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| Next: <a href="Insn-Lengths.html#Insn-Lengths" accesskey="n" rel="next">Insn Lengths</a>, Previous: <a href="Tagging-Insns.html#Tagging-Insns" accesskey="p" rel="prev">Tagging Insns</a>, Up: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="u" rel="up">Insn Attributes</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Example-of-Attribute-Specifications"></a> |
| <h4 class="subsection">16.19.4 Example of Attribute Specifications</h4> |
| <a name="index-attribute-specifications-example"></a> |
| <a name="index-attribute-specifications"></a> |
| |
| <p>The judicious use of defaulting is important in the efficient use of |
| insn attributes. Typically, insns are divided into <em>types</em> and an |
| attribute, customarily called <code>type</code>, is used to represent this |
| value. This attribute is normally used only to define the default value |
| for other attributes. An example will clarify this usage. |
| </p> |
| <p>Assume we have a RISC machine with a condition code and in which only |
| full-word operations are performed in registers. Let us assume that we |
| can divide all insns into loads, stores, (integer) arithmetic |
| operations, floating point operations, and branches. |
| </p> |
| <p>Here we will concern ourselves with determining the effect of an insn on |
| the condition code and will limit ourselves to the following possible |
| effects: The condition code can be set unpredictably (clobbered), not |
| be changed, be set to agree with the results of the operation, or only |
| changed if the item previously set into the condition code has been |
| modified. |
| </p> |
| <p>Here is part of a sample <samp>md</samp> file for such a machine: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define_attr "type" "load,store,arith,fp,branch" (const_string "arith")) |
| |
| (define_attr "cc" "clobber,unchanged,set,change0" |
| (cond [(eq_attr "type" "load") |
| (const_string "change0") |
| (eq_attr "type" "store,branch") |
| (const_string "unchanged") |
| (eq_attr "type" "arith") |
| (if_then_else (match_operand:SI 0 "" "") |
| (const_string "set") |
| (const_string "clobber"))] |
| (const_string "clobber"))) |
| |
| (define_insn "" |
| [(set (match_operand:SI 0 "general_operand" "=r,r,m") |
| (match_operand:SI 1 "general_operand" "r,m,r"))] |
| "" |
| "@ |
| move %0,%1 |
| load %0,%1 |
| store %0,%1" |
| [(set_attr "type" "arith,load,store")]) |
| </pre></div> |
| |
| <p>Note that we assume in the above example that arithmetic operations |
| performed on quantities smaller than a machine word clobber the condition |
| code since they will set the condition code to a value corresponding to the |
| full-word result. |
| </p> |
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