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<h4 class="subsection">3.17.33 S/390 and zSeries Options</h4>

<p><a name="index-S_002f390-and-zSeries-Options-1899"></a>
These are the &lsquo;<samp><span class="samp">-m</span></samp>&rsquo; options defined for the S/390 and zSeries architecture.

     <dl>
<dt><code>-mhard-float</code><dt><code>-msoft-float</code><dd><a name="index-mhard_002dfloat-1900"></a><a name="index-msoft_002dfloat-1901"></a>Use (do not use) the hardware floating-point instructions and registers
for floating-point operations.  When <samp><span class="option">-msoft-float</span></samp> is specified,
functions in <samp><span class="file">libgcc.a</span></samp> will be used to perform floating-point
operations.  When <samp><span class="option">-mhard-float</span></samp> is specified, the compiler
generates IEEE floating-point instructions.  This is the default.

     <br><dt><code>-mhard-dfp</code><dt><code>-mno-hard-dfp</code><dd><a name="index-mhard_002ddfp-1902"></a><a name="index-mno_002dhard_002ddfp-1903"></a>Use (do not use) the hardware decimal-floating-point instructions for
decimal-floating-point operations.  When <samp><span class="option">-mno-hard-dfp</span></samp> is
specified, functions in <samp><span class="file">libgcc.a</span></samp> will be used to perform
decimal-floating-point operations.  When <samp><span class="option">-mhard-dfp</span></samp> is
specified, the compiler generates decimal-floating-point hardware
instructions.  This is the default for <samp><span class="option">-march=z9-ec</span></samp> or higher.

     <br><dt><code>-mlong-double-64</code><dt><code>-mlong-double-128</code><dd><a name="index-mlong_002ddouble_002d64-1904"></a><a name="index-mlong_002ddouble_002d128-1905"></a>These switches control the size of <code>long double</code> type. A size
of 64bit makes the <code>long double</code> type equivalent to the <code>double</code>
type. This is the default.

     <br><dt><code>-mbackchain</code><dt><code>-mno-backchain</code><dd><a name="index-mbackchain-1906"></a><a name="index-mno_002dbackchain-1907"></a>Store (do not store) the address of the caller's frame as backchain pointer
into the callee's stack frame. 
A backchain may be needed to allow debugging using tools that do not understand
DWARF-2 call frame information. 
When <samp><span class="option">-mno-packed-stack</span></samp> is in effect, the backchain pointer is stored
at the bottom of the stack frame; when <samp><span class="option">-mpacked-stack</span></samp> is in effect,
the backchain is placed into the topmost word of the 96/160 byte register
save area.

     <p>In general, code compiled with <samp><span class="option">-mbackchain</span></samp> is call-compatible with
code compiled with <samp><span class="option">-mmo-backchain</span></samp>; however, use of the backchain
for debugging purposes usually requires that the whole binary is built with
<samp><span class="option">-mbackchain</span></samp>.  Note that the combination of <samp><span class="option">-mbackchain</span></samp>,
<samp><span class="option">-mpacked-stack</span></samp> and <samp><span class="option">-mhard-float</span></samp> is not supported.  In order
to build a linux kernel use <samp><span class="option">-msoft-float</span></samp>.

     <p>The default is to not maintain the backchain.

     <br><dt><code>-mpacked-stack</code><dt><code>-mno-packed-stack</code><dd><a name="index-mpacked_002dstack-1908"></a><a name="index-mno_002dpacked_002dstack-1909"></a>Use (do not use) the packed stack layout.  When <samp><span class="option">-mno-packed-stack</span></samp> is
specified, the compiler uses the all fields of the 96/160 byte register save
area only for their default purpose; unused fields still take up stack space. 
When <samp><span class="option">-mpacked-stack</span></samp> is specified, register save slots are densely
packed at the top of the register save area; unused space is reused for other
purposes, allowing for more efficient use of the available stack space. 
However, when <samp><span class="option">-mbackchain</span></samp> is also in effect, the topmost word of
the save area is always used to store the backchain, and the return address
register is always saved two words below the backchain.

     <p>As long as the stack frame backchain is not used, code generated with
<samp><span class="option">-mpacked-stack</span></samp> is call-compatible with code generated with
<samp><span class="option">-mno-packed-stack</span></samp>.  Note that some non-FSF releases of GCC 2.95 for
S/390 or zSeries generated code that uses the stack frame backchain at run
time, not just for debugging purposes.  Such code is not call-compatible
with code compiled with <samp><span class="option">-mpacked-stack</span></samp>.  Also, note that the
combination of <samp><span class="option">-mbackchain</span></samp>,
<samp><span class="option">-mpacked-stack</span></samp> and <samp><span class="option">-mhard-float</span></samp> is not supported.  In order
to build a linux kernel use <samp><span class="option">-msoft-float</span></samp>.

     <p>The default is to not use the packed stack layout.

     <br><dt><code>-msmall-exec</code><dt><code>-mno-small-exec</code><dd><a name="index-msmall_002dexec-1910"></a><a name="index-mno_002dsmall_002dexec-1911"></a>Generate (or do not generate) code using the <code>bras</code> instruction
to do subroutine calls. 
This only works reliably if the total executable size does not
exceed 64k.  The default is to use the <code>basr</code> instruction instead,
which does not have this limitation.

     <br><dt><code>-m64</code><dt><code>-m31</code><dd><a name="index-m64-1912"></a><a name="index-m31-1913"></a>When <samp><span class="option">-m31</span></samp> is specified, generate code compliant to the
GNU/Linux for S/390 ABI.  When <samp><span class="option">-m64</span></samp> is specified, generate
code compliant to the GNU/Linux for zSeries ABI.  This allows GCC in
particular to generate 64-bit instructions.  For the &lsquo;<samp><span class="samp">s390</span></samp>&rsquo;
targets, the default is <samp><span class="option">-m31</span></samp>, while the &lsquo;<samp><span class="samp">s390x</span></samp>&rsquo;
targets default to <samp><span class="option">-m64</span></samp>.

     <br><dt><code>-mzarch</code><dt><code>-mesa</code><dd><a name="index-mzarch-1914"></a><a name="index-mesa-1915"></a>When <samp><span class="option">-mzarch</span></samp> is specified, generate code using the
instructions available on z/Architecture. 
When <samp><span class="option">-mesa</span></samp> is specified, generate code using the
instructions available on ESA/390.  Note that <samp><span class="option">-mesa</span></samp> is
not possible with <samp><span class="option">-m64</span></samp>. 
When generating code compliant to the GNU/Linux for S/390 ABI,
the default is <samp><span class="option">-mesa</span></samp>.  When generating code compliant
to the GNU/Linux for zSeries ABI, the default is <samp><span class="option">-mzarch</span></samp>.

     <br><dt><code>-mmvcle</code><dt><code>-mno-mvcle</code><dd><a name="index-mmvcle-1916"></a><a name="index-mno_002dmvcle-1917"></a>Generate (or do not generate) code using the <code>mvcle</code> instruction
to perform block moves.  When <samp><span class="option">-mno-mvcle</span></samp> is specified,
use a <code>mvc</code> loop instead.  This is the default unless optimizing for
size.

     <br><dt><code>-mdebug</code><dt><code>-mno-debug</code><dd><a name="index-mdebug-1918"></a><a name="index-mno_002ddebug-1919"></a>Print (or do not print) additional debug information when compiling. 
The default is to not print debug information.

     <br><dt><code>-march=</code><var>cpu-type</var><dd><a name="index-march-1920"></a>Generate code that will run on <var>cpu-type</var>, which is the name of a system
representing a certain processor type.  Possible values for
<var>cpu-type</var> are &lsquo;<samp><span class="samp">g5</span></samp>&rsquo;, &lsquo;<samp><span class="samp">g6</span></samp>&rsquo;, &lsquo;<samp><span class="samp">z900</span></samp>&rsquo;, &lsquo;<samp><span class="samp">z990</span></samp>&rsquo;,
&lsquo;<samp><span class="samp">z9-109</span></samp>&rsquo;, &lsquo;<samp><span class="samp">z9-ec</span></samp>&rsquo; and &lsquo;<samp><span class="samp">z10</span></samp>&rsquo;. 
When generating code using the instructions available on z/Architecture,
the default is <samp><span class="option">-march=z900</span></samp>.  Otherwise, the default is
<samp><span class="option">-march=g5</span></samp>.

     <br><dt><code>-mtune=</code><var>cpu-type</var><dd><a name="index-mtune-1921"></a>Tune to <var>cpu-type</var> everything applicable about the generated code,
except for the ABI and the set of available instructions. 
The list of <var>cpu-type</var> values is the same as for <samp><span class="option">-march</span></samp>. 
The default is the value used for <samp><span class="option">-march</span></samp>.

     <br><dt><code>-mtpf-trace</code><dt><code>-mno-tpf-trace</code><dd><a name="index-mtpf_002dtrace-1922"></a><a name="index-mno_002dtpf_002dtrace-1923"></a>Generate code that adds (does not add) in TPF OS specific branches to trace
routines in the operating system.  This option is off by default, even
when compiling for the TPF OS.

     <br><dt><code>-mfused-madd</code><dt><code>-mno-fused-madd</code><dd><a name="index-mfused_002dmadd-1924"></a><a name="index-mno_002dfused_002dmadd-1925"></a>Generate code that uses (does not use) the floating point multiply and
accumulate instructions.  These instructions are generated by default if
hardware floating point is used.

     <br><dt><code>-mwarn-framesize=</code><var>framesize</var><dd><a name="index-mwarn_002dframesize-1926"></a>Emit a warning if the current function exceeds the given frame size.  Because
this is a compile time check it doesn't need to be a real problem when the program
runs.  It is intended to identify functions which most probably cause
a stack overflow.  It is useful to be used in an environment with limited stack
size e.g. the linux kernel.

     <br><dt><code>-mwarn-dynamicstack</code><dd><a name="index-mwarn_002ddynamicstack-1927"></a>Emit a warning if the function calls alloca or uses dynamically
sized arrays.  This is generally a bad idea with a limited stack size.

     <br><dt><code>-mstack-guard=</code><var>stack-guard</var><dt><code>-mstack-size=</code><var>stack-size</var><dd><a name="index-mstack_002dguard-1928"></a><a name="index-mstack_002dsize-1929"></a>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</var>
bytes above the <var>stack-size</var> (remember that the stack on s390 grows downward). 
If the <var>stack-guard</var> 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</var> has to be greater than
<var>stack-guard</var> 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</var>. 
The <var>stack-guard</var> option can only be used in conjunction with <var>stack-size</var>. 
</dl>

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