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annotate docs/HowToUseAttributes.rst @ 107:a03ddd01be7e

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author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Sun, 31 Jan 2016 17:34:49 +0900
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1 =====================
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2 How To Use Attributes
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3 =====================
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4
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5 .. contents::
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6 :local:
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7
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8 Introduction
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9 ============
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10
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11 Attributes in LLVM have changed in some fundamental ways. It was necessary to
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12 do this to support expanding the attributes to encompass more than a handful of
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13 attributes --- e.g. command line options. The old way of handling attributes
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14 consisted of representing them as a bit mask of values. This bit mask was
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15 stored in a "list" structure that was reference counted. The advantage of this
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16 was that attributes could be manipulated with 'or's and 'and's. The
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17 disadvantage of this was that there was limited room for expansion, and
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18 virtually no support for attribute-value pairs other than alignment.
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19
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20 In the new scheme, an ``Attribute`` object represents a single attribute that's
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21 uniqued. You use the ``Attribute::get`` methods to create a new ``Attribute``
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22 object. An attribute can be a single "enum" value (the enum being the
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23 ``Attribute::AttrKind`` enum), a string representing a target-dependent
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24 attribute, or an attribute-value pair. Some examples:
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25
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26 * Target-independent: ``noinline``, ``zext``
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27 * Target-dependent: ``"no-sse"``, ``"thumb2"``
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28 * Attribute-value pair: ``"cpu" = "cortex-a8"``, ``align = 4``
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29
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30 Note: for an attribute value pair, we expect a target-dependent attribute to
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31 have a string for the value.
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32
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33 ``Attribute``
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34 =============
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35 An ``Attribute`` object is designed to be passed around by value.
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36
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37 Because attributes are no longer represented as a bit mask, you will need to
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38 convert any code which does treat them as a bit mask to use the new query
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39 methods on the Attribute class.
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40
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41 ``AttributeSet``
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42 ================
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43
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44 The ``AttributeSet`` class replaces the old ``AttributeList`` class. The
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45 ``AttributeSet`` stores a collection of Attribute objects for each kind of
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46 object that may have an attribute associated with it: the function as a
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47 whole, the return type, or the function's parameters. A function's attributes
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48 are at index ``AttributeSet::FunctionIndex``; the return type's attributes are
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49 at index ``AttributeSet::ReturnIndex``; and the function's parameters'
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50 attributes are at indices 1, ..., n (where 'n' is the number of parameters).
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51 Most methods on the ``AttributeSet`` class take an index parameter.
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52
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53 An ``AttributeSet`` is also a uniqued and immutable object. You create an
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54 ``AttributeSet`` through the ``AttributeSet::get`` methods. You can add and
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55 remove attributes, which result in the creation of a new ``AttributeSet``.
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56
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57 An ``AttributeSet`` object is designed to be passed around by value.
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58
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59 Note: It is advised that you do *not* use the ``AttributeSet`` "introspection"
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60 methods (e.g. ``Raw``, ``getRawPointer``, etc.). These methods break
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61 encapsulation, and may be removed in a future release (i.e. LLVM 4.0).
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62
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63 ``AttrBuilder``
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64 ===============
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65
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66 Lastly, we have a "builder" class to help create the ``AttributeSet`` object
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67 without having to create several different intermediate uniqued
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68 ``AttributeSet`` objects. The ``AttrBuilder`` class allows you to add and
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69 remove attributes at will. The attributes won't be uniqued until you call the
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70 appropriate ``AttributeSet::get`` method.
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71
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72 An ``AttrBuilder`` object is *not* designed to be passed around by value. It
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73 should be passed by reference.
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74
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75 Note: It is advised that you do *not* use the ``AttrBuilder::addRawValue()``
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76 method or the ``AttrBuilder(uint64_t Val)`` constructor. These are for
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77 backwards compatibility and may be removed in a future release (i.e. LLVM 4.0).
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78
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79 And that's basically it! A lot of functionality is hidden behind these classes,
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80 but the interfaces are pretty straight forward.
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81