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<h3 class="section">Options for Code Generation Conventions</h3>
<p>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 <code>-ffoo</code> would be <code>-fno-foo</code>. In the table below, only
one of the forms is listed--the one which is not the default. You
can figure out the other form by either removing <code>no-</code> or adding
it.
<dl>
<dt><code>-fbounds-check</code>
<dd>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 77 front-ends, where
this option defaults to true and false respectively.
<br><dt><code>-ftrapv</code>
<dd>This option generates traps for signed overflow on addition, subtraction,
multiplication operations.
<br><dt><code>-fexceptions</code>
<dd>Enable exception handling. Generates extra code needed to propagate
exceptions. For some targets, this implies GCC will generate 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 will enable it by default for languages like
C++ which normally require exception handling, and disable 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>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>-funwind-tables</code>
<dd>Similar to <code>-fexceptions</code>, except that it will just generate any needed
static data, but will not affect the generated code in any other way.
You will normally not enable this option; instead, a language processor
that needs this handling would enable it on your behalf.
<br><dt><code>-fasynchronous-unwind-tables</code>
<dd>Generate unwind table in dwarf2 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>-fpcc-struct-return</code>
<dd>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>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 <code>-fpcc-struct-return</code>
switch is not binary compatible with code compiled with the
<code>-freg-struct-return</code> switch.
Use it to conform to a non-default application binary interface.
<br><dt><code>-freg-struct-return</code>
<dd>Return <code>struct</code> and <code>union</code> values in registers when possible.
This is more efficient for small structures than
<code>-fpcc-struct-return</code>.
<p>If you specify neither <code>-fpcc-struct-return</code> nor
<code>-freg-struct-return</code>, GCC defaults to whichever convention is
standard for the target. If there is no standard convention, GCC
defaults to <code>-fpcc-struct-return</code>, 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 <code>-freg-struct-return</code>
switch is not binary compatible with code compiled with the
<code>-fpcc-struct-return</code> switch.
Use it to conform to a non-default application binary interface.
<br><dt><code>-fshort-enums</code>
<dd>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
will be equivalent to the smallest integer type which has enough room.
<p><strong>Warning:</strong> the <code>-fshort-enums</code> 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>Use the same size for <code>double</code> as for <code>float</code>.
<p><strong>Warning:</strong> the <code>-fshort-double</code> 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>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 <code>-fshort-wchar</code> 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>-fshared-data</code>
<dd>Requests that the data and non-<code>const</code> variables of this
compilation be shared data rather than private data. The distinction
makes sense only on certain operating systems, where shared data is
shared between processes running the same program, while private data
exists in one copy per process.
<br><dt><code>-fno-common</code>
<dd>In C, allocate even 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 will get an error when you link them.
The only reason this might be useful is if you wish to verify that the
program will work on other systems which always work this way.
<br><dt><code>-fno-ident</code>
<dd>Ignore the <code>#ident</code> directive.
<br><dt><code>-fno-gnu-linker</code>
<dd>Do not output global initializations (such as C++ constructors and
destructors) in the form used by the GNU linker (on systems where the GNU
linker is the standard method of handling them). Use this option when
you want to use a non-GNU linker, which also requires using the
<code>collect2</code> program to make sure the system linker includes
constructors and destructors. (<code>collect2</code> is included in the GCC
distribution.) For systems which <em>must</em> use <code>collect2</code>, the
compiler driver <code>gcc</code> is configured to do this automatically.
<br><dt><code>-finhibit-size-directive</code>
<dd>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 <code>crtstuff.c</code>; you should not need to use it
for anything else.
<br><dt><code>-fverbose-asm</code>
<dd>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
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