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| <a name="Condition-Code-Status"></a> |
| <h3 class="section">17.16 Condition Code Status</h3> |
| <a name="index-condition-code-status"></a> |
| |
| <p>The macros in this section can be split in two families, according to the |
| two ways of representing condition codes in GCC. |
| </p> |
| <p>The first representation is the so called <code>(cc0)</code> representation |
| (see <a href="Jump-Patterns.html#Jump-Patterns">Jump Patterns</a>), where all instructions can have an implicit |
| clobber of the condition codes. The second is the condition code |
| register representation, which provides better schedulability for |
| architectures that do have a condition code register, but on which |
| most instructions do not affect it. The latter category includes |
| most RISC machines. |
| </p> |
| <p>The implicit clobbering poses a strong restriction on the placement of |
| the definition and use of the condition code. In the past the definition |
| and use were always adjacent. However, recent changes to support trapping |
| arithmatic may result in the definition and user being in different blocks. |
| Thus, there may be a <code>NOTE_INSN_BASIC_BLOCK</code> between them. Additionally, |
| the definition may be the source of exception handling edges. |
| </p> |
| <p>These restrictions can prevent important |
| optimizations on some machines. For example, on the IBM RS/6000, there |
| is a delay for taken branches unless the condition code register is set |
| three instructions earlier than the conditional branch. The instruction |
| scheduler cannot perform this optimization if it is not permitted to |
| separate the definition and use of the condition code register. |
| </p> |
| <p>For this reason, it is possible and suggested to use a register to |
| represent the condition code for new ports. If there is a specific |
| condition code register in the machine, use a hard register. If the |
| condition code or comparison result can be placed in any general register, |
| or if there are multiple condition registers, use a pseudo register. |
| Registers used to store the condition code value will usually have a mode |
| that is in class <code>MODE_CC</code>. |
| </p> |
| <p>Alternatively, you can use <code>BImode</code> if the comparison operator is |
| specified already in the compare instruction. In this case, you are not |
| interested in most macros in this section. |
| </p> |
| <table class="menu" border="0" cellspacing="0"> |
| <tr><td align="left" valign="top">• <a href="CC0-Condition-Codes.html#CC0-Condition-Codes" accesskey="1">CC0 Condition Codes</a>:</td><td> </td><td align="left" valign="top">Old style representation of condition codes. |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="MODE_005fCC-Condition-Codes.html#MODE_005fCC-Condition-Codes" accesskey="2">MODE_CC Condition Codes</a>:</td><td> </td><td align="left" valign="top">Modern representation of condition codes. |
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