SSE 内在函数导致正常浮点运算返回 -1.#INV
我在编写执行音频处理的 SSE 方法时遇到问题。我在这里基于英特尔的论文实现了 SSE 随机函数：
我也有一个方法就是也使用 SSE 执行从 Float 到 S16 的转换，转换执行起来非常简单，如下所示：
unsigned int Float_S16LE(float *data, const unsigned int samples, uint8_t *dest)
{
int16_t *dst = (int16_t*)dest;
const __m128 mul = _mm_set_ps1((float)INT16_MAX);
__m128 rand;
const uint32_t even = count & ~0x3;
for(uint32_t i = 0; i < even; i += 4, data += 4, dst += 4)
{
/* random round to dither */
FloatRand4(-0.5f, 0.5f, NULL, &rand);
__m128 rmul = _mm_add_ps(mul, rand);
__m128 in = _mm_mul_ps(_mm_load_ps(data),rmul);
__m64 con = _mm_cvtps_pi16(in);
memcpy(dst, &con, sizeof(int16_t) * 4);
}
}
FloatRand4 定义如下：
static inline void FloatRand4(const float min, const float max, float result[4], __m128 *sseresult = NULL)
{
const float delta = (max - min) / 2.0f;
const float factor = delta / (float)INT32_MAX;
...
}
如果 sseresult != NULL 则返回 __m128 结果，并且结果未使用。 这在第一个循环中表现完美，但在下一个循环中，delta 变为 -1.#INF 而不是 1.0。如果我注释掉行 __m64 con = _mm_cvtps_pi16(in); 问题就会消失。
我认为 FPU 正在进入未知状态或其他状态。
I am having a problem with a SSE method I am writing that performs audio processing. I have implemented a SSE random function based on Intel's paper here:
I also have a method that is performing conversions from Float to S16 using SSE also, the conversion is performed quite simply as follows:
unsigned int Float_S16LE(float *data, const unsigned int samples, uint8_t *dest)
{
int16_t *dst = (int16_t*)dest;
const __m128 mul = _mm_set_ps1((float)INT16_MAX);
__m128 rand;
const uint32_t even = count & ~0x3;
for(uint32_t i = 0; i < even; i += 4, data += 4, dst += 4)
{
/* random round to dither */
FloatRand4(-0.5f, 0.5f, NULL, &rand);
__m128 rmul = _mm_add_ps(mul, rand);
__m128 in = _mm_mul_ps(_mm_load_ps(data),rmul);
__m64 con = _mm_cvtps_pi16(in);
memcpy(dst, &con, sizeof(int16_t) * 4);
}
}
FloatRand4 is defined as follows:
static inline void FloatRand4(const float min, const float max, float result[4], __m128 *sseresult = NULL)
{
const float delta = (max - min) / 2.0f;
const float factor = delta / (float)INT32_MAX;
...
}
If sseresult != NULL the __m128 result is returned and result is unused.
This performs perfectly on the first loop, but on the next loop delta becomes -1.#INF instead of 1.0. If I comment out the line __m64 con = _mm_cvtps_pi16(in); the problem goes away.
I think that the FPU is getting into an unknown state or something.
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混合 SSE 整数算术和（常规）浮点算术。可能会产生奇怪的结果，因为两者都在相同的寄存器上操作。如果使用：
FPU 重置为正确状态。 Microsoft 提供了有关何时使用 EMMS 的指南
Mixing SSE Integer arithmetic and (regular) Floating point math. Can produce weird results because both are operating on the same registers. If you use:
the FPU is reset into a correct state. Microsoft has Guidelines for When to Use EMMS
如果不可能对 __m64 进行未对齐的流或存储，我要么将其保留在 _m128i 中，然后使用 _mm_maskmoveu_si128 进行屏蔽写入，要么手动存储这 8 个字节。
http://msdn.microsoft.com/en-us/library/bytwczae.aspx
If it's impossible to do an unaligned stream or store of an __m64, I'd either keep it inside an _m128i and do a masked write with _mm_maskmoveu_si128 or store those 8 bytes by hand.
http://msdn.microsoft.com/en-us/library/bytwczae.aspx