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ePenguin-Software-Framework/toolchains/avr/share/doc/gcc/Code-Gen-Options.html

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<h3 class="section">3.18 Options for Code Generation Conventions</h3>
<p><a name="index-code-generation-conventions-2789"></a><a name="index-options_002c-code-generation-2790"></a><a name="index-run_002dtime-options-2791"></a>
These machine-independent options control the interface conventions
used in code generation.
<p>Most of them have both positive and negative forms; the negative form
of <samp><span class="option">-ffoo</span></samp> is <samp><span class="option">-fno-foo</span></samp>. In the table below, only
one of the forms is listed&mdash;the one that is not the default. You
can figure out the other form by either removing &lsquo;<samp><span class="samp">no-</span></samp>&rsquo; or adding
it.
<dl>
<dt><code>-fbounds-check</code><dd><a name="index-fbounds_002dcheck-2792"></a>For front ends that support it, generate additional code to check that
indices used to access arrays are within the declared range. This is
currently only supported by the Java and Fortran front ends, where
this option defaults to true and false respectively.
<br><dt><code>-fstack-reuse=</code><var>reuse-level</var><dd><a name="index-fstack_005freuse-2793"></a>This option controls stack space reuse for user declared local/auto variables
and compiler generated temporaries. <var>reuse_level</var> can be &lsquo;<samp><span class="samp">all</span></samp>&rsquo;,
&lsquo;<samp><span class="samp">named_vars</span></samp>&rsquo;, or &lsquo;<samp><span class="samp">none</span></samp>&rsquo;. &lsquo;<samp><span class="samp">all</span></samp>&rsquo; enables stack reuse for all
local variables and temporaries, &lsquo;<samp><span class="samp">named_vars</span></samp>&rsquo; enables the reuse only for
user defined local variables with names, and &lsquo;<samp><span class="samp">none</span></samp>&rsquo; disables stack reuse
completely. The default value is &lsquo;<samp><span class="samp">all</span></samp>&rsquo;. 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>For example,
<pre class="smallexample"> int *p;
{
int local1;
p = &amp;local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}
</pre>
<p>Another example:
<pre class="smallexample">
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A&amp; ar)
{
ap = &amp;ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap-&gt;i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap-&gt;i?
}
</pre>
<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.
<br><dt><code>-ftrapv</code><dd><a name="index-ftrapv-2794"></a>This option generates traps for signed overflow on addition, subtraction,
multiplication operations.
<br><dt><code>-fwrapv</code><dd><a name="index-fwrapv-2795"></a>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.
<br><dt><code>-fexceptions</code><dd><a name="index-fexceptions-2796"></a>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.
<br><dt><code>-fnon-call-exceptions</code><dd><a name="index-fnon_002dcall_002dexceptions-2797"></a>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>.
<br><dt><code>-fdelete-dead-exceptions</code><dd><a name="index-fdelete_002ddead_002dexceptions-2798"></a>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.
<br><dt><code>-funwind-tables</code><dd><a name="index-funwind_002dtables-2799"></a>Similar to <samp><span class="option">-fexceptions</span></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.
<br><dt><code>-fasynchronous-unwind-tables</code><dd><a name="index-fasynchronous_002dunwind_002dtables-2800"></a>Generate unwind table in DWARF 2 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).
<br><dt><code>-fno-gnu-unique</code><dd><a name="index-fno_002dgnu_002dunique-2801"></a>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><span class="option">-fno-gnu-unique</span></samp>.
<br><dt><code>-fpcc-struct-return</code><dd><a name="index-fpcc_002dstruct_002dreturn-2802"></a>Return &ldquo;short&rdquo; <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>The precise convention for returning structures in memory depends
on the target configuration macros.
<p>Short structures and unions are those whose size and alignment match
that of some integer type.
<p><strong>Warning:</strong> code compiled with the <samp><span class="option">-fpcc-struct-return</span></samp>
switch is not binary compatible with code compiled with the
<samp><span class="option">-freg-struct-return</span></samp> switch.
Use it to conform to a non-default application binary interface.
<br><dt><code>-freg-struct-return</code><dd><a name="index-freg_002dstruct_002dreturn-2803"></a>Return <code>struct</code> and <code>union</code> values in registers when possible.
This is more efficient for small structures than
<samp><span class="option">-fpcc-struct-return</span></samp>.
<p>If you specify neither <samp><span class="option">-fpcc-struct-return</span></samp> nor
<samp><span class="option">-freg-struct-return</span></samp>, GCC defaults to whichever convention is
standard for the target. If there is no standard convention, GCC
defaults to <samp><span class="option">-fpcc-struct-return</span></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><strong>Warning:</strong> code compiled with the <samp><span class="option">-freg-struct-return</span></samp>
switch is not binary compatible with code compiled with the
<samp><span class="option">-fpcc-struct-return</span></samp> switch.
Use it to conform to a non-default application binary interface.
<br><dt><code>-fshort-enums</code><dd><a name="index-fshort_002denums-2804"></a>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><strong>Warning:</strong> the <samp><span class="option">-fshort-enums</span></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.
<br><dt><code>-fshort-double</code><dd><a name="index-fshort_002ddouble-2805"></a>Use the same size for <code>double</code> as for <code>float</code>.
<p><strong>Warning:</strong> the <samp><span class="option">-fshort-double</span></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.
<br><dt><code>-fshort-wchar</code><dd><a name="index-fshort_002dwchar-2806"></a>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><strong>Warning:</strong> the <samp><span class="option">-fshort-wchar</span></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.
<br><dt><code>-fno-common</code><dd><a name="index-fno_002dcommon-2807"></a>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><span class="option">-fcommon</span></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><span class="option">-fno-common</span></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><span class="option">-fcommon</span></samp> instead.
Compiling with <samp><span class="option">-fno-common</span></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.
<br><dt><code>-fno-ident</code><dd><a name="index-fno_002dident-2808"></a>Ignore the <code>#ident</code> directive.
<br><dt><code>-finhibit-size-directive</code><dd><a name="index-finhibit_002dsize_002ddirective-2809"></a>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><span class="file">crtstuff.c</span></samp>; you should not need to use it
for anything else.
<br><dt><code>-fverbose-asm</code><dd><a name="index-fverbose_002dasm-2810"></a>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><samp><span class="option">-fno-verbose-asm</span></samp>, the default, causes the
extra information to be omitted and is useful when comparing two assembler
files.
<br><dt><code>-frecord-gcc-switches</code><dd><a name="index-frecord_002dgcc_002dswitches-2811"></a>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><span class="option">-fverbose-asm</span></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><span class="option">-grecord-gcc-switches</span></samp> for another
way of storing compiler options into the object file.
<br><dt><code>-fpic</code><dd><a name="index-fpic-2812"></a><a name="index-global-offset-table-2813"></a><a name="index-PIC-2814"></a>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><span class="option">-fpic</span></samp> does not work; in that case, recompile with <samp><span class="option">-fPIC</span></samp>
instead. (These maximums are 8k on the SPARC and 32k
on the m68k and RS/6000. The x86 has no such limit.)
<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>When this flag is set, the macros <code>__pic__</code> and <code>__PIC__</code>
are defined to 1.
<br><dt><code>-fPIC</code><dd><a name="index-fPIC-2815"></a>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 the m68k,
PowerPC and SPARC.
<p>Position-independent code requires special support, and therefore works
only on certain machines.
<p>When this flag is set, the macros <code>__pic__</code> and <code>__PIC__</code>
are defined to 2.
<br><dt><code>-fpie</code><dt><code>-fPIE</code><dd><a name="index-fpie-2816"></a><a name="index-fPIE-2817"></a>These options are similar to <samp><span class="option">-fpic</span></samp> and <samp><span class="option">-fPIC</span></samp>, but
generated position independent code can be only linked into executables.
Usually these options are used when <samp><span class="option">-pie</span></samp> GCC option is
used during linking.
<p><samp><span class="option">-fpie</span></samp> and <samp><span class="option">-fPIE</span></samp> both define the macros
<code>__pie__</code> and <code>__PIE__</code>. The macros have the value 1
for <samp><span class="option">-fpie</span></samp> and 2 for <samp><span class="option">-fPIE</span></samp>.
<br><dt><code>-fno-jump-tables</code><dd><a name="index-fno_002djump_002dtables-2818"></a>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><span class="option">-fpic</span></samp> or <samp><span class="option">-fPIC</span></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.
<br><dt><code>-ffixed-</code><var>reg</var><dd><a name="index-ffixed-2819"></a>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><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>This flag does not have a negative form, because it specifies a
three-way choice.
<br><dt><code>-fcall-used-</code><var>reg</var><dd><a name="index-fcall_002dused-2820"></a>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>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>This flag does not have a negative form, because it specifies a
three-way choice.
<br><dt><code>-fcall-saved-</code><var>reg</var><dd><a name="index-fcall_002dsaved-2821"></a>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>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>A different sort of disaster results from the use of this flag for
a register in which function values may be returned.
<p>This flag does not have a negative form, because it specifies a
three-way choice.
<br><dt><code>-fpack-struct[=</code><var>n</var><code>]</code><dd><a name="index-fpack_002dstruct-2822"></a>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><strong>Warning:</strong> the <samp><span class="option">-fpack-struct</span></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.
<br><dt><code>-finstrument-functions</code><dd><a name="index-finstrument_002dfunctions-2823"></a>Generate instrumentation calls for entry and exit to functions. Just
after function entry and just before function exit, the following
profiling functions are called with the address of the current
function and its call site. (On some platforms,
<code>__builtin_return_address</code> does not work beyond the current
function, so the call site information may not be available to the
profiling functions otherwise.)
<pre class="smallexample"> void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
</pre>
<p>The first argument is the address of the start of the current function,
which may be looked up exactly in the symbol table.
<p>This instrumentation is also done for functions expanded inline in other
functions. The profiling calls indicate where, conceptually, the
inline function is entered and exited. This means that addressable
versions of such functions must be available. If all your uses of a
function are expanded inline, this may mean an additional expansion of
code size. If you use <code>extern inline</code> in your C code, an
addressable version of such functions must be provided. (This is
normally the case anyway, but if you get lucky and the optimizer always
expands the functions inline, you might have gotten away without
providing static copies.)
<p>A function may be given the attribute <code>no_instrument_function</code>, in
which case this instrumentation is not done. This can be used, for
example, for the profiling functions listed above, high-priority
interrupt routines, and any functions from which the profiling functions
cannot safely be called (perhaps signal handlers, if the profiling
routines generate output or allocate memory).
<br><dt><code>-finstrument-functions-exclude-file-list=</code><var>file</var><code>,</code><var>file</var><code>,...</code><dd><a name="index-finstrument_002dfunctions_002dexclude_002dfile_002dlist-2824"></a>
Set the list of functions that are excluded from instrumentation (see
the description of <samp><span class="option">-finstrument-functions</span></samp>). If the file that
contains a function definition matches with one of <var>file</var>, then
that function is not instrumented. The match is done on substrings:
if the <var>file</var> parameter is a substring of the file name, it is
considered to be a match.
<p>For example:
<pre class="smallexample"> -finstrument-functions-exclude-file-list=/bits/stl,include/sys
</pre>
<p class="noindent">excludes any inline function defined in files whose pathnames
contain <samp><span class="file">/bits/stl</span></samp> or <samp><span class="file">include/sys</span></samp>.
<p>If, for some reason, you want to include letter &lsquo;<samp><span class="samp">,</span></samp>&rsquo; in one of
<var>sym</var>, write &lsquo;<samp><span class="samp">\,</span></samp>&rsquo;. For example,
<samp><span class="option">-finstrument-functions-exclude-file-list='\,\,tmp'</span></samp>
(note the single quote surrounding the option).
<br><dt><code>-finstrument-functions-exclude-function-list=</code><var>sym</var><code>,</code><var>sym</var><code>,...</code><dd><a name="index-finstrument_002dfunctions_002dexclude_002dfunction_002dlist-2825"></a>
This is similar to <samp><span class="option">-finstrument-functions-exclude-file-list</span></samp>,
but this option sets the list of function names to be excluded from
instrumentation. The function name to be matched is its user-visible
name, such as <code>vector&lt;int&gt; blah(const vector&lt;int&gt; &amp;)</code>, not the
internal mangled name (e.g., <code>_Z4blahRSt6vectorIiSaIiEE</code>). The
match is done on substrings: if the <var>sym</var> parameter is a substring
of the function name, it is considered to be a match. For C99 and C++
extended identifiers, the function name must be given in UTF-8, not
using universal character names.
<br><dt><code>-fstack-check</code><dd><a name="index-fstack_002dcheck-2826"></a>Generate code to verify that you do not go beyond the boundary of the
stack. You should specify this flag if you are running in an
environment with multiple threads, but you only rarely need to specify it in
a single-threaded environment since stack overflow is automatically
detected on nearly all systems if there is only one stack.
<p>Note that this switch does not actually cause checking to be done; the
operating system or the language runtime must do that. The switch causes
generation of code to ensure that they see the stack being extended.
<p>You can additionally specify a string parameter: &lsquo;<samp><span class="samp">no</span></samp>&rsquo; means no
checking, &lsquo;<samp><span class="samp">generic</span></samp>&rsquo; means force the use of old-style checking,
&lsquo;<samp><span class="samp">specific</span></samp>&rsquo; means use the best checking method and is equivalent
to bare <samp><span class="option">-fstack-check</span></samp>.
<p>Old-style checking is a generic mechanism that requires no specific
target support in the compiler but comes with the following drawbacks:
<ol type=1 start=1>
<li>Modified allocation strategy for large objects: they are always
allocated dynamically if their size exceeds a fixed threshold.
<li>Fixed limit on the size of the static frame of functions: when it is
topped by a particular function, stack checking is not reliable and
a warning is issued by the compiler.
<li>Inefficiency: because of both the modified allocation strategy and the
generic implementation, code performance is hampered.
</ol>
<p>Note that old-style stack checking is also the fallback method for
&lsquo;<samp><span class="samp">specific</span></samp>&rsquo; if no target support has been added in the compiler.
<br><dt><code>-fstack-limit-register=</code><var>reg</var><dt><code>-fstack-limit-symbol=</code><var>sym</var><dt><code>-fno-stack-limit</code><dd><a name="index-fstack_002dlimit_002dregister-2827"></a><a name="index-fstack_002dlimit_002dsymbol-2828"></a><a name="index-fno_002dstack_002dlimit-2829"></a>Generate code to ensure that the stack does not grow beyond a certain value,
either the value of a register or the address of a symbol. If a larger
stack is required, a signal is raised at run time. For most targets,
the signal is raised before the stack overruns the boundary, so
it is possible to catch the signal without taking special precautions.
<p>For instance, if the stack starts at absolute address &lsquo;<samp><span class="samp">0x80000000</span></samp>&rsquo;
and grows downwards, you can use the flags
<samp><span class="option">-fstack-limit-symbol=__stack_limit</span></samp> and
<samp><span class="option">-Wl,--defsym,__stack_limit=0x7ffe0000</span></samp> to enforce a stack limit
of 128KB. Note that this may only work with the GNU linker.
<br><dt><code>-fsplit-stack</code><dd><a name="index-fsplit_002dstack-2830"></a>Generate code to automatically split the stack before it overflows.
The resulting program has a discontiguous stack which can only
overflow if the program is unable to allocate any more memory. This
is most useful when running threaded programs, as it is no longer
necessary to calculate a good stack size to use for each thread. This
is currently only implemented for the x86 targets running
GNU/Linux.
<p>When code compiled with <samp><span class="option">-fsplit-stack</span></samp> calls code compiled
without <samp><span class="option">-fsplit-stack</span></samp>, there may not be much stack space
available for the latter code to run. If compiling all code,
including library code, with <samp><span class="option">-fsplit-stack</span></samp> is not an option,
then the linker can fix up these calls so that the code compiled
without <samp><span class="option">-fsplit-stack</span></samp> always has a large stack. Support for
this is implemented in the gold linker in GNU binutils release 2.21
and later.
<br><dt><code>-fleading-underscore</code><dd><a name="index-fleading_002dunderscore-2831"></a>This option and its counterpart, <samp><span class="option">-fno-leading-underscore</span></samp>, forcibly
change the way C symbols are represented in the object file. One use
is to help link with legacy assembly code.
<p><strong>Warning:</strong> the <samp><span class="option">-fleading-underscore</span></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.
<br><dt><code>-ftls-model=</code><var>model</var><dd><a name="index-ftls_002dmodel-2832"></a>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 &lsquo;<samp><span class="samp">global-dynamic</span></samp>&rsquo;,
&lsquo;<samp><span class="samp">local-dynamic</span></samp>&rsquo;, &lsquo;<samp><span class="samp">initial-exec</span></samp>&rsquo; or &lsquo;<samp><span class="samp">local-exec</span></samp>&rsquo;.
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><span class="option">-fpic</span></samp> is not given on the command line.
<p>The default without <samp><span class="option">-fpic</span></samp> is &lsquo;<samp><span class="samp">initial-exec</span></samp>&rsquo;; with
<samp><span class="option">-fpic</span></samp> the default is &lsquo;<samp><span class="samp">global-dynamic</span></samp>&rsquo;.
<br><dt><code>-fvisibility=</code><span class="roman">[</span><code>default</code><span class="roman">|</span><code>internal</code><span class="roman">|</span><code>hidden</code><span class="roman">|</span><code>protected</code><span class="roman">]</span><dd><a name="index-fvisibility-2833"></a>Set the default ELF image symbol visibility to the specified option&mdash;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>Despite the nomenclature, &lsquo;<samp><span class="samp">default</span></samp>&rsquo; always means public; i.e.,
available to be linked against from outside the shared object.
&lsquo;<samp><span class="samp">protected</span></samp>&rsquo; and &lsquo;<samp><span class="samp">internal</span></samp>&rsquo; are pretty useless in real-world
usage so the only other commonly used option is &lsquo;<samp><span class="samp">hidden</span></samp>&rsquo;.
The default if <samp><span class="option">-fvisibility</span></samp> isn't specified is
&lsquo;<samp><span class="samp">default</span></samp>&rsquo;, i.e., make every symbol public.
<p>A good explanation of the benefits offered by ensuring ELF
symbols have the correct visibility is given by &ldquo;How To Write
Shared Libraries&rdquo; by Ulrich Drepper (which can be found at
<a href="http://www.akkadia.org/drepper/">http://www.akkadia.org/drepper/</a><!-- /@w -->)&mdash;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><span class="option">-fvisibility=hidden</span></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>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&mdash;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>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><code>extern</code> declarations are not affected by <samp><span class="option">-fvisibility</span></samp>, so
a lot of code can be recompiled with <samp><span class="option">-fvisibility=hidden</span></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>Note that <samp><span class="option">-fvisibility</span></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 &lsquo;<samp><span class="samp">type_info</span></samp>&rsquo; nodes are unified between
the DSOs.
<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>.
<br><dt><code>-fstrict-volatile-bitfields</code><dd><a name="index-fstrict_002dvolatile_002dbitfields-2834"></a>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>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>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>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>The default value of this option is determined by the application binary
interface for the target processor.
<br><dt><code>-fsync-libcalls</code><dd><a name="index-fsync_002dlibcalls-2835"></a>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>The default value of this option is enabled, thus the only useful form
of the option is <samp><span class="option">-fno-sync-libcalls</span></samp>. This option is used in
the implementation of the <samp><span class="file">libatomic</span></samp> runtime library.
</dl>
<!-- man end -->
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