reg</var> must be the name of a register. The register names accepted
|
are machine-specific and are defined in the <code>REGISTER_NAMES</code>
|
macro in the machine description macro file.
|
</p>
|
<p>This flag does not have a negative form, because it specifies a
|
three-way choice.
|
</p>
|
</dd>
|
<dt><code>-fcall-used-<var>reg</var></code></dt>
|
<dd><a name="index-fcall_002dused"></a>
|
<p>Treat the register named <var>reg</var> as an allocable register that is
|
clobbered by function calls. It may be allocated for temporaries or
|
variables that do not live across a call. Functions compiled this way
|
do not save and restore the register <var>reg</var>.
|
</p>
|
<p>It is an error to use this flag with the frame pointer or stack pointer.
|
Use of this flag for other registers that have fixed pervasive roles in
|
the machine’s execution model produces disastrous results.
|
</p>
|
<p>This flag does not have a negative form, because it specifies a
|
three-way choice.
|
</p>
|
</dd>
|
<dt><code>-fcall-saved-<var>reg</var></code></dt>
|
<dd><a name="index-fcall_002dsaved"></a>
|
<p>Treat the register named <var>reg</var> as an allocable register saved by
|
functions. It may be allocated even for temporaries or variables that
|
live across a call. Functions compiled this way save and restore
|
the register <var>reg</var> if they use it.
|
</p>
|
<p>It is an error to use this flag with the frame pointer or stack pointer.
|
Use of this flag for other registers that have fixed pervasive roles in
|
the machine’s execution model produces disastrous results.
|
</p>
|
<p>A different sort of disaster results from the use of this flag for
|
a register in which function values may be returned.
|
</p>
|
<p>This flag does not have a negative form, because it specifies a
|
three-way choice.
|
</p>
|
</dd>
|
<dt><code>-fpack-struct[=<var>n</var>]</code></dt>
|
<dd><a name="index-fpack_002dstruct"></a>
|
<p>Without a value specified, pack all structure members together without
|
holes. When a value is specified (which must be a small power of two), pack
|
structure members according to this value, representing the maximum
|
alignment (that is, objects with default alignment requirements larger than
|
this are output potentially unaligned at the next fitting location.
|
</p>
|
<p><strong>Warning:</strong> the <samp>-fpack-struct</samp> switch causes GCC to generate
|
code that is not binary compatible with code generated without that switch.
|
Additionally, it makes the code suboptimal.
|
Use it to conform to a non-default application binary interface.
|
</p>
|
</dd>
|
<dt><code>-fleading-underscore</code></dt>
|
<dd><a name="index-fleading_002dunderscore"></a>
|
<p>This option and its counterpart, <samp>-fno-leading-underscore</samp>, forcibly
|
change the way C symbols are represented in the object file. One use
|
is to help link with legacy assembly code.
|
</p>
|
<p><strong>Warning:</strong> the <samp>-fleading-underscore</samp> switch causes GCC to
|
generate code that is not binary compatible with code generated without that
|
switch. Use it to conform to a non-default application binary interface.
|
Not all targets provide complete support for this switch.
|
</p>
|
</dd>
|
<dt><code>-ftls-model=<var>model</var></code></dt>
|
<dd><a name="index-ftls_002dmodel"></a>
|
<p>Alter the thread-local storage model to be used (see <a href="Thread_002dLocal.html#Thread_002dLocal">Thread-Local</a>).
|
The <var>model</var> argument should be one of ‘<samp>global-dynamic</samp>’,
|
‘<samp>local-dynamic</samp>’, ‘<samp>initial-exec</samp>’ or ‘<samp>local-exec</samp>’.
|
Note that the choice is subject to optimization: the compiler may use
|
a more efficient model for symbols not visible outside of the translation
|
unit, or if <samp>-fpic</samp> is not given on the command line.
|
</p>
|
<p>The default without <samp>-fpic</samp> is ‘<samp>initial-exec</samp>’; with
|
<samp>-fpic</samp> the default is ‘<samp>global-dynamic</samp>’.
|
</p>
|
</dd>
|
<dt><code>-fvisibility=<span class="roman">[</span>default<span class="roman">|</span>internal<span class="roman">|</span>hidden<span class="roman">|</span>protected<span class="roman">]</span></code></dt>
|
<dd><a name="index-fvisibility"></a>
|
<p>Set the default ELF image symbol visibility to the specified option—all
|
symbols are marked with this unless overridden within the code.
|
Using this feature can very substantially improve linking and
|
load times of shared object libraries, produce more optimized
|
code, provide near-perfect API export and prevent symbol clashes.
|
It is <strong>strongly</strong> recommended that you use this in any shared objects
|
you distribute.
|
</p>
|
<p>Despite the nomenclature, ‘<samp>default</samp>’ always means public; i.e.,
|
available to be linked against from outside the shared object.
|
‘<samp>protected</samp>’ and ‘<samp>internal</samp>’ are pretty useless in real-world
|
usage so the only other commonly used option is ‘<samp>hidden</samp>’.
|
The default if <samp>-fvisibility</samp> isn’t specified is
|
‘<samp>default</samp>’, i.e., make every symbol public.
|
</p>
|
<p>A good explanation of the benefits offered by ensuring ELF
|
symbols have the correct visibility is given by “How To Write
|
Shared Libraries” by Ulrich Drepper (which can be found at
|
<a href="http://www.akkadia.org/drepper/">http://www.akkadia.org/drepper/</a><!-- /@w -->)—however a superior
|
solution made possible by this option to marking things hidden when
|
the default is public is to make the default hidden and mark things
|
public. This is the norm with DLLs on Windows and with <samp>-fvisibility=hidden</samp>
|
and <code>__attribute__ ((visibility("default")))</code> instead of
|
<code>__declspec(dllexport)</code> you get almost identical semantics with
|
identical syntax. This is a great boon to those working with
|
cross-platform projects.
|
</p>
|
<p>For those adding visibility support to existing code, you may find
|
<code>#pragma GCC visibility</code> of use. This works by you enclosing
|
the declarations you wish to set visibility for with (for example)
|
<code>#pragma GCC visibility push(hidden)</code> and
|
<code>#pragma GCC visibility pop</code>.
|
Bear in mind that symbol visibility should be viewed <strong>as
|
part of the API interface contract</strong> and thus all new code should
|
always specify visibility when it is not the default; i.e., declarations
|
only for use within the local DSO should <strong>always</strong> be marked explicitly
|
as hidden as so to avoid PLT indirection overheads—making this
|
abundantly clear also aids readability and self-documentation of the code.
|
Note that due to ISO C++ specification requirements, <code>operator new</code> and
|
<code>operator delete</code> must always be of default visibility.
|
</p>
|
<p>Be aware that headers from outside your project, in particular system
|
headers and headers from any other library you use, may not be
|
expecting to be compiled with visibility other than the default. You
|
may need to explicitly say <code>#pragma GCC visibility push(default)</code>
|
before including any such headers.
|
</p>
|
<p><code>extern</code> declarations are not affected by <samp>-fvisibility</samp>, so
|
a lot of code can be recompiled with <samp>-fvisibility=hidden</samp> with
|
no modifications. However, this means that calls to <code>extern</code>
|
functions with no explicit visibility use the PLT, so it is more
|
effective to use <code>__attribute ((visibility))</code> and/or
|
<code>#pragma GCC visibility</code> to tell the compiler which <code>extern</code>
|
declarations should be treated as hidden.
|
</p>
|
<p>Note that <samp>-fvisibility</samp> does affect C++ vague linkage
|
entities. This means that, for instance, an exception class that is
|
be thrown between DSOs must be explicitly marked with default
|
visibility so that the ‘<samp>type_info</samp>’ nodes are unified between
|
the DSOs.
|
</p>
|
<p>An overview of these techniques, their benefits and how to use them
|
is at <a href="http://gcc.gnu.org/wiki/Visibility">http://gcc.gnu.org/wiki/Visibility</a>.
|
</p>
|
</dd>
|
<dt><code>-fstrict-volatile-bitfields</code></dt>
|
<dd><a name="index-fstrict_002dvolatile_002dbitfields"></a>
|
<p>This option should be used if accesses to volatile bit-fields (or other
|
structure fields, although the compiler usually honors those types
|
anyway) should use a single access of the width of the
|
field’s type, aligned to a natural alignment if possible. For
|
example, targets with memory-mapped peripheral registers might require
|
all such accesses to be 16 bits wide; with this flag you can
|
declare all peripheral bit-fields as <code>unsigned short</code> (assuming short
|
is 16 bits on these targets) to force GCC to use 16-bit accesses
|
instead of, perhaps, a more efficient 32-bit access.
|
</p>
|
<p>If this option is disabled, the compiler uses the most efficient
|
instruction. In the previous example, that might be a 32-bit load
|
instruction, even though that accesses bytes that do not contain
|
any portion of the bit-field, or memory-mapped registers unrelated to
|
the one being updated.
|
</p>
|
<p>In some cases, such as when the <code>packed</code> attribute is applied to a
|
structure field, it may not be possible to access the field with a single
|
read or write that is correctly aligned for the target machine. In this
|
case GCC falls back to generating multiple accesses rather than code that
|
will fault or truncate the result at run time.
|
</p>
|
<p>Note: Due to restrictions of the C/C++11 memory model, write accesses are
|
not allowed to touch non bit-field members. It is therefore recommended
|
to define all bits of the field’s type as bit-field members.
|
</p>
|
<p>The default value of this option is determined by the application binary
|
interface for the target processor.
|
</p>
|
</dd>
|
<dt><code>-fsync-libcalls</code></dt>
|
<dd><a name="index-fsync_002dlibcalls"></a>
|
<p>This option controls whether any out-of-line instance of the <code>__sync</code>
|
family of functions may be used to implement the C++11 <code>__atomic</code>
|
family of functions.
|
</p>
|
<p>The default value of this option is enabled, thus the only useful form
|
of the option is <samp>-fno-sync-libcalls</samp>. This option is used in
|
the implementation of the <samp>libatomic</samp> runtime library.
|
</p>
|
</dd>
|
</dl>
|
|
<hr>
|
<div class="header">
|
<p>
|
Next: <a href="Developer-Options.html#Developer-Options" accesskey="n" rel="next">Developer Options</a>, Previous: <a href="Directory-Options.html#Directory-Options" accesskey="p" rel="prev">Directory Options</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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