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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 ‘<samp><span class="samp">-m</span></samp>’ 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 ‘<samp><span class="samp">s390</span></samp>’ |
| targets, the default is <samp><span class="option">-m31</span></samp>, while the ‘<samp><span class="samp">s390x</span></samp>’ |
| 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 ‘<samp><span class="samp">g5</span></samp>’, ‘<samp><span class="samp">g6</span></samp>’, ‘<samp><span class="samp">z900</span></samp>’, ‘<samp><span class="samp">z990</span></samp>’, |
| ‘<samp><span class="samp">z9-109</span></samp>’, ‘<samp><span class="samp">z9-ec</span></samp>’ and ‘<samp><span class="samp">z10</span></samp>’. |
| 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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