[libc-commits] [libc] [libc][NFC] Simplify FloatProperties implementation (PR #74821)
Guillaume Chatelet via libc-commits
libc-commits at lists.llvm.org
Mon Dec 11 08:34:51 PST 2023
https://github.com/gchatelet updated https://github.com/llvm/llvm-project/pull/74821
>From 32b53e8059191b849ce5e742118fa7c8865badce Mon Sep 17 00:00:00 2001
From: Guillaume Chatelet <gchatelet at google.com>
Date: Tue, 5 Dec 2023 14:59:52 +0000
Subject: [PATCH] [libc][NFC] Simplify FloatProperties implementation
---
libc/src/__support/FPUtil/FloatProperties.h | 168 ++++++++++----------
1 file changed, 85 insertions(+), 83 deletions(-)
diff --git a/libc/src/__support/FPUtil/FloatProperties.h b/libc/src/__support/FPUtil/FloatProperties.h
index 4288cd719a1d8a..ba54e9d1781cab 100644
--- a/libc/src/__support/FPUtil/FloatProperties.h
+++ b/libc/src/__support/FPUtil/FloatProperties.h
@@ -80,27 +80,105 @@ template <> struct FPBaseProperties<FPType::X86_Binary80> {
FPEncoding::X86_ExtendedPrecision;
};
+// TODO: Move this utility elsewhere.
+template <typename T, size_t count> static constexpr T mask_trailing_ones() {
+ static_assert(cpp::is_unsigned_v<T>);
+ constexpr unsigned t_bits = CHAR_BIT * sizeof(T);
+ static_assert(count <= t_bits && "Invalid bit index");
+ return count == 0 ? 0 : (T(-1) >> (t_bits - count));
+}
+
// Derives more properties from 'FPBaseProperties' above.
// This class serves as a halfway point between 'FPBaseProperties' and
// 'FPProperties' below.
template <FPType fp_type>
struct FPCommonProperties : private FPBaseProperties<fp_type> {
+private:
using UP = FPBaseProperties<fp_type>;
- using BitsType = typename UP::UIntType;
+ using UP::EXP_BITS;
+ using UP::SIG_BITS;
+ using UP::TOTAL_BITS;
+ using UIntType = typename UP::UIntType;
+
+ LIBC_INLINE_VAR static constexpr int STORAGE_BITS =
+ sizeof(UIntType) * CHAR_BIT;
+ static_assert(STORAGE_BITS >= TOTAL_BITS);
- LIBC_INLINE_VAR static constexpr uint32_t BIT_WIDTH = UP::TOTAL_BITS;
- LIBC_INLINE_VAR static constexpr uint32_t MANTISSA_WIDTH = UP::SIG_BITS;
- LIBC_INLINE_VAR static constexpr uint32_t EXPONENT_WIDTH = UP::EXP_BITS;
+ // The number of bits to represent sign.
+ // For documentation purpose, always 1.
+ LIBC_INLINE_VAR static constexpr int SIGN_BITS = 1;
+ static_assert(SIGN_BITS + EXP_BITS + SIG_BITS == TOTAL_BITS);
// The exponent bias. Always positive.
+ LIBC_INLINE_VAR static constexpr int32_t EXP_BIAS =
+ (1U << (EXP_BITS - 1U)) - 1U;
+ static_assert(EXP_BIAS > 0);
+
+ // Shifts
+ LIBC_INLINE_VAR static constexpr int SIG_MASK_SHIFT = 0;
+ LIBC_INLINE_VAR static constexpr int EXP_MASK_SHIFT = SIG_BITS;
+ LIBC_INLINE_VAR static constexpr int SIGN_MASK_SHIFT = SIG_BITS + EXP_BITS;
+
+ // Masks
+ LIBC_INLINE_VAR static constexpr UIntType SIG_MASK =
+ mask_trailing_ones<UIntType, SIG_BITS>() << SIG_MASK_SHIFT;
+ LIBC_INLINE_VAR static constexpr UIntType EXP_MASK =
+ mask_trailing_ones<UIntType, EXP_BITS>() << EXP_MASK_SHIFT;
+ // Trailing underscore on SIGN_MASK_ is temporary - it will be removed
+ // once we can replace the public part below with the private one.
+ LIBC_INLINE_VAR static constexpr UIntType SIGN_MASK_ =
+ mask_trailing_ones<UIntType, SIGN_BITS>() << SIGN_MASK_SHIFT;
+ LIBC_INLINE_VAR static constexpr UIntType FP_MASK =
+ mask_trailing_ones<UIntType, TOTAL_BITS>();
+ static_assert((SIG_MASK & EXP_MASK & SIGN_MASK_) == 0, "masks disjoint");
+ static_assert((SIG_MASK | EXP_MASK | SIGN_MASK_) == FP_MASK, "masks covers");
+
+ LIBC_INLINE static constexpr UIntType bit_at(int position) {
+ return UIntType(1) << position;
+ }
+
+ LIBC_INLINE_VAR static constexpr UIntType QNAN_MASK =
+ UP::ENCODING == FPEncoding::X86_ExtendedPrecision
+ ? bit_at(SIG_BITS - 1) | bit_at(SIG_BITS - 2) // 0b1100...
+ : bit_at(SIG_BITS - 1); // 0b1000...
+
+ LIBC_INLINE_VAR static constexpr UIntType SNAN_MASK =
+ UP::ENCODING == FPEncoding::X86_ExtendedPrecision
+ ? bit_at(SIG_BITS - 1) | bit_at(SIG_BITS - 3) // 0b1010...
+ : bit_at(SIG_BITS - 2); // 0b0100...
+
+ // The number of bits after the decimal dot when the number if in normal form.
+ LIBC_INLINE_VAR static constexpr int FRACTION_BITS =
+ UP::ENCODING == FPEncoding::X86_ExtendedPrecision ? SIG_BITS - 1
+ : SIG_BITS;
+
+public:
+ // Public facing API to keep the change local to this file.
+ using BitsType = UIntType;
+
+ LIBC_INLINE_VAR static constexpr uint32_t BIT_WIDTH = TOTAL_BITS;
+ LIBC_INLINE_VAR static constexpr uint32_t MANTISSA_WIDTH = FRACTION_BITS;
+ LIBC_INLINE_VAR static constexpr uint32_t MANTISSA_PRECISION =
+ MANTISSA_WIDTH + 1;
+ LIBC_INLINE_VAR static constexpr BitsType MANTISSA_MASK =
+ mask_trailing_ones<UIntType, MANTISSA_WIDTH>();
+ LIBC_INLINE_VAR static constexpr uint32_t EXPONENT_WIDTH = EXP_BITS;
LIBC_INLINE_VAR static constexpr uint32_t EXPONENT_BIAS =
- (1U << (UP::EXP_BITS - 1U)) - 1U;
- static_assert(EXPONENT_BIAS > 0);
+ static_cast<uint32_t>(EXP_BIAS);
+ LIBC_INLINE_VAR static constexpr BitsType SIGN_MASK = SIGN_MASK_;
+ LIBC_INLINE_VAR static constexpr BitsType EXPONENT_MASK = EXP_MASK;
+ LIBC_INLINE_VAR static constexpr BitsType EXP_MANT_MASK = EXP_MASK | SIG_MASK;
+
+ // If a number x is a NAN, then it is a quiet NAN if:
+ // QuietNaNMask & bits(x) != 0
+ // Else, it is a signalling NAN.
+ static constexpr BitsType QUIET_NAN_MASK = QNAN_MASK;
};
} // namespace internal
-template <FPType> struct FPProperties {};
+template <FPType fp_type>
+struct FPProperties : public internal::FPCommonProperties<fp_type> {};
// ----------------
// Work In Progress
@@ -110,90 +188,14 @@ template <FPType> struct FPProperties {};
// empty, 'FPProperties' declaration can be fully replace with
// 'FPCommonProperties' implementation.
-template <>
-struct FPProperties<FPType::IEEE754_Binary32>
- : public internal::FPCommonProperties<FPType::IEEE754_Binary32> {
- // The mantissa precision includes the implicit bit.
- static constexpr uint32_t MANTISSA_PRECISION = MANTISSA_WIDTH + 1;
- static constexpr BitsType MANTISSA_MASK = (BitsType(1) << MANTISSA_WIDTH) - 1;
- static constexpr BitsType SIGN_MASK = BitsType(1)
- << (EXPONENT_WIDTH + MANTISSA_WIDTH);
- static constexpr BitsType EXPONENT_MASK = ~(SIGN_MASK | MANTISSA_MASK);
- static constexpr BitsType EXP_MANT_MASK = MANTISSA_MASK + EXPONENT_MASK;
- static_assert(EXP_MANT_MASK == ~SIGN_MASK,
- "Exponent and mantissa masks are not as expected.");
-
- // If a number x is a NAN, then it is a quiet NAN if:
- // QuietNaNMask & bits(x) != 0
- // Else, it is a signalling NAN.
- static constexpr BitsType QUIET_NAN_MASK = 0x00400000U;
-};
-
-template <>
-struct FPProperties<FPType::IEEE754_Binary64>
- : public internal::FPCommonProperties<FPType::IEEE754_Binary64> {
- static constexpr uint32_t MANTISSA_PRECISION = MANTISSA_WIDTH + 1;
- static constexpr BitsType MANTISSA_MASK = (BitsType(1) << MANTISSA_WIDTH) - 1;
- static constexpr BitsType SIGN_MASK = BitsType(1)
- << (EXPONENT_WIDTH + MANTISSA_WIDTH);
- static constexpr BitsType EXPONENT_MASK = ~(SIGN_MASK | MANTISSA_MASK);
- static constexpr BitsType EXP_MANT_MASK = MANTISSA_MASK + EXPONENT_MASK;
- static_assert(EXP_MANT_MASK == ~SIGN_MASK,
- "Exponent and mantissa masks are not as expected.");
-
- // If a number x is a NAN, then it is a quiet NAN if:
- // QuietNaNMask & bits(x) != 0
- // Else, it is a signalling NAN.
- static constexpr BitsType QUIET_NAN_MASK = 0x0008000000000000ULL;
-};
-
// Properties for numbers represented in 80 bits long double on non-Windows x86
// platforms.
template <>
struct FPProperties<FPType::X86_Binary80>
: public internal::FPCommonProperties<FPType::X86_Binary80> {
- static constexpr BitsType FULL_WIDTH_MASK = ((BitsType(1) << BIT_WIDTH) - 1);
- static constexpr uint32_t MANTISSA_WIDTH = 63;
- static constexpr uint32_t MANTISSA_PRECISION = MANTISSA_WIDTH + 1;
- static constexpr BitsType MANTISSA_MASK = (BitsType(1) << MANTISSA_WIDTH) - 1;
// The x86 80 bit float represents the leading digit of the mantissa
// explicitly. This is the mask for that bit.
static constexpr BitsType EXPLICIT_BIT_MASK = (BitsType(1) << MANTISSA_WIDTH);
- static constexpr BitsType SIGN_MASK =
- BitsType(1) << (EXPONENT_WIDTH + MANTISSA_WIDTH + 1);
- static constexpr BitsType EXPONENT_MASK =
- ((BitsType(1) << EXPONENT_WIDTH) - 1) << (MANTISSA_WIDTH + 1);
- static constexpr BitsType EXP_MANT_MASK =
- MANTISSA_MASK | EXPLICIT_BIT_MASK | EXPONENT_MASK;
- static_assert(EXP_MANT_MASK == (~SIGN_MASK & FULL_WIDTH_MASK),
- "Exponent and mantissa masks are not as expected.");
-
- // If a number x is a NAN, then it is a quiet NAN if:
- // QuietNaNMask & bits(x) != 0
- // Else, it is a signalling NAN.
- static constexpr BitsType QUIET_NAN_MASK = BitsType(1)
- << (MANTISSA_WIDTH - 1);
-};
-
-// Properties for numbers represented in 128 bits long double on non x86
-// platform.
-template <>
-struct FPProperties<FPType::IEEE754_Binary128>
- : public internal::FPCommonProperties<FPType::IEEE754_Binary128> {
- static constexpr uint32_t MANTISSA_PRECISION = MANTISSA_WIDTH + 1;
- static constexpr BitsType MANTISSA_MASK = (BitsType(1) << MANTISSA_WIDTH) - 1;
- static constexpr BitsType SIGN_MASK = BitsType(1)
- << (EXPONENT_WIDTH + MANTISSA_WIDTH);
- static constexpr BitsType EXPONENT_MASK = ~(SIGN_MASK | MANTISSA_MASK);
- static constexpr BitsType EXP_MANT_MASK = MANTISSA_MASK | EXPONENT_MASK;
- static_assert(EXP_MANT_MASK == ~SIGN_MASK,
- "Exponent and mantissa masks are not as expected.");
-
- // If a number x is a NAN, then it is a quiet NAN if:
- // QuietNaNMask & bits(x) != 0
- // Else, it is a signalling NAN.
- static constexpr BitsType QUIET_NAN_MASK = BitsType(1)
- << (MANTISSA_WIDTH - 1);
};
//-----------------------------------------------------------------------------
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