[libc-commits] [libc] [reland][libc] `FPRep` builders return `FPRep` instead of raw `StorageType` (PR #78978)
via libc-commits
libc-commits at lists.llvm.org
Mon Jan 22 06:15:15 PST 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libc
Author: Guillaume Chatelet (gchatelet)
<details>
<summary>Changes</summary>
Reland #<!-- -->78588
---
Patch is 46.67 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/78978.diff
2 Files Affected:
- (modified) libc/src/__support/FPUtil/FPBits.h (+313-282)
- (modified) libc/test/src/__support/FPUtil/fpbits_test.cpp (+87-84)
``````````diff
diff --git a/libc/src/__support/FPUtil/FPBits.h b/libc/src/__support/FPUtil/FPBits.h
index be700285de82854..2465158bb2cdfc7 100644
--- a/libc/src/__support/FPUtil/FPBits.h
+++ b/libc/src/__support/FPUtil/FPBits.h
@@ -64,38 +64,46 @@ LIBC_INLINE_VAR constexpr Sign Sign::POS = Sign(false);
// └─────────▲─────────┘
// │
// ┌─────────┴─────────┐
-// │ FPRepBase<FPType> │
+// │ FPStorage<FPType> │
// └─────────▲─────────┘
// │
// ┌────────────┴─────────────┐
// │ │
-// ┌────────┴──────┐ ┌─────────────┴──────────────┐
-// │ FPRep<FPType> │ │ FPRep<FPType::X86_Binary80 │
-// └────────▲──────┘ └─────────────▲──────────────┘
+// ┌────────┴─────────┐ ┌──────────────┴──────────────────┐
+// │ FPRepSem<FPType> │ │ FPRepSem<FPType::X86_Binary80 │
+// └────────▲─────────┘ └──────────────▲──────────────────┘
// │ │
// └────────────┬─────────────┘
// │
// ┌─────┴─────┐
+// │ FPRep<T> │
+// └───────────┘
+// │
+// ┌─────┴─────┐
// │ FPBits<T> │
// └───────────┘
//
-// - 'FPLayout' defines only a few constants, namely the 'StorageType' and the
-// length of the sign, the exponent and significand parts.
-// - 'FPRepBase' builds more constants on top of those from 'FPLayout' like
-// exponent bias, shifts and masks. It also defines tools to assemble or test
+// - 'FPLayout' defines only a few constants, namely the 'StorageType' and
+// length of the sign, the exponent, fraction and significand parts.
+// - 'FPStorage' builds more constants on top of those from 'FPLayout' like
+// exponent bias and masks. It also holds the bit representation of the
+// floating point as a 'StorageType' type and defines tools to assemble or test
// these parts.
-// - 'FPRep' defines functions to interact with the floating point
-// representation. The default implementation is the one for 'IEEE754', a
-// specialization is provided for X86 Extended Precision that has a different
-// encoding.
-// - 'FPBits' is templated on the platform floating point types. Contrary to
-// 'FPRep' that is platform agnostic 'FPBits' is architecture dependent.
+// - 'FPRepSem' defines functions to interact semantically with the floating
+// point representation. The default implementation is the one for 'IEEE754', a
+// specialization is provided for X86 Extended Precision.
+// - 'FPRep' derives from 'FPRepSem' and adds functions that are common to all
+// implementations.
+// - 'FPBits' exposes all functions from 'FPRep' but operates on the native C++
+// floating point type instead of 'FPType'.
namespace internal {
// Defines the layout (sign, exponent, significand) of a floating point type in
// memory. It also defines its associated StorageType, i.e., the unsigned
// integer type used to manipulate its representation.
+// Additionally we provide the fractional part length, i.e., the number of bits
+// after the decimal dot when the number is in normal form.
template <FPType> struct FPLayout {};
template <> struct FPLayout<FPType::IEEE754_Binary16> {
@@ -103,6 +111,7 @@ template <> struct FPLayout<FPType::IEEE754_Binary16> {
LIBC_INLINE_VAR static constexpr int SIGN_LEN = 1;
LIBC_INLINE_VAR static constexpr int EXP_LEN = 5;
LIBC_INLINE_VAR static constexpr int SIG_LEN = 10;
+ LIBC_INLINE_VAR static constexpr int FRACTION_LEN = SIG_LEN;
};
template <> struct FPLayout<FPType::IEEE754_Binary32> {
@@ -110,6 +119,7 @@ template <> struct FPLayout<FPType::IEEE754_Binary32> {
LIBC_INLINE_VAR static constexpr int SIGN_LEN = 1;
LIBC_INLINE_VAR static constexpr int EXP_LEN = 8;
LIBC_INLINE_VAR static constexpr int SIG_LEN = 23;
+ LIBC_INLINE_VAR static constexpr int FRACTION_LEN = SIG_LEN;
};
template <> struct FPLayout<FPType::IEEE754_Binary64> {
@@ -117,6 +127,7 @@ template <> struct FPLayout<FPType::IEEE754_Binary64> {
LIBC_INLINE_VAR static constexpr int SIGN_LEN = 1;
LIBC_INLINE_VAR static constexpr int EXP_LEN = 11;
LIBC_INLINE_VAR static constexpr int SIG_LEN = 52;
+ LIBC_INLINE_VAR static constexpr int FRACTION_LEN = SIG_LEN;
};
template <> struct FPLayout<FPType::IEEE754_Binary128> {
@@ -124,6 +135,7 @@ template <> struct FPLayout<FPType::IEEE754_Binary128> {
LIBC_INLINE_VAR static constexpr int SIGN_LEN = 1;
LIBC_INLINE_VAR static constexpr int EXP_LEN = 15;
LIBC_INLINE_VAR static constexpr int SIG_LEN = 112;
+ LIBC_INLINE_VAR static constexpr int FRACTION_LEN = SIG_LEN;
};
template <> struct FPLayout<FPType::X86_Binary80> {
@@ -131,23 +143,22 @@ template <> struct FPLayout<FPType::X86_Binary80> {
LIBC_INLINE_VAR static constexpr int SIGN_LEN = 1;
LIBC_INLINE_VAR static constexpr int EXP_LEN = 15;
LIBC_INLINE_VAR static constexpr int SIG_LEN = 64;
+ LIBC_INLINE_VAR static constexpr int FRACTION_LEN = SIG_LEN - 1;
};
-} // namespace internal
-
-// FPRepBase derives useful constants from the FPLayout.
-template <FPType fp_type>
-struct FPRepBase : public internal::FPLayout<fp_type> {
-private:
- using UP = internal::FPLayout<fp_type>;
+// FPStorage derives useful constants from the FPLayout above.
+template <FPType fp_type> struct FPStorage : public FPLayout<fp_type> {
+ using UP = FPLayout<fp_type>;
-public:
using UP::EXP_LEN; // The number of bits for the *exponent* part
using UP::SIG_LEN; // The number of bits for the *significand* part
using UP::SIGN_LEN; // The number of bits for the *sign* part
// For convenience, the sum of `SIG_LEN`, `EXP_LEN`, and `SIGN_LEN`.
LIBC_INLINE_VAR static constexpr int TOTAL_LEN = SIGN_LEN + EXP_LEN + SIG_LEN;
+ // The number of bits after the decimal dot when the number is in normal form.
+ using UP::FRACTION_LEN;
+
// An unsigned integer that is wide enough to contain all of the floating
// point bits.
using StorageType = typename UP::StorageType;
@@ -162,41 +173,30 @@ struct FPRepBase : public internal::FPLayout<fp_type> {
(1U << (EXP_LEN - 1U)) - 1U;
static_assert(EXP_BIAS > 0);
-protected:
- // The shift amount to get the *significand* part to the least significant
- // bit. Always `0` but kept for consistency.
- LIBC_INLINE_VAR static constexpr int SIG_MASK_SHIFT = 0;
- // The shift amount to get the *exponent* part to the least significant bit.
- LIBC_INLINE_VAR static constexpr int EXP_MASK_SHIFT = SIG_LEN;
- // The shift amount to get the *sign* part to the least significant bit.
- LIBC_INLINE_VAR static constexpr int SIGN_MASK_SHIFT = SIG_LEN + EXP_LEN;
-
// The bit pattern that keeps only the *significand* part.
LIBC_INLINE_VAR static constexpr StorageType SIG_MASK =
- mask_trailing_ones<StorageType, SIG_LEN>() << SIG_MASK_SHIFT;
-
-public:
+ mask_trailing_ones<StorageType, SIG_LEN>();
// The bit pattern that keeps only the *exponent* part.
LIBC_INLINE_VAR static constexpr StorageType EXP_MASK =
- mask_trailing_ones<StorageType, EXP_LEN>() << EXP_MASK_SHIFT;
+ mask_trailing_ones<StorageType, EXP_LEN>() << SIG_LEN;
// The bit pattern that keeps only the *sign* part.
LIBC_INLINE_VAR static constexpr StorageType SIGN_MASK =
- mask_trailing_ones<StorageType, SIGN_LEN>() << SIGN_MASK_SHIFT;
+ mask_trailing_ones<StorageType, SIGN_LEN>() << (EXP_LEN + SIG_LEN);
// The bit pattern that keeps only the *exponent + significand* part.
LIBC_INLINE_VAR static constexpr StorageType EXP_SIG_MASK =
mask_trailing_ones<StorageType, EXP_LEN + SIG_LEN>();
// The bit pattern that keeps only the *sign + exponent + significand* part.
LIBC_INLINE_VAR static constexpr StorageType FP_MASK =
mask_trailing_ones<StorageType, TOTAL_LEN>();
+ // The bit pattern that keeps only the *fraction* part.
+ // i.e., the *significand* without the leading one.
+ LIBC_INLINE_VAR static constexpr StorageType FRACTION_MASK =
+ mask_trailing_ones<StorageType, FRACTION_LEN>();
static_assert((SIG_MASK & EXP_MASK & SIGN_MASK) == 0, "masks disjoint");
static_assert((SIG_MASK | EXP_MASK | SIGN_MASK) == FP_MASK, "masks cover");
protected:
- LIBC_INLINE static constexpr StorageType bit_at(int position) {
- return StorageType(1) << position;
- }
-
// A stongly typed integer that prevents mixing and matching integers with
// different semantics.
template <typename T> struct TypedInt {
@@ -248,7 +248,7 @@ struct FPRepBase : public internal::FPLayout<fp_type> {
// An opaque type to store a floating point significand.
// We define special values but it is valid to create arbitrary values as long
- // as they are in the range [BITS_ALL_ZEROES, BITS_ALL_ONES].
+ // as they are in the range [ZERO, BITS_ALL_ONES].
// Note that the semantics of the Significand are implementation dependent.
// Values greater than BITS_ALL_ONES are truncated.
struct Significand : public TypedInt<StorageType> {
@@ -277,10 +277,8 @@ struct FPRepBase : public internal::FPLayout<fp_type> {
return Significand(StorageType(1));
}
LIBC_INLINE static constexpr auto MSB() {
- return Significand(StorageType(bit_at(SIG_LEN - 1)));
+ return Significand(StorageType(1) << (SIG_LEN - 1));
}
- // Aliases
- LIBC_INLINE static constexpr auto BITS_ALL_ZEROES() { return ZERO(); }
LIBC_INLINE static constexpr auto BITS_ALL_ONES() {
return Significand(SIG_MASK);
}
@@ -306,204 +304,112 @@ struct FPRepBase : public internal::FPLayout<fp_type> {
return encode(exp, sig);
}
+ // The floating point number representation as an unsigned integer.
+ StorageType bits{};
+
+ LIBC_INLINE constexpr FPStorage() : bits(0) {}
+ LIBC_INLINE constexpr FPStorage(StorageType value) : bits(value) {}
+
+ // Observers
LIBC_INLINE constexpr StorageType exp_bits() const { return bits & EXP_MASK; }
LIBC_INLINE constexpr StorageType sig_bits() const { return bits & SIG_MASK; }
LIBC_INLINE constexpr StorageType exp_sig_bits() const {
return bits & EXP_SIG_MASK;
}
-
-private:
- // Merge bits from 'a' and 'b' values according to 'mask'.
- // Use 'a' bits when corresponding 'mask' bits are zeroes and 'b' bits when
- // corresponding bits are ones.
- LIBC_INLINE static constexpr StorageType merge(StorageType a, StorageType b,
- StorageType mask) {
- // https://graphics.stanford.edu/~seander/bithacks.html#MaskedMerge
- return a ^ ((a ^ b) & mask);
- }
-
-protected:
- // The number of bits after the decimal dot when the number is in normal form.
- LIBC_INLINE_VAR static constexpr int FRACTION_LEN =
- fp_type == FPType::X86_Binary80 ? SIG_LEN - 1 : SIG_LEN;
- LIBC_INLINE_VAR static constexpr uint32_t MANTISSA_PRECISION =
- FRACTION_LEN + 1;
- LIBC_INLINE_VAR static constexpr StorageType FRACTION_MASK =
- mask_trailing_ones<StorageType, FRACTION_LEN>();
-
- // The floating point number representation as an unsigned integer.
- StorageType bits = 0;
-
-public:
- LIBC_INLINE constexpr Sign sign() const {
- return (bits & SIGN_MASK) ? Sign::NEG : Sign::POS;
- }
-
- LIBC_INLINE constexpr void set_sign(Sign signVal) {
- if (sign() != signVal)
- bits ^= SIGN_MASK;
- }
-
- LIBC_INLINE constexpr StorageType get_mantissa() const {
- return bits & FRACTION_MASK;
- }
-
- LIBC_INLINE constexpr void set_mantissa(StorageType mantVal) {
- bits = merge(bits, mantVal, FRACTION_MASK);
- }
-
- LIBC_INLINE constexpr uint16_t get_biased_exponent() const {
- return uint16_t((bits & EXP_MASK) >> EXP_MASK_SHIFT);
- }
-
- LIBC_INLINE constexpr void set_biased_exponent(StorageType biased) {
- bits = merge(bits, biased << EXP_MASK_SHIFT, EXP_MASK);
- }
-
- LIBC_INLINE constexpr int get_exponent() const {
- return int(get_biased_exponent()) - EXP_BIAS;
- }
-
- // If the number is subnormal, the exponent is treated as if it were the
- // minimum exponent for a normal number. This is to keep continuity between
- // the normal and subnormal ranges, but it causes problems for functions where
- // values are calculated from the exponent, since just subtracting the bias
- // will give a slightly incorrect result. Additionally, zero has an exponent
- // of zero, and that should actually be treated as zero.
- LIBC_INLINE constexpr int get_explicit_exponent() const {
- const int biased_exp = int(get_biased_exponent());
- if (is_zero()) {
- return 0;
- } else if (biased_exp == 0) {
- return 1 - EXP_BIAS;
- } else {
- return biased_exp - EXP_BIAS;
- }
- }
-
- LIBC_INLINE constexpr StorageType uintval() const { return bits & FP_MASK; }
- LIBC_INLINE constexpr void set_uintval(StorageType value) {
- bits = (value & FP_MASK);
- }
-
- LIBC_INLINE constexpr bool is_zero() const { return exp_sig_bits() == 0; }
-
- LIBC_INLINE
- constexpr bool is_subnormal() const {
- return exp_bits() == encode(BiasedExponent::BITS_ALL_ZEROES());
- }
-
- LIBC_INLINE constexpr bool is_neg() const { return sign().is_neg(); }
- LIBC_INLINE constexpr bool is_pos() const { return sign().is_pos(); }
};
-namespace internal {
-
-// Manipulates the representation of a floating point number defined by its
-// FPType. This layer is architecture agnostic and does not handle C++ floating
-// point types directly ('float', 'double' and 'long double'). Use the FPBits
-// below if needed.
-//
-// TODO: Specialize this class for FPType::X86_Binary80 and remove ad-hoc logic
-// from FPRepBase.
-template <FPType fp_type> struct FPRep : public FPRepBase<fp_type> {
- using UP = FPRepBase<fp_type>;
+// This layer defines all functions that are specific to how the the floating
+// point type is encoded. It enables constructions, modification and observation
+// of values manipulated as 'StorageType'.
+template <FPType fp_type, typename RetT>
+struct FPRepSem : public FPStorage<fp_type> {
+ using UP = FPStorage<fp_type>;
using typename UP::StorageType;
using UP::FRACTION_LEN;
using UP::FRACTION_MASK;
- using UP::MANTISSA_PRECISION;
protected:
- using typename UP::BiasedExponent;
- using typename UP::Exponent;
- using typename UP::Significand;
+ using BiasedExp = typename UP::BiasedExponent;
+ using Exp = typename UP::Exponent;
+ using Sig = typename UP::Significand;
using UP::encode;
using UP::exp_bits;
using UP::exp_sig_bits;
using UP::sig_bits;
+ using UP::UP;
public:
- LIBC_INLINE constexpr bool is_nan() const {
- return exp_sig_bits() >
- encode(BiasedExponent::BITS_ALL_ONES(), Significand::ZERO());
+ // Builders
+ LIBC_INLINE static constexpr RetT one(Sign sign = Sign::POS) {
+ return RetT(encode(sign, Exp::ZERO(), Sig::ZERO()));
}
- LIBC_INLINE constexpr bool is_quiet_nan() const {
- return exp_sig_bits() >=
- encode(BiasedExponent::BITS_ALL_ONES(), Significand::MSB());
+ LIBC_INLINE static constexpr RetT min_subnormal(Sign sign = Sign::POS) {
+ return RetT(encode(sign, BiasedExp::BITS_ALL_ZEROES(), Sig::LSB()));
}
- LIBC_INLINE constexpr bool is_signaling_nan() const {
- return is_nan() && !is_quiet_nan();
+ LIBC_INLINE static constexpr RetT max_subnormal(Sign sign = Sign::POS) {
+ return RetT(
+ encode(sign, BiasedExp::BITS_ALL_ZEROES(), Sig::BITS_ALL_ONES()));
}
- LIBC_INLINE constexpr bool is_inf() const {
- return exp_sig_bits() ==
- encode(BiasedExponent::BITS_ALL_ONES(), Significand::ZERO());
+ LIBC_INLINE static constexpr RetT min_normal(Sign sign = Sign::POS) {
+ return RetT(encode(sign, Exp::MIN(), Sig::ZERO()));
}
- LIBC_INLINE constexpr bool is_finite() const {
- return exp_bits() != encode(BiasedExponent::BITS_ALL_ONES());
+ LIBC_INLINE static constexpr RetT max_normal(Sign sign = Sign::POS) {
+ return RetT(encode(sign, Exp::MAX(), Sig::BITS_ALL_ONES()));
}
- LIBC_INLINE constexpr bool is_normal() const {
- return is_finite() && !UP::is_subnormal();
+ LIBC_INLINE static constexpr RetT inf(Sign sign = Sign::POS) {
+ return RetT(encode(sign, BiasedExp::BITS_ALL_ONES(), Sig::ZERO()));
}
-
- LIBC_INLINE static constexpr StorageType zero(Sign sign = Sign::POS) {
- return encode(sign, BiasedExponent::BITS_ALL_ZEROES(), Significand::ZERO());
+ LIBC_INLINE static constexpr RetT build_nan(Sign sign = Sign::POS,
+ StorageType v = 0) {
+ return RetT(encode(sign, BiasedExp::BITS_ALL_ONES(),
+ (v ? Sig(v) : (Sig::MSB() >> 1))));
}
- LIBC_INLINE static constexpr StorageType one(Sign sign = Sign::POS) {
- return encode(sign, Exponent::ZERO(), Significand::ZERO());
+ LIBC_INLINE static constexpr RetT build_quiet_nan(Sign sign = Sign::POS,
+ StorageType v = 0) {
+ return RetT(encode(sign, BiasedExp::BITS_ALL_ONES(), Sig::MSB() | Sig(v)));
}
- LIBC_INLINE static constexpr StorageType
- min_subnormal(Sign sign = Sign::POS) {
- return encode(sign, BiasedExponent::BITS_ALL_ZEROES(), Significand::LSB());
+
+ // Observers
+ LIBC_INLINE constexpr bool is_nan() const {
+ return exp_sig_bits() > encode(BiasedExp::BITS_ALL_ONES(), Sig::ZERO());
}
- LIBC_INLINE static constexpr StorageType
- max_subnormal(Sign sign = Sign::POS) {
- return encode(sign, BiasedExponent::BITS_ALL_ZEROES(),
- Significand::BITS_ALL_ONES());
+ LIBC_INLINE constexpr bool is_quiet_nan() const {
+ return exp_sig_bits() >= encode(BiasedExp::BITS_ALL_ONES(), Sig::MSB());
}
- LIBC_INLINE static constexpr StorageType min_normal(Sign sign = Sign::POS) {
- return encode(sign, Exponent::MIN(), Significand::ZERO());
+ LIBC_INLINE constexpr bool is_signaling_nan() const {
+ return is_nan() && !is_quiet_nan();
}
- LIBC_INLINE static constexpr StorageType max_normal(Sign sign = Sign::POS) {
- return encode(sign, Exponent::MAX(), Significand::BITS_ALL_ONES());
+ LIBC_INLINE constexpr bool is_inf() const {
+ return exp_sig_bits() == encode(BiasedExp::BITS_ALL_ONES(), Sig::ZERO());
}
- LIBC_INLINE static constexpr StorageType inf(Sign sign = Sign::POS) {
- return encode(sign, BiasedExponent::BITS_ALL_ONES(), Significand::ZERO());
+ LIBC_INLINE constexpr bool is_finite() const {
+ return exp_bits() != encode(BiasedExp::BITS_ALL_ONES());
}
- LIBC_INLINE static constexpr StorageType build_nan(Sign sign = Sign::POS,
- StorageType v = 0) {
- return encode(sign, BiasedExponent::BITS_ALL_ONES(),
- (v ? Significand(v) : (Significand::MSB() >> 1)));
+ LIBC_INLINE
+ constexpr bool is_subnormal() const {
+ return exp_bits() == encode(BiasedExp::BITS_ALL_ZEROES());
}
- LIBC_INLINE static constexpr StorageType
- build_quiet_nan(Sign sign = Sign::POS, StorageType v = 0) {
- return encode(sign, BiasedExponent::BITS_ALL_ONES(),
- Significand::MSB() | Significand(v));
+ LIBC_INLINE constexpr bool is_normal() const {
+ return is_finite() && !UP::is_subnormal();
}
-
- // The function return mantissa with the implicit bit set iff the current
+ // Returns the mantissa with the implicit bit set iff the current
// value is a valid normal number.
LIBC_INLINE constexpr StorageType get_explicit_mantissa() {
- if (UP::is_subnormal())
+ if (is_subnormal())
return sig_bits();
return (StorageType(1) << UP::SIG_LEN) | sig_bits();
}
};
// Specialization for the X86 Extended Precision type.
-template <>
-struct FPRep<FPType::X86_Binary80> : public FPRepBase<FPType::X86_Binary80> {
- using UP = FPRepBase<FPType::X86_Binary80>;
+template <typename RetT>
+struct FPRepSem<FPType::X86_Binary80, RetT>
+ : public FPStorage<FPType::X86_Binary80> {
+ using UP = FPStorage<FPType::X86_Binary80>;
using typename UP::StorageType;
using UP::FRACTION_LEN;
using UP::FRACTION_MASK;
- using UP::MANTISSA_PRECISION;
-
-protected:
- using typename UP::BiasedExponent;
- using typename UP::Significand;
- using UP::encode;
-public:
// The x86 80 bit float represents the leading digit of the mantissa
// explicitly. This is the mask for that bit.
static constexpr StorageType EXPLICIT_BIT_MASK = StorageType(1)
@@ -515,6 +421,45 @@ struct FPRep<...
[truncated]
``````````
</details>
https://github.com/llvm/llvm-project/pull/78978
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