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<title>MODE_CC Condition Codes - GNU Compiler Collection (GCC) Internals</title>
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<a name="MODE_CC-Condition-Codes"></a>
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<a name="MODE_005fCC-Condition-Codes"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="CC0-Condition-Codes.html#CC0-Condition-Codes">CC0 Condition Codes</a>,
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Up: <a rel="up" accesskey="u" href="Condition-Code.html#Condition-Code">Condition Code</a>
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<h4 class="subsection">17.15.2 Representation of condition codes using registers</h4>
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<p><a name="index-CCmode-4433"></a><a name="index-MODE_005fCC-4434"></a>
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<div class="defun">
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— Macro: <b>SELECT_CC_MODE</b> (<var>op, x, y</var>)<var><a name="index-SELECT_005fCC_005fMODE-4435"></a></var><br>
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<blockquote><p>On many machines, the condition code may be produced by other instructions
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than compares, for example the branch can use directly the condition
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code set by a subtract instruction. However, on some machines
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when the condition code is set this way some bits (such as the overflow
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bit) are not set in the same way as a test instruction, so that a different
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branch instruction must be used for some conditional branches. When
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this happens, use the machine mode of the condition code register to
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record different formats of the condition code register. Modes can
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also be used to record which compare instruction (e.g. a signed or an
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unsigned comparison) produced the condition codes.
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<p>If other modes than <code>CCmode</code> are required, add them to
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<samp><var>machine</var><span class="file">-modes.def</span></samp> and define <code>SELECT_CC_MODE</code> to choose
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a mode given an operand of a compare. This is needed because the modes
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have to be chosen not only during RTL generation but also, for example,
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by instruction combination. The result of <code>SELECT_CC_MODE</code> should
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be consistent with the mode used in the patterns; for example to support
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the case of the add on the SPARC discussed above, we have the pattern
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<pre class="smallexample"> (define_insn ""
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[(set (reg:CC_NOOV 0)
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(compare:CC_NOOV
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(plus:SI (match_operand:SI 0 "register_operand" "%r")
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(match_operand:SI 1 "arith_operand" "rI"))
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(const_int 0)))]
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""
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"...")
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</pre>
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<p class="noindent">together with a <code>SELECT_CC_MODE</code> that returns <code>CC_NOOVmode</code>
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for comparisons whose argument is a <code>plus</code>:
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<pre class="smallexample"> #define SELECT_CC_MODE(OP,X,Y) \
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(GET_MODE_CLASS (GET_MODE (X)) == MODE_FLOAT \
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? ((OP == LT || OP == LE || OP == GT || OP == GE) \
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? CCFPEmode : CCFPmode) \
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: ((GET_CODE (X) == PLUS || GET_CODE (X) == MINUS \
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|| GET_CODE (X) == NEG || GET_CODE (x) == ASHIFT) \
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? CC_NOOVmode : CCmode))
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</pre>
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<p>Another reason to use modes is to retain information on which operands
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were used by the comparison; see <code>REVERSIBLE_CC_MODE</code> later in
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this section.
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<p>You should define this macro if and only if you define extra CC modes
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in <samp><var>machine</var><span class="file">-modes.def</span></samp>.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: void <b>TARGET_CANONICALIZE_COMPARISON</b> (<var>int *code, rtx *op0, rtx *op1, bool op0_preserve_value</var>)<var><a name="index-TARGET_005fCANONICALIZE_005fCOMPARISON-4436"></a></var><br>
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<blockquote><p>On some machines not all possible comparisons are defined, but you can
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convert an invalid comparison into a valid one. For example, the Alpha
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does not have a <code>GT</code> comparison, but you can use an <code>LT</code>
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comparison instead and swap the order of the operands.
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<p>On such machines, implement this hook to do any required conversions.
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<var>code</var> is the initial comparison code and <var>op0</var> and <var>op1</var>
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are the left and right operands of the comparison, respectively. If
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<var>op0_preserve_value</var> is <code>true</code> the implementation is not
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allowed to change the value of <var>op0</var> since the value might be used
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in RTXs which aren't comparisons. E.g. the implementation is not
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allowed to swap operands in that case.
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<p>GCC will not assume that the comparison resulting from this macro is
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valid but will see if the resulting insn matches a pattern in the
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<samp><span class="file">md</span></samp> file.
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<p>You need not to implement this hook if it would never change the
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comparison code or operands.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>REVERSIBLE_CC_MODE</b> (<var>mode</var>)<var><a name="index-REVERSIBLE_005fCC_005fMODE-4437"></a></var><br>
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<blockquote><p>A C expression whose value is one if it is always safe to reverse a
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comparison whose mode is <var>mode</var>. If <code>SELECT_CC_MODE</code>
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can ever return <var>mode</var> for a floating-point inequality comparison,
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then <code>REVERSIBLE_CC_MODE (</code><var>mode</var><code>)</code> must be zero.
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<p>You need not define this macro if it would always returns zero or if the
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floating-point format is anything other than <code>IEEE_FLOAT_FORMAT</code>.
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For example, here is the definition used on the SPARC, where floating-point
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inequality comparisons are given either <code>CCFPEmode</code> or <code>CCFPmode</code>:
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<pre class="smallexample"> #define REVERSIBLE_CC_MODE(MODE) \
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((MODE) != CCFPEmode && (MODE) != CCFPmode)
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</pre>
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</blockquote></div>
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<div class="defun">
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— Macro: <b>REVERSE_CONDITION</b> (<var>code, mode</var>)<var><a name="index-REVERSE_005fCONDITION-4438"></a></var><br>
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<blockquote><p>A C expression whose value is reversed condition code of the <var>code</var> for
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comparison done in CC_MODE <var>mode</var>. The macro is used only in case
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<code>REVERSIBLE_CC_MODE (</code><var>mode</var><code>)</code> is nonzero. Define this macro in case
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machine has some non-standard way how to reverse certain conditionals. For
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instance in case all floating point conditions are non-trapping, compiler may
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freely convert unordered compares to ordered ones. Then definition may look
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like:
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<pre class="smallexample"> #define REVERSE_CONDITION(CODE, MODE) \
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((MODE) != CCFPmode ? reverse_condition (CODE) \
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: reverse_condition_maybe_unordered (CODE))
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</pre>
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</blockquote></div>
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<div class="defun">
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— Target Hook: bool <b>TARGET_FIXED_CONDITION_CODE_REGS</b> (<var>unsigned int *p1, unsigned int *p2</var>)<var><a name="index-TARGET_005fFIXED_005fCONDITION_005fCODE_005fREGS-4439"></a></var><br>
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<blockquote><p>On targets which do not use <code>(cc0)</code>, and which use a hard
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register rather than a pseudo-register to hold condition codes, the
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regular CSE passes are often not able to identify cases in which the
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hard register is set to a common value. Use this hook to enable a
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small pass which optimizes such cases. This hook should return true
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to enable this pass, and it should set the integers to which its
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arguments point to the hard register numbers used for condition codes.
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When there is only one such register, as is true on most systems, the
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integer pointed to by <var>p2</var> should be set to
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<code>INVALID_REGNUM</code>.
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<p>The default version of this hook returns false.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: machine_mode <b>TARGET_CC_MODES_COMPATIBLE</b> (<var>machine_mode m1, machine_mode m2</var>)<var><a name="index-TARGET_005fCC_005fMODES_005fCOMPATIBLE-4440"></a></var><br>
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<blockquote><p>On targets which use multiple condition code modes in class
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<code>MODE_CC</code>, it is sometimes the case that a comparison can be
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validly done in more than one mode. On such a system, define this
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target hook to take two mode arguments and to return a mode in which
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both comparisons may be validly done. If there is no such mode,
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return <code>VOIDmode</code>.
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<p>The default version of this hook checks whether the modes are the
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same. If they are, it returns that mode. If they are different, it
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returns <code>VOIDmode</code>.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: unsigned int <b>TARGET_FLAGS_REGNUM</b><var><a name="index-TARGET_005fFLAGS_005fREGNUM-4441"></a></var><br>
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<blockquote><p>If the target has a dedicated flags register, and it needs to use the post-reload comparison elimination pass, then this value should be set appropriately.
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</p></blockquote></div>
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