c++ - 如何在不使用任何 SSE 指令的情况下设置 __m128i？

Question

我有很多函数使用相同的常量 __m128i 值。例如:

const __m128i K8 = _mm_setr_epi8(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16);
const __m128i K16 = _mm_setr_epi16(1, 2, 3, 4, 5, 6, 7, 8);
const __m128i K32 = _mm_setr_epi32(1, 2, 3, 4);

所以我想将所有这些常量存储在一个地方。但是有一个问题:我在运行时检查现有的 CPU 扩展。如果 CPU 不支持例如 SSE(或 AVX)，那么在常量初始化期间程序将崩溃。

那么是否可以在不使用 SSE 的情况下初始化这些常量？

Answer 1

不使用 SSE 指令初始化 __m128i vector 是可能的，但这取决于编译器如何定义 __m128i。

对于 Microsoft Visual Studio，您可以定义下一个宏(它将 __m128i 定义为 char[16]):

template <class T> inline char GetChar(T value, size_t index)
{
    return ((char*)&value)[index];
}

#define AS_CHAR(a) char(a)

#define AS_2CHARS(a) \
    GetChar(int16_t(a), 0), GetChar(int16_t(a), 1)

#define AS_4CHARS(a) \
    GetChar(int32_t(a), 0), GetChar(int32_t(a), 1), \
    GetChar(int32_t(a), 2), GetChar(int32_t(a), 3)

#define _MM_SETR_EPI8(a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, aa, ab, ac, ad, ae, af) \
    {AS_CHAR(a0), AS_CHAR(a1), AS_CHAR(a2), AS_CHAR(a3), \
     AS_CHAR(a4), AS_CHAR(a5), AS_CHAR(a6), AS_CHAR(a7), \
     AS_CHAR(a8), AS_CHAR(a9), AS_CHAR(aa), AS_CHAR(ab), \
     AS_CHAR(ac), AS_CHAR(ad), AS_CHAR(ae), AS_CHAR(af)}

#define _MM_SETR_EPI16(a0, a1, a2, a3, a4, a5, a6, a7) \
    {AS_2CHARS(a0), AS_2CHARS(a1), AS_2CHARS(a2), AS_2CHARS(a3), \
     AS_2CHARS(a4), AS_2CHARS(a5), AS_2CHARS(a6), AS_2CHARS(a7)}

#define _MM_SETR_EPI32(a0, a1, a2, a3) \
    {AS_4CHARS(a0), AS_4CHARS(a1), AS_4CHARS(a2), AS_4CHARS(a3)}       

对于 GCC 它将是(它将 __m128i 定义为 long long[2]):

#define CHAR_AS_LONGLONG(a) (((long long)a) & 0xFF)

#define SHORT_AS_LONGLONG(a) (((long long)a) & 0xFFFF)

#define INT_AS_LONGLONG(a) (((long long)a) & 0xFFFFFFFF)

#define LL_SETR_EPI8(a, b, c, d, e, f, g, h) \
    CHAR_AS_LONGLONG(a) | (CHAR_AS_LONGLONG(b) << 8) | \
    (CHAR_AS_LONGLONG(c) << 16) | (CHAR_AS_LONGLONG(d) << 24) | \
    (CHAR_AS_LONGLONG(e) << 32) | (CHAR_AS_LONGLONG(f) << 40) | \
    (CHAR_AS_LONGLONG(g) << 48) | (CHAR_AS_LONGLONG(h) << 56)

#define LL_SETR_EPI16(a, b, c, d) \
    SHORT_AS_LONGLONG(a) | (SHORT_AS_LONGLONG(b) << 16) | \
    (SHORT_AS_LONGLONG(c) << 32) | (SHORT_AS_LONGLONG(d) << 48)

#define LL_SETR_EPI32(a, b) \
    INT_AS_LONGLONG(a) | (INT_AS_LONGLONG(b) << 32)        

#define _MM_SETR_EPI8(a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, aa, ab, ac, ad, ae, af) \
    {LL_SETR_EPI8(a0, a1, a2, a3, a4, a5, a6, a7), LL_SETR_EPI8(a8, a9, aa, ab, ac, ad, ae, af)}

#define _MM_SETR_EPI16(a0, a1, a2, a3, a4, a5, a6, a7) \
    {LL_SETR_EPI16(a0, a1, a2, a3), LL_SETR_EPI16(a4, a5, a6, a7)}

#define _MM_SETR_EPI32(a0, a1, a2, a3) \
    {LL_SETR_EPI32(a0, a1), LL_SETR_EPI32(a2, a3)}        

因此在您的代码中，__m128i 常量的初始化将如下所示:

const __m128i K8 = _MM_SETR_EPI8(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16);
const __m128i K16 = _MM_SETR_EPI16(1, 2, 3, 4, 5, 6, 7, 8);
const __m128i K32 = _MM_SETR_EPI32(1, 2, 3, 4);