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LibSoftGPU: Add log2_approximate()

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This commit is contained in:
Stephan Unverwerth 2022-02-19 02:15:59 +01:00 • committed by Linus Groh
parent be15cf5457
commit dd900570fd
1 changed files with 21 additions and 0 deletions
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21
Userland/Libraries/LibSoftGPU/SIMD.h
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@ -103,4 +103,25 @@ ALWAYS_INLINE static Vector2<AK::SIMD::f32x4> ddy(Vector2<AK::SIMD::f32x4> const
};
}
// Calculates a quadratic approximation of log2, exploiting the fact that IEEE754 floats are represented as mantissa * 2^exponent.
// See https://stackoverflow.com/questions/9411823/fast-log2float-x-implementation-c
ALWAYS_INLINE static AK::SIMD::f32x4 log2_approximate(AK::SIMD::f32x4 v)
{
union {
AK::SIMD::f32x4 float_val;
AK::SIMD::i32x4 int_val;
} u { v };
// Extract just the exponent minus 1, giving a lower integral bound for log2.
auto log = AK::SIMD::to_f32x4(((u.int_val >> 23) & 255) - 128);
// Replace the exponent with 0, giving a value between 1 and 2.
u.int_val &= ~(255 << 23);
u.int_val |= 127 << 23;
// Approximate log2 by adding a quadratic function of u to the integral part.
log += (-0.34484843f * u.float_val + 2.02466578f) * u.float_val - 0.67487759f;
return log;
}
}