GCC Builtin

#include <stdio.h>
#define likely(x) __builtin_expect(!!(x), 1)
#define unlikely(x) __builtin_expect(!!(x), 0)

int foo(int x) {
    if (unlikely(x > 0)) {
        return x + 1;
    } else {
        return -x + 1;
    }
}

int main(int argc, char *argv[]) {
    int ret = 0;
    for (int i = 0; i < 1000; i++) {
        ret += foo(argc);
    }
    printf("%d\n", ret);
    return 0;
}

int foo(int x) {
    if (x > 0) {
        return x + 1;
    } else {
        return -x + 1;
    }
}

rtx expand_builtin(
    tree exp, rtx target, rtx subtarget, machine_mode mode, int ignore) {
    /* ... */
    case BUILT_IN_DUMMY:
        target = expand_builtin_dummy(exp, target, target_mode);
        if (target) return target;
        break;
    /* ... */
}

static rtx expand_builtin_dummy(
    tree exp, rtx target, machine_mode target_mode) {
    rtx op0 = expand_normal(CALL_EXPR_ARG(exp, 0));
    return expand_unop(GET_MODE(op0), dummy_optab, op0, target, 0);
}

enum riscv_function_type {
#define DEF_RISCV_FTYPE(NARGS, LIST) RISCV_FTYPE_NAME##NARGS LIST,
#include "config/riscv/riscv-ftypes-v.def"
#undef DEF_RISCV_FTYPE
  RISCV_MAX_FTYPE_MAX
};

/* config/riscv/riscv-ftypes-v.def */
DEF_RISCV_FTYPE (1, (SI, SI))
DEF_RISCV_FTYPE (1, (UV2HI, UV2HI))
DEF_RISCV_FTYPE (2, (DI, DI, DI))
DEF_RISCV_FTYPE (2, (V4HI, V4QI, V4QI))
/* ... */

/* riscv-builtins.c */
#define RISCV_FTYPE_NAME1(A, B) RISCV_##A##_FTYPE_##B

/* 以 DEF_RISCV_FTYPE (1, (SI, SI)) 为例, 它会展开为:
 * RISCV_SI_FTYPE_SI, 表示该函数原型是 int x(int) */

static tree riscv_build_function_type(enum riscv_function_type type) {
    static tree types[(int)RISCV_MAX_FTYPE_MAX];

    if (types[(int)type] == NULL_TREE) switch (type) {
#define DEF_RISCV_FTYPE(NUM, ARGS)                                             \
    case RISCV_FTYPE_NAME##NUM ARGS:                                           \
        types[(int)type] =                                                     \
            build_function_type_list(RISCV_FTYPE_ATYPES##NUM ARGS, NULL_TREE); \
        break;
#include "config/riscv/riscv-ftypes-p.def"
#undef DEF_RISCV_FTYPE
            default:
                gcc_unreachable();
        }

    return types[(int)type];
}

// 以 DEF_RISCV_FTYPE (1, (SI, SI)) 为例, RISCV_FTYPE_NAME##... 展开后为:
case RISCV_SI_FTYPE_SI:
  types[RISCV_SI_FTYPE_SI] = build_function_type_list(RISCV_ATYPE_SI, RISCV_ATYPE_SI, NULL_TREE);

// RISCV_ATYPE_SI 开始与 frontend 有关联, 因为 intSI_type_node 是 frontend 代表 int 的 tree:
#define RISCV_ATYPE_SI intSI_type_node

// build_function_type_list 用来生成对应 int x(int) 的 tree. 这个函数在 frontend
// 解析源码时也会调用, 从而导致 frontend 解析 int x(int) 时可以找到之前
// riscv_build_function_type 时生成的 tree
build_function_type_list(tree return_type, ...);
  return build_function_type_list_1 (false, return_type, p);
    return build_function_type (return_type, args);
      // 生成一个 function type tree node, 其中 FUNCTION_TYPE 定义在 tree.def 中
      t = make_node (FUNCTION_TYPE);
      TREE_TYPE (t) = value_type;
      TYPE_ARG_TYPES (t) = arg_types;
      /* If we already have such a type, use the old one.  */
      hashval_t hash = type_hash_canon_hash (t);
      t = type_hash_canon (hash, t);
      // ...
      return t

void riscv_init_builtins ():
 for (size_t i = 0; i < ARRAY_SIZE (riscv_builtins); i++) {
      const struct riscv_builtin_description *d = &riscv_builtins[i];
      if (d->avail ()):
        // 前面提到的 riscv_build_function_type
        tree type = riscv_build_function_type (d->prototype);
        size_t fcode = i;
        // NOTE: add_builtin_function 是 backend 与 frontend 关联最重要的一步:它
        // 定义了 int hello(int) 函数对应的 fndecl 是一个 builtin, 并把 fndecl
        // 通过 pushdecl 添加到 name lookup
        riscv_builtin_decls[i]
              = add_builtin_function (d->name, type, fcode, BUILT_IN_MD, NULL, NULL);
        riscv_builtin_decl_index[d->icode] = i;

static const struct riscv_builtin_description riscv_builtins[] = {
  #include "config/riscv/riscv-builtins-v.def"
};

/* config/riscv/riscv-builtins-v.def */
DIRECT_BUILTIN (ave_si, RISCV_SI_FTYPE_SI_SI, dsp32),
/* ... */

/* DIRECT_BUILTIN 展开后为: */
{ CODE_FOR_riscv_ave_si, "__builtin_riscv_ave_si", RISCV_BUILTIN_DIRECT, RISCV_SI_FTYPE_SI_SI, riscv_builtin_avail_dsp32}
/*|d->icode              |d->name                                        |d->prototype         |d->avail()   */

/* CODE_FOR_riscv_ave_si 直接对应着 md 中 insn 的定义, 这个 enum 由 genopinit 生成 */
(define_insn "riscv_ave_<mode>"
  [(set (match_operand:GPR 0 "register_operand" "=r")
     (unspec:GPR
      [(match_operand:GPR 1 "register_operand" "r")
       (match_operand:GPR 2 "register_operand" "r")]
       UNSPEC_AVE))]
  "TARGET_XTHEAD_DSP"
  "ave\\t%0,%1,%2"
)

/* expr.c */
expand_expr_real_1
  // NOTE: fndecl 对应前面 add_builtin_function 生成的 tree, 其中
  // fndecl_built_in_p 是 DECL_BUILT_IN_CLASS (node) != NOT_BUILT_IN, 参考
  // add_builtin_function 时使用的 BUILT_IN_MD 参数
  if (fndecl && fndecl_built_in_p (fndecl)):
    return expand_builtin (exp, target, subtarget, tmode, ignore);

rtx riscv_expand_builtin (tree exp, rtx target, rtx subtarget ATTRIBUTE_UNUSED,
              machine_mode mode ATTRIBUTE_UNUSED,
              int ignore ATTRIBUTE_UNUSED):
  tree name_id = DECL_NAME (fndecl);
  size_t i;

  unsigned int fcode = DECL_FUNCTION_CODE (fndecl);
  /* NOTE: fcode 参考前面 add_builtin_function 时的 fcode 参数  */
  d = &riscv_builtins[fcode];

  switch (d->builtin_type):
    case RISCV_BUILTIN_DIRECT:
      // d->icode 即是 CODE_FOR_xxx, 用它可以和 md 关联
      return riscv_expand_builtin_direct (d->icode, target, exp, true);