什么是在创建抽象语法树的上下文中合成的属性？ [英] What are synthesized attributes in the context of creating an abstract syntax tree?

问题描述

编译器解析源代码并构建抽象语法树。用于构造抽象语法树的函数返回构成合成属性的指针。它们是什么，它们与继承的属性有什么不同？

Compilers parse source code and build an abstract syntax tree. The functions used to construct an abstract syntax tree return pointers which constitute synthesized attributes. What are they and how do they differ from inherited attributes.?

编辑：我不知道这是否可以帮助，

edit: I don't know if this can help, but I originally heard of these terms in a French context: Attributs synthétisés, attributs hérités.

推荐答案

属性是与中心感兴趣的内容相关联的附加值，与法律语境中的这些术语相关联：Attributessynthétisés，attributionshérités。 。在AST的情况下，您可以将它们认为是与每个AST节点相关联的对（attribute_name，attribute_value），其中属性名称对应于一些有趣的事实类型，并且属性值对应于该事实的实际状态（例如，（constants_in_subtree_count，12））。

Attributes are additional values associated with something of central interest. In the case of ASTs, you can think of them as pairs (attribute_name, attribute_value) associated with each AST node, where the attribute name corresponds to some interesting fact-type, and the attribute value corresponds to the actual state of that fact (e.g., "(constants_in_subtree_count,12)").

继承和是用于描述属性值对于每个AST节点计算，通常与使用其孩子产生AST节点的语法规则相关联。

Inherited and synthesized are terms for describing how the attribute values are computed for each AST node, usually associated with the grammar rule that produces the AST node using its children.

合成是根据来自子节点的属性值计算的，并且被向上传递树。通常，组合合成属性的值以产生父节点的属性。如果AST节点有两个子节点，每个子节点都有自己的属性（constants_in_subtree_count，5）和（constants_in_subtree_count，7），那么通过将这些属性向上传递，父节点可以计算出其对应的属性（constants_in_subtree_count，12）。

Synthesized attributes are those whose value is computed from attribute values from children nodes, and are being passed up the tree. Often, values of synthesized attributes are combined to produce an attribute for the parent node. If an AST node has two children, each of which have their own attributes (constants_in_subtree_count,5) and (constants_in_subtree_count,7), then by passing those attributes up, the parent can compute his corresponding attribute (constants_in_subtree_count,12).

继承的属性是从父级传递给子级的属性。如果函数AST的根知道函数返回类型是（return_type，integer）作为属性，它可以将返回类型传递给函数根的子节点，例如。到函数体。在那棵树的深处是一个实际的return语句;如果它接收到继承属性（return_type，X），
它可以检查它计算的结果是否是正确的类型。

Inherited attributes are those passed from the parent down to the child. If the root of a function AST "knows" the function return type is (return_type,integer) as an attribute, it can pass the return type to the children of the function root, e.g. to the function body. Someplace deep down in that tree is an actual return statement; if it receives the inherited attribute (return_type,X), it can check that the result it is computing is the correct type.

能够为节点定义任意属性集合，并且为了处理AST（构建符号表，构造控制流图，进行类型检查，计算度量，...）所需的多个目的将它们上下传递。 属性语法 生成器是一种解析器生成器，将生成语法规则，属性集定义和规则，以及如何为每个规则中涉及的节点计算合成和继承属性，并生成解析器和计算所有属性的AST walker。

In practice, you want to be able to define arbitrary sets of attributes for nodes, and pass them up and down the tree for the multiple purposes required to process ASTs (building symbol tables, constructing control flow graphs, doing type checking, computing metrics, ...). An attribute grammar generator is a kind of parser generator that will take grammar rules, sets of attribute definitions, and rules about how to compute synthesized and inherited attributes for the nodes involved in each rule, and generates both a parser and an AST walker that computes all the attributes.

这个想法的价值在于它提供了一种由自动化支持的组织原理，可以用来以常规方式计算关于AST的许多有趣的东西。否则，您可以使用特殊代码对所有内容进行编码。

The value of this idea is it provides an organizing principle supported by automation, that can be used to compute many interesting things about ASTs in a regular way. Otherwise you get to code all that stuff using ad hoc code.

我们的 DMS软件重构工具包是一个AST操作系统（实际上是源到源程序转换），它大量使用并行属性计算来计算各种有用的AST分析：常规度量，符号表，类型检查（类似于上面描述的返回类型检查），从代码中提取控制和数据流，以及其他不那么容易描述但是在子树上计算的有用结果（这个表达式）。为什么要并行？嗯，在子树中的属性计算本质上是独立的，所以并行性已经存在，当你处理真正的大树性能问题。 DMS经常处理数千个编译单元，每个编译单元产生一个（可能是大的）AST。

Our DMS Software Reengineering Toolkit is an AST manipulation system (actually source-to-source program transformations) that heavily uses parallel attribute evaluation to compute all kinds of useful analyses over ASTs: conventional metrics, symbol tables, type checks (like the return type check I described above), control and data flow extraction from code, as well as other not-so-easily described but useful results computed over subtrees ("list of side effecting assignments in this expression"). Why parallel? Well, attribute computations in subtrees are essentially independent, so the parallelism is already there, and when you deal with really big trees performance matters. DMS often deals with thousands of compilation units, each producing a (possibly big) AST.

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