Revert "[libc] FPRep builders return FPRep instead of raw StorageType"

[gchatelet]

Reverts #78588

[llvmbot]

@llvm/pr-subscribers-libc

Author: Guillaume Chatelet (gchatelet)

Changes

Reverts llvm/llvm-project#78588

---

Patch is 46.41 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/78974.diff

2 Files Affected:

• (modified) libc/src/__support/FPUtil/FPBits.h (+281-312)
• (modified) libc/test/src/__support/FPUtil/fpbits_test.cpp (+84-87)

diff --git a/libc/src/__support/FPUtil/FPBits.h b/libc/src/__support/FPUtil/FPBits.h
index bc6b19b6ebf1089..be700285de82854 100644
--- a/libc/src/__support/FPUtil/FPBits.h
+++ b/libc/src/__support/FPUtil/FPBits.h
@@ -64,46 +64,38 @@ LIBC_INLINE_VAR constexpr Sign Sign::POS = Sign(false);
 //             └─────────▲─────────┘
 //                       │
 //             ┌─────────┴─────────┐
-//             │ FPStorage<FPType> │
+//             │ FPRepBase<FPType> │
 //             └─────────▲─────────┘
 //                       │
 //          ┌────────────┴─────────────┐
 //          │                          │
-// ┌────────┴─────────┐ ┌──────────────┴──────────────────┐
-// │ FPRepSem<FPType> │ │  FPRepSem<FPType::X86_Binary80  │
-// └────────▲─────────┘ └──────────────▲──────────────────┘
+// ┌────────┴──────┐     ┌─────────────┴──────────────┐
+// │ FPRep<FPType> │     │ FPRep<FPType::X86_Binary80 │
+// └────────▲──────┘     └─────────────▲──────────────┘
 //          │                          │
 //          └────────────┬─────────────┘
 //                       │
 //                 ┌─────┴─────┐
-//                 │  FPRep<T> │
-//                 └───────────┘
-//                       │
-//                 ┌─────┴─────┐
 //                 │ FPBits<T> │
 //                 └───────────┘
 //
-// - '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
+// - '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
 // these parts.
-// - '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'.
+// - '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.
 
 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> {
@@ -111,7 +103,6 @@ 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> {
@@ -119,7 +110,6 @@ 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> {
@@ -127,7 +117,6 @@ 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> {
@@ -135,7 +124,6 @@ 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> {
@@ -143,22 +131,23 @@ 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;
 };
 
-// FPStorage derives useful constants from the FPLayout above.
-template <FPType fp_type> struct FPStorage : public FPLayout<fp_type> {
-  using UP = FPLayout<fp_type>;
+} // 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>;
 
+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;
@@ -173,30 +162,41 @@ template <FPType fp_type> struct FPStorage : public 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>();
+      mask_trailing_ones<StorageType, SIG_LEN>() << SIG_MASK_SHIFT;
+
+public:
   // The bit pattern that keeps only the *exponent* part.
   LIBC_INLINE_VAR static constexpr StorageType EXP_MASK =
-      mask_trailing_ones<StorageType, EXP_LEN>() << SIG_LEN;
+      mask_trailing_ones<StorageType, EXP_LEN>() << EXP_MASK_SHIFT;
   // The bit pattern that keeps only the *sign* part.
   LIBC_INLINE_VAR static constexpr StorageType SIGN_MASK =
-      mask_trailing_ones<StorageType, SIGN_LEN>() << (EXP_LEN + SIG_LEN);
+      mask_trailing_ones<StorageType, SIGN_LEN>() << SIGN_MASK_SHIFT;
   // 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 @@ template <FPType fp_type> struct FPStorage : public 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 [ZERO, BITS_ALL_ONES].
+  // as they are in the range [BITS_ALL_ZEROES, 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> {
@@ -277,8 +277,10 @@ template <FPType fp_type> struct FPStorage : public FPLayout<fp_type> {
       return Significand(StorageType(1));
     }
     LIBC_INLINE static constexpr auto MSB() {
-      return Significand(StorageType(1) << (SIG_LEN - 1));
+      return Significand(StorageType(bit_at(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);
     }
@@ -304,95 +306,181 @@ template <FPType fp_type> struct FPStorage : public 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;
   }
-};
 
-// 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;
+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:
-  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;
+  // 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:
-  // Builders
-  LIBC_INLINE static constexpr RetT one(Sign sign = Sign::POS) {
-    return RetT(encode(sign, Exp::ZERO(), Sig::ZERO()));
+  LIBC_INLINE constexpr Sign sign() const {
+    return (bits & SIGN_MASK) ? Sign::NEG : Sign::POS;
   }
-  LIBC_INLINE static constexpr RetT min_subnormal(Sign sign = Sign::POS) {
-    return RetT(encode(sign, BiasedExp::BITS_ALL_ZEROES(), Sig::LSB()));
+
+  LIBC_INLINE constexpr void set_sign(Sign signVal) {
+    if (sign() != signVal)
+      bits ^= SIGN_MASK;
   }
-  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 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 static constexpr RetT min_normal(Sign sign = Sign::POS) {
-    return RetT(encode(sign, Exp::MIN(), Sig::ZERO()));
+
+  LIBC_INLINE constexpr void set_biased_exponent(StorageType biased) {
+    bits = merge(bits, biased << EXP_MASK_SHIFT, EXP_MASK);
   }
-  LIBC_INLINE static constexpr RetT max_normal(Sign sign = Sign::POS) {
-    return RetT(encode(sign, Exp::MAX(), Sig::BITS_ALL_ONES()));
+
+  LIBC_INLINE constexpr int get_exponent() const {
+    return int(get_biased_exponent()) - EXP_BIAS;
   }
-  LIBC_INLINE static constexpr RetT inf(Sign sign = Sign::POS) {
-    return RetT(encode(sign, BiasedExp::BITS_ALL_ONES(), Sig::ZERO()));
+
+  // 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 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 constexpr StorageType uintval() const { return bits & FP_MASK; }
+  LIBC_INLINE constexpr void set_uintval(StorageType value) {
+    bits = (value & FP_MASK);
   }
-  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 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());
   }
 
-  // Observers
+  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>;
+  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 UP::encode;
+  using UP::exp_bits;
+  using UP::exp_sig_bits;
+  using UP::sig_bits;
+
+public:
   LIBC_INLINE constexpr bool is_nan() const {
-    return exp_sig_bits() > encode(BiasedExp::BITS_ALL_ONES(), Sig::ZERO());
+    return exp_sig_bits() >
+           encode(BiasedExponent::BITS_ALL_ONES(), Significand::ZERO());
   }
   LIBC_INLINE constexpr bool is_quiet_nan() const {
-    return exp_sig_bits() >= encode(BiasedExp::BITS_ALL_ONES(), Sig::MSB());
+    return exp_sig_bits() >=
+           encode(BiasedExponent::BITS_ALL_ONES(), Significand::MSB());
   }
   LIBC_INLINE constexpr bool is_signaling_nan() const {
     return is_nan() && !is_quiet_nan();
   }
   LIBC_INLINE constexpr bool is_inf() const {
-    return exp_sig_bits() == encode(BiasedExp::BITS_ALL_ONES(), Sig::ZERO());
+    return exp_sig_bits() ==
+           encode(BiasedExponent::BITS_ALL_ONES(), Significand::ZERO());
   }
   LIBC_INLINE constexpr bool is_finite() const {
-    return exp_bits() != encode(BiasedExp::BITS_ALL_ONES());
-  }
-  LIBC_INLINE
-  constexpr bool is_subnormal() const {
-    return exp_bits() == encode(BiasedExp::BITS_ALL_ZEROES());
+    return exp_bits() != encode(BiasedExponent::BITS_ALL_ONES());
   }
   LIBC_INLINE constexpr bool is_normal() const {
     return is_finite() && !UP::is_subnormal();
   }
-  // Returns the mantissa with the implicit bit set iff the current
+
+  LIBC_INLINE static constexpr StorageType zero(Sign sign = Sign::POS) {
+    return encode(sign, BiasedExponent::BITS_ALL_ZEROES(), Significand::ZERO());
+  }
+  LIBC_INLINE static constexpr StorageType one(Sign sign = Sign::POS) {
+    return encode(sign, Exponent::ZERO(), Significand::ZERO());
+  }
+  LIBC_INLINE static constexpr StorageType
+  min_subnormal(Sign sign = Sign::POS) {
+    return encode(sign, BiasedExponent::BITS_ALL_ZEROES(), Significand::LSB());
+  }
+  LIBC_INLINE static constexpr StorageType
+  max_subnormal(Sign sign = Sign::POS) {
+    return encode(sign, BiasedExponent::BITS_ALL_ZEROES(),
+                  Significand::BITS_ALL_ONES());
+  }
+  LIBC_INLINE static constexpr StorageType min_normal(Sign sign = Sign::POS) {
+    return encode(sign, Exponent::MIN(), Significand::ZERO());
+  }
+  LIBC_INLINE static constexpr StorageType max_normal(Sign sign = Sign::POS) {
+    return encode(sign, Exponent::MAX(), Significand::BITS_ALL_ONES());
+  }
+  LIBC_INLINE static constexpr StorageType inf(Sign sign = Sign::POS) {
+    return encode(sign, BiasedExponent::BITS_ALL_ONES(), Significand::ZERO());
+  }
+  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 static constexpr StorageType
+  build_quiet_nan(Sign sign = Sign::POS, StorageType v = 0) {
+    return encode(sign, BiasedExponent::BITS_ALL_ONES(),
+                  Significand::MSB() | Significand(v));
+  }
+
+  // The function return 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())
@@ -402,14 +490,20 @@ struct FPRepSem : public FPStorage<fp_type> {
 };
 
 // Specialization for the X86 Extended Precision type.
-template <typename RetT>
-struct FPRepSem<FPType::X86_Binary80, RetT>
-    : public FPStorage<FPType::X86_Binary80> {
-  using UP = FPStorage<FPType::X86_Binary80>;
+template <>
+struct FPRep<FPType::X86_Binary80> : public FPRepBase<FPType::X86_Binary80> {
+  using UP = FPRepBase<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)
@@ -421,45 +515,6 @@ struct FPRepSem<FPType::X86_Binary80, RetT>
                 "the explicit bit and the fractional part should cover the "
                 "whole significand");
 
-protected:
-  using BiasedExp = ty...
[truncated]