Bazel 以什么顺序链接所有文件？

Question

我知道通常当链接器在 C++ 中运行时，命令中链接参数的顺序很重要（如此处所述）。

Bazel 以什么顺序链接文件？它是否只是从构建文件的顶部到底部，有没有办法自定义它？

Answer 1

Bazel 使用“拓扑”顺序进行链接。来自文档；

“拓扑”（以前称为“链接”）：从根到叶的拓扑排序。没有从左到右的保证。

此场景中的叶子是所有库依赖项，以树的形式表示。链接顺序与 BUILD 文件中列出的目标的顺序无关。

您可以通过检查源代码来确认这一点。源代码中的文档字符串比 depset 文档提供了更好的解释。

 * <p>LINK_ORDER: a variation of left-to-right preorder that enforces topological sorting. In
 * Starlark it is called "topological"; its older deprecated name is "link".
 *
 * <p>For example, for the nested set {A, C, {B, D}}, the iteration order is "A C B D"
 * (parent-first).
 *
 * <p>This type of set would typically be used for artifacts where elements of nested sets go after
 * the direct members of a set, for example when providing a list of libraries to the C++ compiler.
 *
 * <p>The custom ordering has the property that elements of nested sets always come before elements
 * of descendant nested sets. Left-to-right order is preserved if possible, both for items and for
 * references to nested sets.
 *
 * <p>The left-to-right pre-order-like ordering is implemented by running a right-to-left postorder
 * traversal and then reversing the result.

有一种方法可以定制它。尽管这并不简单，并且涉及编写您自己的 starlark 插件。

例如，如果您修改my_c_archive.bzl示例，为每次调用 depset 构造函数添加不同的排序参数，理论上您可以更改链接的顺序。例如 这个

 * <p>LINK_ORDER: a variation of left-to-right preorder that enforces topological sorting. In
 * Starlark it is called "topological"; its older deprecated name is "link".
 *
 * <p>For example, for the nested set {A, C, {B, D}}, the iteration order is "A C B D"
 * (parent-first).
 *
 * <p>This type of set would typically be used for artifacts where elements of nested sets go after
 * the direct members of a set, for example when providing a list of libraries to the C++ compiler.
 *
 * <p>The custom ordering has the property that elements of nested sets always come before elements
 * of descendant nested sets. Left-to-right order is preserved if possible, both for items and for
 * references to nested sets.
 *
 * <p>The left-to-right pre-order-like ordering is implemented by running a right-to-left postorder
 * traversal and then reversing the result.

会变成这样；

        libraries = depset(direct = [
            cc_common.create_library_to_link(
                actions = ctx.actions,
                feature_configuration = feature_configuration,
                cc_toolchain = cc_toolchain,
                static_library = output_file,
            ),
        ]),