如何在 Neon 内在函数中使用 if 条件进行并行操作？

Question

我问了一个关于 vclt_s8 比较的问题。 Does anybody know how to use Neon intrinsics uint8x8_t vclt_s8 (int8x8_t, int8x8_t)

但是，如果我们有这样的代码：

if(a > b + c) {
    a = b + c;
} else if(a < b - c) {
    a = b - c;
}

如何将其转换为 Neon 内在函数？在这种情况下，我们似乎无法进行 8 个算子并行操作。不是吗？

Answer 1

显然你不能使用 SIMD 进行分支，所以你必须看看如何使用掩码以无分支的方式实现这种逻辑。我只是给出伪代码，所以你会明白一般的想法 - 编码应该相当简单：

bc = b + c       ; get `(b + c)` in a vector register
mask = a > bc    ; use compare instruction to generate mask (-1 = true, 0 = false)
bc = bc & mask   ; use bitwise AND to zero out elements of `(b + c)` which we do not want
a = a & ~mask    ; use bitwise ANDC to zero out elements of `a` which we do not want
a = a | bc       ; combine required elements into `a` using bitwise OR

bc = b - c       ; get `(b - c)` in a vector register
mask = a < bc    ; use compare instruction to generate mask (-1 = true, 0 = false)
bc = bc & mask   ; use bitwise AND to zero out elements of `(b - c)` which we do not want
a = a & ~mask    ; use bitwise ANDC to zero out elements of `a` which we do not want
a = a | bc       ; combine required elements into `a` using bitwise OR

请注意，我在这里作弊并从您的标量代码中省略了else（假设这两个分支是互斥的）所以我实现的实际上相当于：

if (a > b + c) {
    a = b + c;
}
if (a < b - c) {
    a = b - c;
}

如果这是一个错误的假设，那么您将需要执行一些额外的按位运算来实现逻辑 else。