[libc][math][c23] Add expf16 C23 math function (#100632)

Part of #95250.
llvm · Jul 30, 2024 · 971a1ac · 971a1ac
1 parent d01c051
commit 971a1ac
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Showing 15 changed files with 404 additions and 21 deletions.
diff --git a/libc/config/linux/aarch64/entrypoints.txt b/libc/config/linux/aarch64/entrypoints.txt
@@ -540,6 +540,7 @@ if(LIBC_TYPES_HAS_FLOAT16)
     libc.src.math.canonicalizef16
     libc.src.math.ceilf16
     libc.src.math.copysignf16
+    libc.src.math.expf16
     libc.src.math.f16add
     libc.src.math.f16addf
     libc.src.math.f16div

diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
@@ -562,6 +562,7 @@ if(LIBC_TYPES_HAS_FLOAT16)
     libc.src.math.canonicalizef16
     libc.src.math.ceilf16
     libc.src.math.copysignf16
+    libc.src.math.expf16
     libc.src.math.f16add
     libc.src.math.f16addf
     libc.src.math.f16addl

diff --git a/libc/docs/math/index.rst b/libc/docs/math/index.rst
@@ -284,7 +284,7 @@ Higher Math Functions
 +-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+
 | erfc      |                  |                 |                        |                      |                        | 7.12.8.2               | F.10.5.2                   |
 +-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+
-| exp       | |check|          | |check|         |                        |                      |                        | 7.12.6.1               | F.10.3.1                   |
+| exp       | |check|          | |check|         |                        | |check|              |                        | 7.12.6.1               | F.10.3.1                   |
 +-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+
 | exp10     | |check|          | |check|         |                        |                      |                        | 7.12.6.2               | F.10.3.2                   |
 +-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+

diff --git a/libc/spec/stdc.td b/libc/spec/stdc.td
@@ -573,6 +573,7 @@ def StdC : StandardSpec<"stdc"> {
 
           FunctionSpec<"exp", RetValSpec<DoubleType>, [ArgSpec<DoubleType>]>,
           FunctionSpec<"expf", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,
+          GuardedFunctionSpec<"expf16", RetValSpec<Float16Type>, [ArgSpec<Float16Type>], "LIBC_TYPES_HAS_FLOAT16">,
 
           FunctionSpec<"exp2", RetValSpec<DoubleType>, [ArgSpec<DoubleType>]>,
           FunctionSpec<"exp2f", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,

diff --git a/libc/src/math/CMakeLists.txt b/libc/src/math/CMakeLists.txt
@@ -97,6 +97,7 @@ add_math_entrypoint_object(erff)
 
 add_math_entrypoint_object(exp)
 add_math_entrypoint_object(expf)
+add_math_entrypoint_object(expf16)
 
 add_math_entrypoint_object(exp2)
 add_math_entrypoint_object(exp2f)

diff --git a/libc/src/math/expf16.h b/libc/src/math/expf16.h
@@ -0,0 +1,21 @@
+//===-- Implementation header for expf16 ------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_MATH_EXPF16_H
+#define LLVM_LIBC_SRC_MATH_EXPF16_H
+
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/properties/types.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+float16 expf16(float16 x);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_MATH_EXPF16_H
diff --git a/libc/src/math/generic/CMakeLists.txt b/libc/src/math/generic/CMakeLists.txt
@@ -1226,6 +1226,27 @@ add_entrypoint_object(
     -O3
 )
 
+add_entrypoint_object(
+  expf16
+  SRCS
+    expf16.cpp
+  HDRS
+    ../expf16.h
+  DEPENDS
+    libc.hdr.errno_macros
+    libc.hdr.fenv_macros
+    libc.src.__support.CPP.array
+    libc.src.__support.FPUtil.fenv_impl
+    libc.src.__support.FPUtil.fp_bits
+    libc.src.__support.FPUtil.polyeval
+    libc.src.__support.FPUtil.multiply_add
+    libc.src.__support.FPUtil.nearest_integer
+    libc.src.__support.FPUtil.rounding_mode
+    libc.src.__support.macros.optimization
+  COMPILE_OPTIONS
+    -O3
+)
+
 add_entrypoint_object(
   exp2
   SRCS

diff --git a/libc/src/math/generic/expf16.cpp b/libc/src/math/generic/expf16.cpp
@@ -0,0 +1,173 @@
+//===-- Half-precision e^x function ---------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/math/expf16.h"
+#include "hdr/errno_macros.h"
+#include "hdr/fenv_macros.h"
+#include "src/__support/CPP/array.h"
+#include "src/__support/FPUtil/FEnvImpl.h"
+#include "src/__support/FPUtil/FPBits.h"
+#include "src/__support/FPUtil/PolyEval.h"
+#include "src/__support/FPUtil/except_value_utils.h"
+#include "src/__support/FPUtil/multiply_add.h"
+#include "src/__support/FPUtil/nearest_integer.h"
+#include "src/__support/FPUtil/rounding_mode.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/optimization.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+static constexpr fputil::ExceptValues<float16, 2> EXPF16_EXCEPTS_LO = {{
+    // (input, RZ output, RU offset, RD offset, RN offset)
+    // x = 0x1.de4p-8, expf16(x) = 0x1.01cp+0 (RZ)
+    {0x1f79U, 0x3c07U, 1U, 0U, 0U},
+    // x = 0x1.73cp-6, expf16(x) = 0x1.05cp+0 (RZ)
+    {0x25cfU, 0x3c17U, 1U, 0U, 0U},
+}};
+
+static constexpr fputil::ExceptValues<float16, 3> EXPF16_EXCEPTS_HI = {{
+    // (input, RZ output, RU offset, RD offset, RN offset)
+    // x = 0x1.c34p+0, expf16(x) = 0x1.74cp+2 (RZ)
+    {0x3f0dU, 0x45d3U, 1U, 0U, 1U},
+    // x = -0x1.488p-5, expf16(x) = 0x1.ebcp-1 (RZ)
+    {0xa922U, 0x3bafU, 1U, 0U, 0U},
+    // x = -0x1.55p-5, expf16(x) = 0x1.ebp-1 (RZ)
+    {0xa954U, 0x3bacU, 1U, 0U, 0U},
+}};
+
+// Generated by Sollya with the following commands:
+//   > display = hexadecimal;
+//   > for i from -18 to 12 do print(round(exp(i), SG, RN));
+static constexpr cpp::array<float, 31> EXP_HI = {
+    0x1.05a628p-26f, 0x1.639e32p-25f, 0x1.e355bcp-24f, 0x1.4875cap-22f,
+    0x1.be6c7p-21f,  0x1.2f6054p-19f, 0x1.9c54c4p-18f, 0x1.183542p-16f,
+    0x1.7cd79cp-15f, 0x1.02cf22p-13f, 0x1.5fc21p-12f,  0x1.de16bap-11f,
+    0x1.44e52p-9f,   0x1.b993fep-8f,  0x1.2c155cp-6f,  0x1.97db0cp-5f,
+    0x1.152aaap-3f,  0x1.78b564p-2f,  0x1p+0f,         0x1.5bf0a8p+1f,
+    0x1.d8e64cp+2f,  0x1.415e5cp+4f,  0x1.b4c902p+5f,  0x1.28d38ap+7f,
+    0x1.936dc6p+8f,  0x1.122886p+10f, 0x1.749ea8p+11f, 0x1.fa7158p+12f,
+    0x1.5829dcp+14f, 0x1.d3c448p+15f, 0x1.3de166p+17f,
+};
+
+// Generated by Sollya with the following commands:
+//   > display = hexadecimal;
+//   > for i from 0 to 7 do print(round(exp(i * 2^-3), SG, RN));
+static constexpr cpp::array<float, 8> EXP_MID = {
+    0x1p+0f,        0x1.221604p+0f, 0x1.48b5e4p+0f, 0x1.747a52p+0f,
+    0x1.a61298p+0f, 0x1.de455ep+0f, 0x1.0ef9dcp+1f, 0x1.330e58p+1f,
+};
+
+LLVM_LIBC_FUNCTION(float16, expf16, (float16 x)) {
+  using FPBits = fputil::FPBits<float16>;
+  FPBits x_bits(x);
+
+  uint16_t x_u = x_bits.uintval();
+  uint16_t x_abs = x_u & 0x7fffU;
+
+  // When 0 < |x| <= 2^(-5), or |x| >= 12, or x is NaN.
+  if (LIBC_UNLIKELY(x_abs <= 0x2800U || x_abs >= 0x4a00U)) {
+    // exp(NaN) = NaN
+    if (x_bits.is_nan()) {
+      if (x_bits.is_signaling_nan()) {
+        fputil::raise_except_if_required(FE_INVALID);
+        return FPBits::quiet_nan().get_val();
+      }
+
+      return x;
+    }
+
+    // When x >= 12.
+    if (x_bits.is_pos() && x_abs >= 0x4a00U) {
+      // exp(+inf) = +inf
+      if (x_bits.is_inf())
+        return FPBits::inf().get_val();
+
+      switch (fputil::quick_get_round()) {
+      case FE_TONEAREST:
+      case FE_UPWARD:
+        fputil::set_errno_if_required(ERANGE);
+        fputil::raise_except_if_required(FE_OVERFLOW);
+        return FPBits::inf().get_val();
+      default:
+        return FPBits::max_normal().get_val();
+      }
+    }
+
+    // When x <= -18.
+    if (x_u >= 0xcc80U) {
+      // exp(-inf) = +0
+      if (x_bits.is_inf())
+        return FPBits::zero().get_val();
+
+      fputil::set_errno_if_required(ERANGE);
+      fputil::raise_except_if_required(FE_UNDERFLOW | FE_INEXACT);
+
+      switch (fputil::quick_get_round()) {
+      case FE_UPWARD:
+        return FPBits::min_subnormal().get_val();
+      default:
+        return FPBits::zero().get_val();
+      }
+    }
+
+    // When 0 < |x| <= 2^(-5).
+    if (x_abs <= 0x2800U && !x_bits.is_zero()) {
+      if (auto r = EXPF16_EXCEPTS_LO.lookup(x_u); LIBC_UNLIKELY(r.has_value()))
+        return r.value();
+
+      float xf = x;
+      // Degree-3 minimax polynomial generated by Sollya with the following
+      // commands:
+      //   > display = hexadecimal;
+      //   > P = fpminimax(expm1(x)/x, 2, [|SG...|], [-2^-5, 2^-5]);
+      //   > 1 + x * P;
+      float r =
+          fputil::polyeval(xf, 0x1p+0f, 0x1p+0f, 0x1.0004p-1f, 0x1.555778p-3f);
+      return static_cast<float16>(r);
+    }
+  }
+
+  if (auto r = EXPF16_EXCEPTS_HI.lookup(x_u); LIBC_UNLIKELY(r.has_value()))
+    return r.value();
+
+  // For -18 < x < 12, to compute exp(x), we perform the following range
+  // reduction: find hi, mid, lo, such that:
+  //   x = hi + mid + lo, in which
+  //     hi is an integer,
+  //     mid * 2^3 is an integer,
+  //     -2^(-4) <= lo < 2^(-4).
+  // In particular,
+  //   hi + mid = round(x * 2^3) * 2^(-3).
+  // Then,
+  //   exp(x) = exp(hi + mid + lo) = exp(hi) * exp(mid) * exp(lo).
+  // We store exp(hi) and exp(mid) in the lookup tables EXP_HI and EXP_MID
+  // respectively.  exp(lo) is computed using a degree-3 minimax polynomial
+  // generated by Sollya.
+
+  float xf = static_cast<float>(x);
+  float kf = fputil::nearest_integer(xf * 0x1.0p+3f);
+  int x_hi_mid = static_cast<int>(kf);
+  int x_hi = x_hi_mid >> 3;
+  int x_mid = x_hi_mid & 0x7;
+  // lo = x - (hi + mid) = round(x * 2^3) * (-2^(-3)) + x
+  float lo = fputil::multiply_add(kf, -0x1.0p-3f, xf);
+
+  float exp_hi = EXP_HI[x_hi + 18];
+  float exp_mid = EXP_MID[x_mid];
+  // Degree-3 minimax polynomial generated by Sollya with the following
+  // commands:
+  //   > display = hexadecimal;
+  //   > P = fpminimax(expm1(x)/x, 2, [|SG...|], [-2^-4, 2^-4]);
+  //   > 1 + x * P;
+  float exp_lo =
+      fputil::polyeval(lo, 0x1p+0f, 0x1p+0f, 0x1.001p-1f, 0x1.555ddep-3f);
+  return static_cast<float16>(exp_hi * exp_mid * exp_lo);
+}
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/test/UnitTest/FPMatcher.h b/libc/test/UnitTest/FPMatcher.h
@@ -74,7 +74,8 @@ template <typename T> struct FPTest : public Test {
   static constexpr T inf = FPBits::inf(Sign::POS).get_val();
   static constexpr T neg_inf = FPBits::inf(Sign::NEG).get_val();
   static constexpr T min_normal = FPBits::min_normal().get_val();
-  static constexpr T max_normal = FPBits::max_normal().get_val();
+  static constexpr T max_normal = FPBits::max_normal(Sign::POS).get_val();
+  static constexpr T neg_max_normal = FPBits::max_normal(Sign::NEG).get_val();
   static constexpr T min_denormal = FPBits::min_subnormal().get_val();
   static constexpr T max_denormal = FPBits::max_subnormal().get_val();
 

diff --git a/libc/test/src/math/CMakeLists.txt b/libc/test/src/math/CMakeLists.txt
@@ -888,6 +888,19 @@ add_fp_unittest(
     libc.src.__support.FPUtil.fp_bits
 )
 
+add_fp_unittest(
+  exp_test
+  NEED_MPFR
+  SUITE
+    libc-math-unittests
+  SRCS
+    exp_test.cpp
+  DEPENDS
+    libc.src.errno.errno
+    libc.src.math.exp
+    libc.src.__support.FPUtil.fp_bits
+)
+
 add_fp_unittest(
   expf_test
   NEED_MPFR
@@ -902,16 +915,14 @@ add_fp_unittest(
 )
 
 add_fp_unittest(
- exp_test
- NEED_MPFR
- SUITE
-   libc-math-unittests
- SRCS
-   exp_test.cpp
- DEPENDS
-   libc.src.errno.errno
-   libc.src.math.exp
-   libc.src.__support.FPUtil.fp_bits
+  expf16_test
+  NEED_MPFR
+  SUITE
+    libc-math-unittests
+  SRCS
+    expf16_test.cpp
+  DEPENDS
+    libc.src.math.expf16
 )
 
 add_fp_unittest(

diff --git a/libc/test/src/math/expf16_test.cpp b/libc/test/src/math/expf16_test.cpp
@@ -0,0 +1,40 @@
+//===-- Exhaustive test for expf16 ----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/math/expf16.h"
+#include "test/UnitTest/FPMatcher.h"
+#include "test/UnitTest/Test.h"
+#include "utils/MPFRWrapper/MPFRUtils.h"
+
+using LlvmLibcExpf16Test = LIBC_NAMESPACE::testing::FPTest<float16>;
+
+namespace mpfr = LIBC_NAMESPACE::testing::mpfr;
+
+// Range: [0, Inf];
+static constexpr uint16_t POS_START = 0x0000U;
+static constexpr uint16_t POS_STOP = 0x7c00U;
+
+// Range: [-Inf, 0];
+static constexpr uint16_t NEG_START = 0x8000U;
+static constexpr uint16_t NEG_STOP = 0xfc00U;
+
+TEST_F(LlvmLibcExpf16Test, PositiveRange) {
+  for (uint16_t v = POS_START; v <= POS_STOP; ++v) {
+    float16 x = FPBits(v).get_val();
+    EXPECT_MPFR_MATCH_ALL_ROUNDING(mpfr::Operation::Exp, x,
+                                   LIBC_NAMESPACE::expf16(x), 0.5);
+  }
+}
+
+TEST_F(LlvmLibcExpf16Test, NegativeRange) {
+  for (uint16_t v = NEG_START; v <= NEG_STOP; ++v) {
+    float16 x = FPBits(v).get_val();
+    EXPECT_MPFR_MATCH_ALL_ROUNDING(mpfr::Operation::Exp, x,
+                                   LIBC_NAMESPACE::expf16(x), 0.5);
+  }
+}
diff --git a/libc/test/src/math/performance_testing/CMakeLists.txt b/libc/test/src/math/performance_testing/CMakeLists.txt
@@ -175,6 +175,17 @@ add_perf_binary(
     -fno-builtin
 )
 
+add_perf_binary(
+  expf16_perf
+  SRCS
+    expf16_perf.cpp
+  DEPENDS
+    .single_input_single_output_diff
+    libc.src.math.expf16
+  COMPILE_OPTIONS
+    -fno-builtin
+)
+
 add_perf_binary(
   fabsf_perf
   SRCS