@table @emph
@item Overall Options
@xref{Overall Options,,Options Controlling the Kind of Output}.
-@gccoptlist{-c -S -E -o @var{file} -combine -pipe -pass-exit-codes @gol
--x @var{language} -v -### --help --target-help --version @@@var{file}}
+@gccoptlist{-c -S -E -o @var{file} -combine -pipe -pass-exit-codes @gol
+-x @var{language} -v -### --help@r{[}=@var{class}@r{]} --target-help @gol
+--version @@@var{file}}
@item C Language Options
@xref{C Dialect Options,,Options Controlling C Dialect}.
-@gccoptlist{-ansi -std=@var{standard} -aux-info @var{filename} @gol
+@gccoptlist{-ansi -std=@var{standard} -fgnu89-inline @gol
+-aux-info @var{filename} @gol
-fno-asm -fno-builtin -fno-builtin-@var{function} @gol
-fhosted -ffreestanding -fopenmp -fms-extensions @gol
-trigraphs -no-integrated-cpp -traditional -traditional-cpp @gol
@item Warning Options
@xref{Warning Options,,Options to Request or Suppress Warnings}.
@gccoptlist{-fsyntax-only -pedantic -pedantic-errors @gol
--w -Wextra -Wall -Waggregate-return -Walways-true -Warray-bounds @gol
+-w -Wextra -Wall -Waddress -Waggregate-return -Warray-bounds @gol
-Wno-attributes -Wc++-compat -Wc++0x-compat -Wcast-align -Wcast-qual @gol
-Wchar-subscripts -Wclobbered -Wcomment @gol
-Wconversion -Wcoverage-mismatch -Wno-deprecated-declarations @gol
-Wdisabled-optimization -Wno-div-by-zero @gol
-Wempty-body -Wno-endif-labels @gol
--Werror -Werror-* -Werror-implicit-function-declaration @gol
+-Werror -Werror=* @gol
-Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 @gol
-Wno-format-extra-args -Wformat-nonliteral @gol
-Wformat-security -Wformat-y2k @gol
-Wimplicit -Wimplicit-function-declaration -Wimplicit-int @gol
-Wimport -Wno-import -Winit-self -Winline @gol
--Wno-int-to-pointer-cast @gol
--Wno-invalid-offsetof -Winvalid-pch @gol
--Wlarger-than-@var{len} -Wunsafe-loop-optimizations -Wlong-long @gol
+-Wno-int-to-pointer-cast -Wno-invalid-offsetof @gol
+-Winvalid-pch -Wlarger-than-@var{len} -Wunsafe-loop-optimizations @gol
+-Wlogical-op -Wlong-long @gol
-Wmain -Wmissing-braces -Wmissing-field-initializers @gol
-Wmissing-format-attribute -Wmissing-include-dirs @gol
-Wmissing-noreturn @gol
-Wreturn-type -Wsequence-point -Wshadow @gol
-Wsign-compare -Wstack-protector @gol
-Wstrict-aliasing -Wstrict-aliasing=2 @gol
--Wstring-literal-comparison @gol
+-Wstrict-overflow -Wstrict-overflow=@var{n} @gol
-Wswitch -Wswitch-default -Wswitch-enum @gol
-Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized @gol
-Wunknown-pragmas -Wno-pragmas -Wunreachable-code @gol
-Wunused -Wunused-function -Wunused-label -Wunused-parameter @gol
--Wunused-value -Wunused-variable -Wvariadic-macros @gol
+-Wunused-value -Wunused-variable @gol
+-Wvariadic-macros -Wvla @gol
-Wvolatile-register-var -Wwrite-strings}
@item C-only Warning Options
-p -pg -print-file-name=@var{library} -print-libgcc-file-name @gol
-print-multi-directory -print-multi-lib @gol
-print-prog-name=@var{program} -print-search-dirs -Q @gol
+-print-sysroot-headers-suffix @gol
-save-temps -time}
@item Optimization Options
-fvariable-expansion-in-unroller @gol
-ftree-pre -ftree-ccp -ftree-dce -ftree-loop-optimize @gol
-ftree-loop-linear -ftree-loop-im -ftree-loop-ivcanon -fivopts @gol
+-fcheck-data-deps @gol
-ftree-dominator-opts -ftree-dse -ftree-copyrename -ftree-sink @gol
--ftree-ch -ftree-sra -ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize @gol
+-ftree-ch -ftree-sra -ftree-ter -ftree-fre -ftree-vectorize @gol
-ftree-vect-loop-version -ftree-salias -fipa-pta -fweb @gol
-ftree-copy-prop -ftree-store-ccp -ftree-store-copy-prop -fwhole-program @gol
--param @var{name}=@var{value}
-dynamic -dynamiclib -exported_symbols_list @gol
-filelist -flat_namespace -force_cpusubtype_ALL @gol
-force_flat_namespace -headerpad_max_install_names @gol
+-iframework @gol
-image_base -init -install_name -keep_private_externs @gol
-multi_module -multiply_defined -multiply_defined_unused @gol
-noall_load -no_dead_strip_inits_and_terms @gol
-mips16 -mno-mips16 -mabi=@var{abi} -mabicalls -mno-abicalls @gol
-mshared -mno-shared -mxgot -mno-xgot -mgp32 -mgp64 @gol
-mfp32 -mfp64 -mhard-float -msoft-float @gol
--msingle-float -mdouble-float -mdsp -mpaired-single -mips3d @gol
+-msingle-float -mdouble-float -mdsp -mdspr2 -mpaired-single -mips3d @gol
-mlong64 -mlong32 -msym32 -mno-sym32 @gol
-G@var{num} -membedded-data -mno-embedded-data @gol
-muninit-const-in-rodata -mno-uninit-const-in-rodata @gol
-mpowerpc-gpopt -mno-powerpc-gpopt @gol
-mpowerpc-gfxopt -mno-powerpc-gfxopt @gol
-mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mfprnd -mno-fprnd @gol
--mmfpgpr -mno-mfpgpr @gol
+-mcmpb -mno-cmpb -mmfpgpr -mno-mfpgpr -mdfp -mno-dfp @gol
-mnew-mnemonics -mold-mnemonics @gol
-mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc @gol
-m64 -m32 -mxl-compat -mno-xl-compat -mpe @gol
understood by @command{gcc}. If the @option{-v} option is also specified
then @option{--help} will also be passed on to the various processes
invoked by @command{gcc}, so that they can display the command line options
-they accept. If the @option{-Wextra} option is also specified then command
-line options which have no documentation associated with them will also
-be displayed.
+they accept. If the @option{-Wextra} option has also been specified
+(prior to the @option{--help} option), then command line options which
+have no documentation associated with them will also be displayed.
@item --target-help
@opindex target-help
-Print (on the standard output) a description of target specific command
+Print (on the standard output) a description of target-specific command
line options for each tool.
+@item --help=@var{class}@r{[},@var{qualifier}@r{]}
+Print (on the standard output) a description of the command line
+options understood by the compiler that fit into a specific class.
+The class can be one of @var{optimizers}, @var{warnings}, @var{target}
+or @var{params}:
+
+@table @gcctabopt
+@item @var{optimizers}
+This will display all of the optimization options supported by the
+compiler.
+
+@item @var{warnings}
+This will display all of the options controlling warning messages
+produced by the compiler.
+
+@item @var{target}
+This will display target-specific options. Unlike the
+@option{--target-help} option however, target-specific options of the
+linker and assembler will not be displayed. This is because those
+tools do not currently support the extended @option{--help=} syntax.
+
+@item @var{params}
+This will display the values recognized by the @option{--param}
+option.
+@end table
+
+It is possible to further refine the output of the @option{--help=}
+option by adding a comma separated list of qualifiers after the
+class. These can be any from the following list:
+
+@table @gcctabopt
+@item undocumented
+Display only those options which are undocumented.
+
+@item joined
+Display options which take an argument that appears after an equal
+sign in the same continuous piece of text, such as:
+@samp{--help=target}.
+
+@item separate
+Display options which take an argument that appears as a separate word
+following the original option, such as: @samp{-o output-file}.
+@end table
+
+Thus for example to display all the undocumented target-specific
+switches supported by the compiler the following can be used:
+
+@smallexample
+--help=target,undocumented
+@end smallexample
+
+The sense of a qualifier can be inverted by prefixing it with the
+@var{^} character, so for example to display all binary warning
+options (i.e. ones that are either on or off and that do not take an
+argument), which have a description the following can be used:
+
+@smallexample
+--help=warnings,^joined,^undocumented
+@end smallexample
+
+A class can also be used as a qualifier, although this usually
+restricts the output by so much that there is nothing to display. One
+case where it does work however is when one of the classes is
+@var{target}. So for example to display all the target-specific
+optimization options the following can be used:
+
+@smallexample
+--help=target,optimizers
+@end smallexample
+
+The @option{--help=} option can be repeated on the command line. Each
+successive use will display its requested class of options, skipping
+those that have already been displayed.
+
+If the @option{-Q} option appears on the command line before the
+@option{--help=} option, then the descriptive text displayed by
+@option{--help=} is changed. Instead of describing the displayed
+options, an indication is given as to whether the option is enabled,
+disabled or set to a specific value (assuming that the compiler
+knows this at the point where the @option{--help=} option is used).
+
+Here is a truncated example from the ARM port of @command{gcc}:
+
+@smallexample
+ % gcc -Q -mabi=2 --help=target -c
+ The following options are target specific:
+ -mabi= 2
+ -mabort-on-noreturn [disabled]
+ -mapcs [disabled]
+@end smallexample
+
+The output is sensitive to the effects of previous command line
+options, so for example it is possible to find out which optimizations
+are enabled at @option{-O2} by using:
+
+@smallexample
+-O2 --help=optimizers
+@end smallexample
+
+Alternatively you can discover which binary optimizations are enabled
+by @option{-O3} by using:
+
+@smallexample
+gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
+gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
+diff /tmp/O2-opts /tmp/O3-opts | grep enabled
+@end smallexample
+
@item --version
@opindex version
Display the version number and copyrights of the invoked GCC@.
@xref{Standards,,Language Standards Supported by GCC}, for details of
these standard versions.
+@item -fgnu89-inline
+@opindex fgnu89-inline
+The option @option{-fgnu89-inline} tells GCC to use the traditional
+GNU semantics for @code{inline} functions when in C99 mode.
+@xref{Inline,,An Inline Function is As Fast As a Macro}. This option
+is accepted and ignored by GCC versions 4.1.3 up to but not including
+4.3. In GCC versions 4.3 and later it changes the behavior of GCC in
+C99 mode. Using this option is roughly equivalent to adding the
+@code{gnu_inline} function attribute to all inline functions
+(@pxref{Function Attributes}).
+
+The option @option{-fno-gnu89-inline} explicitly tells GCC to use the
+C99 semantics for @code{inline} when in C99 or gnu99 mode (i.e., it
+specifies the default behavior). This option was first supported in
+GCC 4.3. This option is not supported in C89 or gnu89 mode.
+
+The preprocesor macros @code{__GNUC_GNU_INLINE__} and
+@code{__GNUC_STDC_INLINE__} may be used to check which semantics are
+in effect for @code{inline} functions. @xref{Common Predefined
+Macros,,,cpp,The C Preprocessor}.
+
@item -aux-info @var{filename}
@opindex aux-info
Output to the given filename prototyped declarations for all functions
This warning is enabled by @option{-Wall}.
@item -Wimplicit-function-declaration
-@itemx -Werror-implicit-function-declaration
@opindex Wimplicit-function-declaration
-@opindex Werror-implicit-function-declaration
-Give a warning (or error) whenever a function is used before being
-declared. The form @option{-Wno-error-implicit-function-declaration}
-is not supported.
-This warning is enabled by @option{-Wall} (as a warning, not an error).
+@opindex Wno-implicit-function-declaration
+Give a warning whenever a function is used before being declared. In
+C99 mode (@option{-std=c99} or @option{-std=gnu99}), this warning is
+enabled by default and it is made into an error by
+@option{-pedantic-errors}. This warning is also enabled by
+@option{-Wall}.
@item -Wimplicit
@opindex Wimplicit
@item -Wunused-value
@opindex Wunused-value
-Warn whenever a statement computes a result that is explicitly not used.
-This warning is enabled by @option{-Wall}.
+Warn whenever a statement computes a result that is explicitly not
+used. To suppress this warning cast the unused expression to
+@samp{void}. This includes an expression-statement or the left-hand
+side of a comma expression that contains no side effects. For example,
+an expression such as @samp{x[i,j]} will cause a warning, while
+@samp{x[(void)i,j]} will not.
-To suppress this warning cast the expression to @samp{void}.
+This warning is enabled by @option{-Wall}.
@item -Wunused
@opindex Wunused
These warnings are possible only in optimizing compilation,
because they require data flow information that is computed only
-when optimizing. If you do not specify @option{-O}, you will not get
+when optimizing. If you do not specify @option{-O}, you will not get
these warnings. Instead, GCC will issue a warning about @option{-Wuninitialized}
requiring @option{-O}.
@option{-Wstrict-aliasing}, but it will also give a warning for some ambiguous
cases that are safe.
+@item -Wstrict-overflow
+@item -Wstrict-overflow=@var{n}
+@opindex -Wstrict-overflow
+This option is only active when @option{-fstrict-overflow} is active.
+It warns about cases where the compiler optimizes based on the
+assumption that signed overflow does not occur. Note that it does not
+warn about all cases where the code might overflow: it only warns
+about cases where the compiler implements some optimization. Thus
+this warning depends on the optimization level.
+
+An optimization which assumes that signed overflow does not occur is
+perfectly safe if the values of the variables involved are such that
+overflow never does, in fact, occur. Therefore this warning can
+easily give a false positive: a warning about code which is not
+actually a problem. To help focus on important issues, several
+warning levels are defined.
+
+@table @option
+@item -Wstrict-overflow=1
+Warn about cases which are both questionable and easy to avoid. For
+example: @code{x + 1 > x}; with @option{-fstrict-overflow}, the
+compiler will simplify this to @code{1}. This level of
+@option{-Wstrict-overflow} is enabled by @option{-Wall}; higher levels
+are not, and must be explicitly requested.
+
+@item -Wstrict-overflow=2
+Also warn about other cases where a comparison is simplified to a
+constant. For example: @code{abs (x) >= 0}. This can only be
+simplified when @option{-fstrict-overflow} is in effect, because
+@code{abs (INT_MIN)} overflows to @code{INT_MIN}, which is less than
+zero. @option{-Wstrict-overflow} (with no level) is the same as
+@option{-Wstrict-overflow=2}.
+
+@item -Wstrict-overflow=3
+Also warn about other cases where a comparison is simplified. For
+example: @code{x + 1 > 1} will be simplified to @code{x > 0}.
+
+@item -Wstrict-overflow=4
+Also warn about other simplifications not covered by the above cases.
+For example: @code{(x * 10) / 5} will be simplified to @code{x * 2}.
+
+@item -Wstrict-overflow=5
+Also warn about cases where the compiler reduces the magnitude of a
+constant involved in a comparison. For example: @code{x + 2 > y} will
+be simplified to @code{x + 1 >= y}. This is reported only at the
+highest warning level because this simplification applies to many
+comparisons, so this warning level will give a very large number of
+false positives.
+@end table
+
@item -Warray-bounds
@opindex Wno-array-bounds
@opindex Warray-bounds
@end smallexample
@item
-An expression-statement or the left-hand side of a comma expression
-contains no side effects.
-To suppress the warning, cast the unused expression to void.
-For example, an expression such as @samp{x[i,j]} will cause a warning,
-but @samp{x[(void)i,j]} will not.
-
-@item
An unsigned value is compared against zero with @samp{<} or @samp{>=}.
@item @r{(C only)}
This warning is also enabled by @option{-Wextra}; to get the other warnings
of @option{-Wextra} without this warning, use @samp{-Wextra -Wno-sign-compare}.
+@item -Waddress
+@opindex Waddress
+@opindex Wno-address
+Warn about suspicious uses of memory addresses. These include using
+the address of a function in a conditional expression, such as
+@code{void func(void); if (func)}, and comparisons against the memory
+address of a string literal, such as @code{if (x == "abc")}. Such
+uses typically indicate a programmer error: the address of a function
+always evaluates to true, so their use in a conditional usually
+indicate that the programmer forgot the parentheses in a function
+call; and comparisons against string literals result in unspecified
+behavior and are not portable in C, so they usually indicate that the
+programmer intended to use @code{strcmp}. This warning is enabled by
+@option{-Wall}.
+
+@item -Wlogical-op
+@opindex Wlogical-op
+@opindex Wno-logical-op
+Warn about suspicious uses of logical operators in expressions.
+This includes using logical operators in contexts where a
+bit-wise operator is likely to be expected.
+
@item -Waggregate-return
@opindex Waggregate-return
Warn if any functions that return structures or unions are defined or
called. (In languages where you can return an array, this also elicits
a warning.)
-@item -Walways-true
-@opindex Walways-true
-Warn about comparisons which are always true such as testing if
-unsigned values are greater than or equal to zero. This warning is
-enabled by @option{-Wall}.
-
@item -Wno-attributes
@opindex Wno-attributes
@opindex Wattributes
alternate syntax when in pedantic ISO C99 mode. This is default.
To inhibit the warning messages, use @option{-Wno-variadic-macros}.
+@item -Wvla
+@opindex Wvla
+@opindex Wno-vla
+Warn if variable length array is used in the code.
+@option{-Wno-vla} will prevent the @option{-pedantic} warning of
+the variable length array.
+
@item -Wvolatile-register-var
@opindex Wvolatile-register-var
@opindex Wno-volatile-register-var
This option is only active when @option{-fstack-protector} is active. It
warns about functions that will not be protected against stack smashing.
-@item -Wstring-literal-comparison
-@opindex Wstring-literal-comparison
-Warn about suspicious comparisons to string literal constants. In C,
-direct comparisons against the memory address of a string literal, such
-as @code{if (x == "abc")}, typically indicate a programmer error, and
-even when intentional, result in unspecified behavior and are not portable.
-Usually these warnings alert that the programmer intended to use
-@code{strcmp}. This warning is enabled by @option{-Wall}.
-
@item -Woverlength-strings
@opindex Woverlength-strings
Warn about string constants which are longer than the ``minimum
Don't forget the trailing @samp{/}.
@xref{Environment Variables}.
+@item -print-sysroot-headers-suffix
+@opindex print-sysroot-headers-suffix
+Print the suffix added to the target sysroot when searching for
+headers, or give an error if the compiler is not configured with such
+a suffix---and don't do anything else.
+
@item -dumpmachine
@opindex dumpmachine
Print the compiler's target machine (for example,
-ftree-dominator-opts @gol
-ftree-dse @gol
-ftree-ter @gol
--ftree-lrs @gol
-ftree-sra @gol
-ftree-copyrename @gol
-ftree-fre @gol
Perform linear loop transformations on tree. This flag can improve cache
performance and allow further loop optimizations to take place.
+@item -fcheck-data-deps
+Compare the results of several data dependence analyzers. This option
+is used for debugging the data dependence analyzers.
+
@item -ftree-loop-im
Perform loop invariant motion on trees. This pass moves only invariants that
would be hard to handle at RTL level (function calls, operations that expand to
much more complex trees to work on resulting in better RTL generation. This is
enabled by default at @option{-O} and higher.
-@item -ftree-lrs
-Perform live range splitting during the SSA->normal phase. Distinct live
-ranges of a variable are split into unique variables, allowing for better
-optimization later. This is enabled by default at @option{-O} and higher.
-
@item -ftree-vectorize
Perform loop vectorization on trees.
Bound on size of expressions used in the scalar evolutions analyzer.
Large expressions slow the analyzer.
+@item omega-max-vars
+The maximum number of variables in an Omega constraint system.
+The default value is 128.
+
+@item omega-max-geqs
+The maximum number of inequalities in an Omega constraint system.
+The default value is 256.
+
+@item omega-max-eqs
+The maximum number of equalities in an Omega constraint system.
+The default value is 128.
+
+@item omega-max-wild-cards
+The maximum number of wildcard variables that the Omega solver will
+be able to insert. The default value is 18.
+
+@item omega-hash-table-size
+The size of the hash table in the Omega solver. The default value is
+550.
+
+@item omega-max-keys
+The maximal number of keys used by the Omega solver. The default
+value is 500.
+
+@item omega-eliminate-redundant-constraints
+When set to 1, use expensive methods to eliminate all redundant
+constraints. The default value is 0.
+
@item vect-max-version-checks
The maximum number of runtime checks that can be performed when doing
loop versioning in the vectorizer. See option ftree-vect-loop-version
The following built-in spec functions are provided:
@table @code
+@item @code{getenv}
+The @code{getenv} spec function takes two arguments: an environment
+variable name and a string. If the environment variable is not
+defined, a fatal error is issued. Otherwise, the return value is the
+value of the environment variable concatenated with the string. For
+example, if @env{TOPDIR} is defined as @file{/path/to/top}, then:
+
+@smallexample
+%:getenv(TOPDIR /include)
+@end smallexample
+
+expands to @file{/path/to/top/include}.
+
@item @code{if-exists}
The @code{if-exists} spec function takes one argument, an absolute
pathname to a file. If the file exists, @code{if-exists} returns the
the name of the framework and header.h is found in the
@samp{"PrivateHeaders"} or @samp{"Headers"} directory.
+@item -iframework@var{dir}
+@opindex iframework
+Like @option{-F} except the directory is a treated as a system
+directory. The main difference between this @option{-iframework} and
+@option{-F} is that with @option{-iframework} the compiler does not
+warn about constructs contained within header files found via
+@var{dir}. This option is valid only for the C family of languages.
+
@item -gused
@opindex -gused
Emit debugging information for symbols that are used. For STABS
@item -mcmodel=large
@opindex mcmodel=large
Generate code for the large model: This model makes no assumptions
-about addresses and sizes of sections. Currently GCC does not implement
-this model.
+about addresses and sizes of sections.
@end table
@node IA-64 Options
@opindex mno-dsp
Use (do not use) the MIPS DSP ASE. @xref{MIPS DSP Built-in Functions}.
+@itemx -mdspr2
+@itemx -mno-dspr2
+@opindex mdspr2
+@opindex mno-dspr2
+Use (do not use) the MIPS DSP ASE REV 2. @xref{MIPS DSP Built-in Functions}.
+The option @option{-mdspr2} implies @option{-mdsp}.
+
@itemx -mpaired-single
@itemx -mno-paired-single
@opindex mpaired-single
@itemx -mno-popcntb
@itemx -mfprnd
@itemx -mno-fprnd
+@itemx -mcmpb
+@itemx -mno-cmpb
@itemx -mmfpgpr
@itemx -mno-mfpgpr
+@itemx -mdfp
+@itemx -mno-dfp
@opindex mpower
@opindex mno-power
@opindex mpower2
@opindex mno-popcntb
@opindex mfprnd
@opindex mno-fprnd
+@opindex mcmpb
+@opindex mno-cmpb
@opindex mmfpgpr
@opindex mno-mfpgpr
+@opindex mdfp
+@opindex mno-dfp
GCC supports two related instruction set architectures for the
RS/6000 and PowerPC@. The @dfn{POWER} instruction set are those
instructions supported by the @samp{rios} chip set used in the original
The @option{-mfprnd} option allows GCC to generate the FP round to
integer instructions implemented on the POWER5+ processor and other
processors that support the PowerPC V2.03 architecture.
+The @option{-mcmpb} option allows GCC to generate the compare bytes
+instruction implemented on the POWER6 processor and other processors
+that support the PowerPC V2.05 architecture.
The @option{-mmfpgpr} option allows GCC to generate the FP move to/from
general purpose register instructions implemented on the POWER6X
processor and other processors that support the extended PowerPC V2.05
architecture.
+The @option{-mdfp} option allows GCC to generate the decimal floating
+point instructions implemented on some POWER processors.
The @option{-mpowerpc64} option allows GCC to generate the additional
64-bit instructions that are found in the full PowerPC64 architecture
@item -mstack-size=@var{stack-size}
@opindex mstack-guard
@opindex mstack-size
-These arguments always have to be used in conjunction. If they are present the s390
-back end emits additional instructions in the function prologue which trigger a trap
-if the stack size is @var{stack-guard} bytes above the @var{stack-size}
-(remember that the stack on s390 grows downward). These options are intended to
-be used to help debugging stack overflow problems. The additionally emitted code
-causes only little overhead and hence can also be used in production like systems
-without greater performance degradation. The given values have to be exact
-powers of 2 and @var{stack-size} has to be greater than @var{stack-guard} without
-exceeding 64k.
+If these options are provided the s390 back end emits additional instructions in
+the function prologue which trigger a trap if the stack size is @var{stack-guard}
+bytes above the @var{stack-size} (remember that the stack on s390 grows downward).
+If the @var{stack-guard} option is omitted the smallest power of 2 larger than
+the frame size of the compiled function is chosen.
+These options are intended to be used to help debugging stack overflow problems.
+The additionally emitted code causes only little overhead and hence can also be
+used in production like systems without greater performance degradation. The given
+values have to be exact powers of 2 and @var{stack-size} has to be greater than
+@var{stack-guard} without exceeding 64k.
In order to be efficient the extra code makes the assumption that the stack starts
at an address aligned to the value given by @var{stack-size}.
+The @var{stack-guard} option can only be used in conjunction with @var{stack-size}.
@end table
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