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Fix ldexp implementations.

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The previous implementations produced garbage values if the exponent did
not fit within the exponent bits.  See #2131 for a complete discussion,
and !375 for other possible implementations.

Here we implement the 4-factor version. See `pldexp_impl` in
`GenericPacketMathFunctions.h` for a full description.

The SSE `pcmp*` methods were moved down since `pcmp_le<Packet4i>`
requires `por`.

Left as a "TODO" is to delegate to a faster version if we know the
exponent does fit within the exponent bits.

Fixes #2131.
This commit is contained in:
Antonio Sanchez
2020-10-12 12:24:08 +01:00
committed by Rasmus Munk Larsen
parent 7eb07da538
commit 4cb563a01e
8 changed files with 354 additions and 112 deletions
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44
Eigen/src/Core/arch/AVX/PacketMath.h
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@@ -273,6 +273,15 @@ template<> EIGEN_STRONG_INLINE Packet8i padd<Packet8i>(const Packet8i& a, const
template<> EIGEN_STRONG_INLINE Packet8f psub<Packet8f>(const Packet8f& a, const Packet8f& b) { return _mm256_sub_ps(a,b); }
template<> EIGEN_STRONG_INLINE Packet4d psub<Packet4d>(const Packet4d& a, const Packet4d& b) { return _mm256_sub_pd(a,b); }
template<> EIGEN_STRONG_INLINE Packet8i psub<Packet8i>(const Packet8i& a, const Packet8i& b) {
#ifdef EIGEN_VECTORIZE_AVX2
return _mm256_sub_epi32(a,b);
#else
__m128i lo = _mm_sub_epi32(_mm256_extractf128_si256(a, 0), _mm256_extractf128_si256(b, 0));
__m128i hi = _mm_sub_epi32(_mm256_extractf128_si256(a, 1), _mm256_extractf128_si256(b, 1));
return _mm256_insertf128_si256(_mm256_castsi128_si256(lo), (hi), 1);
#endif
}
template<> EIGEN_STRONG_INLINE Packet8f pnegate(const Packet8f& a)
{
@@ -379,6 +388,7 @@ template<> EIGEN_STRONG_INLINE Packet4d pmin<Packet4d>(const Packet4d& a, const
return _mm256_min_pd(b,a);
#endif
}
template<> EIGEN_STRONG_INLINE Packet8f pmax<Packet8f>(const Packet8f& a, const Packet8f& b) {
#if EIGEN_COMP_GNUC && EIGEN_COMP_GNUC < 63
// See pmin above
@@ -756,17 +766,29 @@ template<> EIGEN_STRONG_INLINE Packet8f pldexp<Packet8f>(const Packet8f& a, cons
}
template<> EIGEN_STRONG_INLINE Packet4d pldexp<Packet4d>(const Packet4d& a, const Packet4d& exponent) {
// Build e=2^n by constructing the exponents in a 128-bit vector and
// shifting them to where they belong in double-precision values.
Packet4i cst_1023 = pset1<Packet4i>(1023);
__m128i emm0 = _mm256_cvtpd_epi32(exponent);
emm0 = _mm_add_epi32(emm0, cst_1023);
emm0 = _mm_shuffle_epi32(emm0, _MM_SHUFFLE(3, 1, 2, 0));
__m128i lo = _mm_slli_epi64(emm0, 52);
__m128i hi = _mm_slli_epi64(_mm_srli_epi64(emm0, 32), 52);
__m256i e = _mm256_insertf128_si256(_mm256_setzero_si256(), lo, 0);
e = _mm256_insertf128_si256(e, hi, 1);
return pmul(a,_mm256_castsi256_pd(e));
// Clamp exponent to [-2099, 2099]
const Packet4d max_exponent = pset1<Packet4d>(2099.0);
const Packet4i e = _mm256_cvtpd_epi32(pmin(pmax(exponent, pnegate(max_exponent)), max_exponent));
// Split 2^e into four factors and multiply.
const Packet4i bias = pset1<Packet4i>(1023);
Packet4i b = parithmetic_shift_right<2>(e); // floor(e/4)
// 2^b
Packet4i hi = vec4i_swizzle1(padd(b, bias), 0, 2, 1, 3);
Packet4i lo = _mm_slli_epi64(hi, 52);
hi = _mm_slli_epi64(_mm_srli_epi64(hi, 32), 52);
Packet4d c = _mm256_castsi256_pd(_mm256_insertf128_si256(_mm256_castsi128_si256(lo), hi, 1));
Packet4d out = pmul(pmul(pmul(a, c), c), c); // a * 2^(3b)
// 2^(e - 3b)
b = psub(psub(psub(e, b), b), b); // e - 3b
hi = vec4i_swizzle1(padd(b, bias), 0, 2, 1, 3);
lo = _mm_slli_epi64(hi, 52);
hi = _mm_slli_epi64(_mm_srli_epi64(hi, 32), 52);
c = _mm256_castsi256_pd(_mm256_insertf128_si256(_mm256_castsi128_si256(lo), hi, 1));
out = pmul(out, c); // a * 2^e
return out;
}
template<> EIGEN_STRONG_INLINE float predux<Packet8f>(const Packet8f& a)