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dsp1: Use clz CPU instruction when normalising fixed-point numbers

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Use the gcc built-in function, making the operation constant-time and
branch-free.
This commit is contained in:
jSTE0 2022-02-22 22:45:14 +00:00
parent be1a37dcbe
commit 397e5eb4fb
1 changed files with 39 additions and 2 deletions
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41
src/dsp.c
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@ -456,11 +456,19 @@ static void DSP1_Inverse (int16 Coefficient, int16 Exponent, int16 *iCoefficient
}
/* Step Three: Normalize*/
#ifdef __GNUC__
{
const int shift = __builtin_clz(Coefficient) - (8 * sizeof(int) - 15);
Coefficient <<= shift;
Exponent -= shift;
}
#else
while (Coefficient < 0x4000)
{
Coefficient <<= 1;
Exponent--;
}
#endif
/* Step Four: Special Case*/
if (Coefficient == 0x4000)
@ -537,9 +545,18 @@ static void DSP1_Normalize (int16 m, int16 *Coefficient, int16 *Exponent)
{
int16 i, e;
i = 0x4000;
e = 0;
#ifdef __GNUC__
i = m < 0 ? ~m : m;
if (i == 0)
e = 15;
else
e = __builtin_clz(i) - (8 * sizeof(int) - 15);
#else
i = 0x4000;
if (m < 0)
{
while ((m & i) && i)
@ -556,6 +573,7 @@ static void DSP1_Normalize (int16 m, int16 *Coefficient, int16 *Exponent)
e++;
}
}
#endif
if (e > 0)
*Coefficient = m * DSP1ROM[0x21 + e] << 1;
@ -571,9 +589,18 @@ static void DSP1_NormalizeDouble (int32 Product, int16 *Coefficient, int16 *Expo
n = Product & 0x7fff;
m = Product >> 15;
i = 0x4000;
e = 0;
#ifdef __GNUC__
i = m < 0 ? ~m : m;
if (i == 0)
e = 15;
else
e = __builtin_clz(i) - (8 * sizeof(int) - 15);
#else
i = 0x4000;
if (m < 0)
{
while ((m & i) && i)
@ -590,6 +617,7 @@ static void DSP1_NormalizeDouble (int32 Product, int16 *Coefficient, int16 *Expo
e++;
}
}
#endif
if (e > 0)
{
@ -599,6 +627,14 @@ static void DSP1_NormalizeDouble (int32 Product, int16 *Coefficient, int16 *Expo
*Coefficient += n * DSP1ROM[0x0040 - e] >> 15;
else
{
#ifdef __GNUC__
i = m < 0 ? ~(n | 0x8000) : n;
if (i == 0)
e += 15;
else
e += __builtin_clz(i) - (8 * sizeof(int) - 15);
#else
i = 0x4000;
if (m < 0)
@ -617,6 +653,7 @@ static void DSP1_NormalizeDouble (int32 Product, int16 *Coefficient, int16 *Expo
e++;
}
}
#endif
if (e > 15)
*Coefficient = n * DSP1ROM[0x0012 + e] << 1;