CPU调度程序如何工作？#

NumPy 调度程序基于多源编译，这意味着采用特定源并使用不同的编译器标志以及影响代码路径的不同C定义对其进行多次编译。这根据所需的优化为每个编译对象启用了某些指令集，并最终将返回的对象链接在一起。

这种机制应该支持所有编译器，并且不需要任何特定于编译器的扩展，但同时它向正常编译添加了一些步骤，解释如下。

1-配置#

在开始通过两个命令参数构建源文件之前，用户配置所需的优化，如上所述：

--cpu-baseline：所需优化的最小集合。
--cpu-dispatch：调度了一组额外的优化。

2- 发现环境#

在这一部分中，我们检查编译器和平台架构并缓存一些中间结果以加快重建速度。

3-验证请求的优化#

通过针对编译器测试它们，并查看编译器可以根据请求的优化支持什么。

4-生成主配置头#

生成的标头_cpu_dispatch.h包含在上一步中已验证的所需优化的指令集的所有定义和标头。

它还包含额外的 C 定义，用于定义 NumPy 的 Python 级模块属性__cpu_baseline__和__cpu_dispatch__.

这个标题里有什么？

示例标头是由 X86 计算机上的 gcc 动态生成的。编译器支持和，结果如下。--cpu-baseline="sse sse2 sse3"--cpu-dispatch="ssse3 sse41"

// The header should be located at numpy/numpy/core/src/common/_cpu_dispatch.h
/**NOTE
 ** C definitions prefixed with "NPY_HAVE_" represent
 ** the required optimizations.
 **
 ** C definitions prefixed with 'NPY__CPU_TARGET_' are protected and
 ** shouldn't be used by any NumPy C sources.
 */
/******* baseline features *******/
/** SSE **/
#define NPY_HAVE_SSE 1
#include <xmmintrin.h>
/** SSE2 **/
#define NPY_HAVE_SSE2 1
#include <emmintrin.h>
/** SSE3 **/
#define NPY_HAVE_SSE3 1
#include <pmmintrin.h>

/******* dispatch-able features *******/
#ifdef NPY__CPU_TARGET_SSSE3
  /** SSSE3 **/
  #define NPY_HAVE_SSSE3 1
  #include <tmmintrin.h>
#endif
#ifdef NPY__CPU_TARGET_SSE41
  /** SSE41 **/
  #define NPY_HAVE_SSE41 1
  #include <smmintrin.h>
#endif

基线功能是通过配置的最小所需优化集--cpu-baseline。它们没有预处理器保护并且始终处于打开状态，这意味着它们可以在任何源中使用。

这是否意味着 NumPy 的基础结构将编译器的基线功能标志传递给所有源？

绝对是的。但可调度源的处理方式有所不同。

如果用户在构建期间指定了某些基线功能，但在运行时机器甚至不支持这些功能怎么办？编译后的代码是否会通过这些定义之一调用，或者编译器本身是否会根据提供的命令行编译器标志自动生成/向量化某些代码？

在加载 NumPy 模块期间，有一个验证步骤可以检测此行为。它将引发 Python 运行时错误来通知用户。这是为了防止 CPU 出现非法指令错误，从而导致段错误。

可调度的功能是我们通过配置的调度的一组附加优化--cpu-dispatch。默认情况下它们不会激活，并且始终受到其他前缀为的 C 定义的保护 NPY__CPU_TARGET_。 C 定义仅在可分派源NPY__CPU_TARGET_中启用。

5-可调度源和配置语句#

可分派源是特殊的C文件，可以使用不同的编译器标志以及不同的C 定义多次编译。这些会影响代码路径，以根据“配置语句”为每个编译对象启用某些指令集，这些语句必须在C 注释之间声明，并在每个可分派源顶部(/**/)以特殊标记@targets开头。同时，如果通过命令参数禁用优化，则可调度源将被视为普通C--disable-optimization源。

什么是配置语句？

配置语句是组合在一起的关键字，以确定可分派源所需的优化。

例子：

/*@targets avx2 avx512f vsx2 vsx3 asimd asimdhp */
// C code

关键字主要代表通过配置的额外优化--cpu-dispatch，但也可以代表其他选项，例如：

目标组：预配置的配置语句，用于从可分派源外部管理所需的优化。
策略：用于更改默认行为或强制编译器执行某些操作的选项集合。
“baseline”：一个唯一的关键字，表示通过配置的最小优化--cpu-baseline

Numpy 的基础设施通过四个步骤处理可调度源：

(A) 识别：就像源模板和 F2PY 一样，可分派源需要特殊的扩展名*.dispatch.c 来标记 C 可分派源文件，并且对于 C++ *.dispatch.cpp或*.dispatch.cxx 注意：尚不支持 C++。
(B) 解析和验证：在这一步中，通过配置语句对上一步过滤的可调度源进行逐一解析和验证，以确定所需的优化。

(C) 包装：这是 NumPy 基础设施所采用的方法，事实证明，该方法足够灵活，可以使用影响代码路径的不同C定义和标志多次编译单个源。该过程是通过为与附加优化相关的每个所需优化创建临时C源代码来实现的，其中包含C定义的声明并通过C指令#include包含所涉及的源代码。如需更多说明，请查看以下 AVX512F 代码：

/*
 * this definition is used by NumPy utilities as suffixes for the
 * exported symbols
 */
#define NPY__CPU_TARGET_CURRENT AVX512F
/*
 * The following definitions enable
 * definitions of the dispatch-able features that are defined within the main
 * configuration header. These are definitions for the implied features.
 */
#define NPY__CPU_TARGET_SSE
#define NPY__CPU_TARGET_SSE2
#define NPY__CPU_TARGET_SSE3
#define NPY__CPU_TARGET_SSSE3
#define NPY__CPU_TARGET_SSE41
#define NPY__CPU_TARGET_POPCNT
#define NPY__CPU_TARGET_SSE42
#define NPY__CPU_TARGET_AVX
#define NPY__CPU_TARGET_F16C
#define NPY__CPU_TARGET_FMA3
#define NPY__CPU_TARGET_AVX2
#define NPY__CPU_TARGET_AVX512F
// our dispatch-able source
#include "/the/absuolate/path/of/hello.dispatch.c"

(D) 可调度配置标头：基础设施为每个可调度源生成一个配置标头，该标头主要包含两个抽象C宏，用于标识生成的对象，因此它们可用于运行时从生成的对象中调度某些符号任何C源的对象。它也用于前向声明。

生成的标头在排除扩展名后采用可调度源的名称并将其替换为.h，例如假设我们有一个名为的可调度源hello.dispatch.c并包含以下内容：

// hello.dispatch.c
/*@targets baseline sse42 avx512f */
#include <stdio.h>
#include "numpy/utils.h" // NPY_CAT, NPY_TOSTR

#ifndef NPY__CPU_TARGET_CURRENT
  // wrapping the dispatch-able source only happens to the additional optimizations
  // but if the keyword 'baseline' provided within the configuration statements,
  // the infrastructure will add extra compiling for the dispatch-able source by
  // passing it as-is to the compiler without any changes.
  #define CURRENT_TARGET(X) X
  #define NPY__CPU_TARGET_CURRENT baseline // for printing only
#else
  // since we reach to this point, that's mean we're dealing with
    // the additional optimizations, so it could be SSE42 or AVX512F
  #define CURRENT_TARGET(X) NPY_CAT(NPY_CAT(X, _), NPY__CPU_TARGET_CURRENT)
#endif
// Macro 'CURRENT_TARGET' adding the current target as suffux to the exported symbols,
// to avoid linking duplications, NumPy already has a macro called
// 'NPY_CPU_DISPATCH_CURFX' similar to it, located at
// numpy/numpy/core/src/common/npy_cpu_dispatch.h
// NOTE: we tend to not adding suffixes to the baseline exported symbols
void CURRENT_TARGET(simd_whoami)(const char *extra_info)
{
    printf("I'm " NPY_TOSTR(NPY__CPU_TARGET_CURRENT) ", %s\n", extra_info);
}

现在假设您将hello.dispatch.c附加到源树，那么基础设施应该生成一个名为 hello.dispatch.h的临时配置头，源树中的任何源都可以访问它，并且它应该包含以下代码：

#ifndef NPY__CPU_DISPATCH_EXPAND_
  // To expand the macro calls in this header
    #define NPY__CPU_DISPATCH_EXPAND_(X) X
#endif
// Undefining the following macros, due to the possibility of including config headers
// multiple times within the same source and since each config header represents
// different required optimizations according to the specified configuration
// statements in the dispatch-able source that derived from it.
#undef NPY__CPU_DISPATCH_BASELINE_CALL
#undef NPY__CPU_DISPATCH_CALL
// nothing strange here, just a normal preprocessor callback
// enabled only if 'baseline' specified within the configuration statements
#define NPY__CPU_DISPATCH_BASELINE_CALL(CB, ...) \
  NPY__CPU_DISPATCH_EXPAND_(CB(__VA_ARGS__))
// 'NPY__CPU_DISPATCH_CALL' is an abstract macro is used for dispatching
// the required optimizations that specified within the configuration statements.
//
// @param CHK, Expected a macro that can be used to detect CPU features
// in runtime, which takes a CPU feature name without string quotes and
// returns the testing result in a shape of boolean value.
// NumPy already has macro called "NPY_CPU_HAVE", which fits this requirement.
//
// @param CB, a callback macro that expected to be called multiple times depending
// on the required optimizations, the callback should receive the following arguments:
//  1- The pending calls of @param CHK filled up with the required CPU features,
//     that need to be tested first in runtime before executing call belong to
//     the compiled object.
//  2- The required optimization name, same as in 'NPY__CPU_TARGET_CURRENT'
//  3- Extra arguments in the macro itself
//
// By default the callback calls are sorted depending on the highest interest
// unless the policy "$keep_sort" was in place within the configuration statements
// see "Dive into the CPU dispatcher" for more clarification.
#define NPY__CPU_DISPATCH_CALL(CHK, CB, ...) \
  NPY__CPU_DISPATCH_EXPAND_(CB((CHK(AVX512F)), AVX512F, __VA_ARGS__)) \
  NPY__CPU_DISPATCH_EXPAND_(CB((CHK(SSE)&&CHK(SSE2)&&CHK(SSE3)&&CHK(SSSE3)&&CHK(SSE41)), SSE41, __VA_ARGS__))

根据上述内容使用配置标头的示例：

// NOTE: The following macros are only defined for demonstration purposes only.
// NumPy already has a collections of macros located at
// numpy/numpy/core/src/common/npy_cpu_dispatch.h, that covers all dispatching
// and declarations scenarios.

#include "numpy/npy_cpu_features.h" // NPY_CPU_HAVE
#include "numpy/utils.h" // NPY_CAT, NPY_EXPAND

// An example for setting a macro that calls all the exported symbols at once
// after checking if they're supported by the running machine.
#define DISPATCH_CALL_ALL(FN, ARGS) \
    NPY__CPU_DISPATCH_CALL(NPY_CPU_HAVE, DISPATCH_CALL_ALL_CB, FN, ARGS) \
    NPY__CPU_DISPATCH_BASELINE_CALL(DISPATCH_CALL_BASELINE_ALL_CB, FN, ARGS)
// The preprocessor callbacks.
// The same suffixes as we define it in the dispatch-able source.
#define DISPATCH_CALL_ALL_CB(CHECK, TARGET_NAME, FN, ARGS) \
  if (CHECK) { NPY_CAT(NPY_CAT(FN, _), TARGET_NAME) ARGS; }
#define DISPATCH_CALL_BASELINE_ALL_CB(FN, ARGS) \
  FN NPY_EXPAND(ARGS);

// An example for setting a macro that calls the exported symbols of highest
// interest optimization, after checking if they're supported by the running machine.
#define DISPATCH_CALL_HIGH(FN, ARGS) \
  if (0) {} \
    NPY__CPU_DISPATCH_CALL(NPY_CPU_HAVE, DISPATCH_CALL_HIGH_CB, FN, ARGS) \
    NPY__CPU_DISPATCH_BASELINE_CALL(DISPATCH_CALL_BASELINE_HIGH_CB, FN, ARGS)
// The preprocessor callbacks
// The same suffixes as we define it in the dispatch-able source.
#define DISPATCH_CALL_HIGH_CB(CHECK, TARGET_NAME, FN, ARGS) \
  else if (CHECK) { NPY_CAT(NPY_CAT(FN, _), TARGET_NAME) ARGS; }
#define DISPATCH_CALL_BASELINE_HIGH_CB(FN, ARGS) \
  else { FN NPY_EXPAND(ARGS); }

// NumPy has a macro called 'NPY_CPU_DISPATCH_DECLARE' can be used
// for forward declarations any kind of prototypes based on
// 'NPY__CPU_DISPATCH_CALL' and 'NPY__CPU_DISPATCH_BASELINE_CALL'.
// However in this example, we just handle it manually.
void simd_whoami(const char *extra_info);
void simd_whoami_AVX512F(const char *extra_info);
void simd_whoami_SSE41(const char *extra_info);

void trigger_me(void)
{
    // bring the auto-generated config header
    // which contains config macros 'NPY__CPU_DISPATCH_CALL' and
    // 'NPY__CPU_DISPATCH_BASELINE_CALL'.
    // it is highly recommended to include the config header before executing
  // the dispatching macros in case if there's another header in the scope.
    #include "hello.dispatch.h"
    DISPATCH_CALL_ALL(simd_whoami, ("all"))
    DISPATCH_CALL_HIGH(simd_whoami, ("the highest interest"))
    // An example of including multiple config headers in the same source
    // #include "hello2.dispatch.h"
    // DISPATCH_CALL_HIGH(another_function, ("the highest interest"))
}