[libc][math][c23] Add exp2f16 C23 math function (llvm#101217)

Part of llvm#95250.
banach-space · Aug 7, 2024 · 76c79aa · 76c79aa
1 parent b900731
commit 76c79aa
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Showing 13 changed files with 361 additions and 20 deletions.
diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
@@ -576,6 +576,7 @@ if(LIBC_TYPES_HAS_FLOAT16)
     libc.src.math.canonicalizef16
     libc.src.math.ceilf16
     libc.src.math.copysignf16
+    libc.src.math.exp2f16
     libc.src.math.expf16
     libc.src.math.f16add
     libc.src.math.f16addf

diff --git a/libc/docs/math/index.rst b/libc/docs/math/index.rst
@@ -290,7 +290,7 @@ Higher Math Functions
 +-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+
 | exp10m1   |                  |                 |                        |                      |                        | 7.12.6.3               | F.10.3.3                   |
 +-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+
-| exp2      | |check|          | |check|         |                        |                      |                        | 7.12.6.4               | F.10.3.4                   |
+| exp2      | |check|          | |check|         |                        | |check|              |                        | 7.12.6.4               | F.10.3.4                   |
 +-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+
 | exp2m1    | |check|          |                 |                        |                      |                        | 7.12.6.5               | F.10.3.5                   |
 +-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+

diff --git a/libc/spec/stdc.td b/libc/spec/stdc.td
@@ -582,6 +582,7 @@ def StdC : StandardSpec<"stdc"> {
 
           FunctionSpec<"exp2", RetValSpec<DoubleType>, [ArgSpec<DoubleType>]>,
           FunctionSpec<"exp2f", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,
+          GuardedFunctionSpec<"exp2f16", RetValSpec<Float16Type>, [ArgSpec<Float16Type>], "LIBC_TYPES_HAS_FLOAT16">,
 
           FunctionSpec<"exp2m1f", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,
 

diff --git a/libc/src/math/CMakeLists.txt b/libc/src/math/CMakeLists.txt
@@ -110,6 +110,7 @@ add_math_entrypoint_object(expf16)
 
 add_math_entrypoint_object(exp2)
 add_math_entrypoint_object(exp2f)
+add_math_entrypoint_object(exp2f16)
 
 add_math_entrypoint_object(exp2m1f)
 

diff --git a/libc/src/math/exp2f16.h b/libc/src/math/exp2f16.h
@@ -0,0 +1,21 @@
+//===-- Implementation header for exp2f16 -----------------------*- 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_EXP2F16_H
+#define LLVM_LIBC_SRC_MATH_EXP2F16_H
+
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/properties/types.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+float16 exp2f16(float16 x);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_MATH_EXP2F16_H
diff --git a/libc/src/math/generic/CMakeLists.txt b/libc/src/math/generic/CMakeLists.txt
@@ -1395,6 +1395,28 @@ add_entrypoint_object(
     -O3
 )
 
+add_entrypoint_object(
+  exp2f16
+  SRCS
+    exp2f16.cpp
+  HDRS
+    ../exp2f16.h
+  DEPENDS
+    libc.hdr.errno_macros
+    libc.hdr.fenv_macros
+    libc.src.__support.CPP.array
+    libc.src.__support.FPUtil.except_value_utils
+    libc.src.__support.FPUtil.fenv_impl
+    libc.src.__support.FPUtil.fp_bits
+    libc.src.__support.FPUtil.multiply_add
+    libc.src.__support.FPUtil.nearest_integer
+    libc.src.__support.FPUtil.polyeval
+    libc.src.__support.FPUtil.rounding_mode
+    libc.src.__support.macros.optimization
+  COMPILE_OPTIONS
+    -O3
+)
+
 add_entrypoint_object(
   exp2m1f
   SRCS

diff --git a/libc/src/math/generic/exp2f16.cpp b/libc/src/math/generic/exp2f16.cpp
@@ -0,0 +1,134 @@
+//===-- Half-precision 2^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/exp2f16.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, 3> EXP2F16_EXCEPTS = {{
+    // (input, RZ output, RU offset, RD offset, RN offset)
+    // x = 0x1.714p-11, exp2f16(x) = 0x1p+0 (RZ)
+    {0x11c5U, 0x3c00U, 1U, 0U, 1U},
+    // x = -0x1.558p-4, exp2f16(x) = 0x1.e34p-1 (RZ)
+    {0xad56U, 0x3b8dU, 1U, 0U, 0U},
+    // x = -0x1.d5cp-4, exp2f16(x) = 0x1.d8cp-1 (RZ)
+    {0xaf57U, 0x3b63U, 1U, 0U, 0U},
+}};
+
+// Generated by Sollya with the following commands:
+//   > display = hexadecimal;
+//   > for i from 0 to 7 do printsingle(round(2^(i * 2^-3), SG, RN));
+static constexpr cpp::array<uint32_t, 8> EXP2_MID_BITS = {
+    0x3f80'0000U, 0x3f8b'95c2U, 0x3f98'37f0U, 0x3fa5'fed7U,
+    0x3fb5'04f3U, 0x3fc5'672aU, 0x3fd7'44fdU, 0x3fea'c0c7U,
+};
+
+LLVM_LIBC_FUNCTION(float16, exp2f16, (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 |x| >= 16, or x is NaN.
+  if (LIBC_UNLIKELY(x_abs >= 0x4c00U)) {
+    // exp2(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 >= 16.
+    if (x_bits.is_pos()) {
+      // exp2(+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 <= -25.
+    if (x_u >= 0xce40U) {
+      // exp2(-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);
+
+      if (fputil::fenv_is_round_up())
+        return FPBits::min_subnormal().get_val();
+      return FPBits::zero().get_val();
+    }
+  }
+
+  if (auto r = EXP2F16_EXCEPTS.lookup(x_u); LIBC_UNLIKELY(r.has_value()))
+    return r.value();
+
+  // For -25 < x < 16, to compute 2^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,
+  //   2^x = 2^(hi + mid + lo) = 2^hi * 2^mid * 2^lo.
+  // We store 2^mid in the lookup table EXP2_MID_BITS, and compute 2^hi * 2^mid
+  // by adding hi to the exponent field of 2^mid.  2^lo is computed using a
+  // degree-3 minimax polynomial generated by Sollya.
+
+  float xf = 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);
+
+  uint32_t exp2_hi_mid_bits =
+      EXP2_MID_BITS[x_mid] +
+      static_cast<uint32_t>(x_hi << fputil::FPBits<float>::FRACTION_LEN);
+  float exp2_hi_mid = fputil::FPBits<float>(exp2_hi_mid_bits).get_val();
+  // Degree-3 minimax polynomial generated by Sollya with the following
+  // commands:
+  //   > display = hexadecimal;
+  //   > P = fpminimax((2^x - 1)/x, 2, [|SG...|], [-2^-4, 2^-4]);
+  //   > 1 + x * P;
+  float exp2_lo = fputil::polyeval(lo, 0x1p+0f, 0x1.62e43p-1f, 0x1.ec0aa6p-3f,
+                                   0x1.c6b4a6p-5f);
+  return static_cast<float16>(exp2_hi_mid * exp2_lo);
+}
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/test/src/math/CMakeLists.txt b/libc/test/src/math/CMakeLists.txt
@@ -939,6 +939,19 @@ add_fp_unittest(
     libc.src.math.expf16
 )
 
+add_fp_unittest(
+  exp2_test
+  NEED_MPFR
+  SUITE
+    libc-math-unittests
+  SRCS
+    exp2_test.cpp
+  DEPENDS
+    libc.src.errno.errno
+    libc.src.math.exp2
+    libc.src.__support.FPUtil.fp_bits
+)
+
 add_fp_unittest(
   exp2f_test
   NEED_MPFR
@@ -953,16 +966,14 @@ add_fp_unittest(
 )
 
 add_fp_unittest(
- exp2_test
- NEED_MPFR
- SUITE
-   libc-math-unittests
- SRCS
-   exp2_test.cpp
- DEPENDS
-   libc.src.errno.errno
-   libc.src.math.exp2
-   libc.src.__support.FPUtil.fp_bits
+  exp2f16_test
+  NEED_MPFR
+  SUITE
+    libc-math-unittests
+  SRCS
+    exp2f16_test.cpp
+  DEPENDS
+    libc.src.math.exp2f16
 )
 
 add_fp_unittest(

diff --git a/libc/test/src/math/exp2f16_test.cpp b/libc/test/src/math/exp2f16_test.cpp
@@ -0,0 +1,40 @@
+//===-- Exhaustive test for exp2f16 ---------------------------------------===//
+//
+// 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/exp2f16.h"
+#include "test/UnitTest/FPMatcher.h"
+#include "test/UnitTest/Test.h"
+#include "utils/MPFRWrapper/MPFRUtils.h"
+
+using LlvmLibcExp2f16Test = 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(LlvmLibcExp2f16Test, PositiveRange) {
+  for (uint16_t v = POS_START; v <= POS_STOP; ++v) {
+    float16 x = FPBits(v).get_val();
+    EXPECT_MPFR_MATCH_ALL_ROUNDING(mpfr::Operation::Exp2, x,
+                                   LIBC_NAMESPACE::exp2f16(x), 0.5);
+  }
+}
+
+TEST_F(LlvmLibcExp2f16Test, NegativeRange) {
+  for (uint16_t v = NEG_START; v <= NEG_STOP; ++v) {
+    float16 x = FPBits(v).get_val();
+    EXPECT_MPFR_MATCH_ALL_ROUNDING(mpfr::Operation::Exp2, x,
+                                   LIBC_NAMESPACE::exp2f16(x), 0.5);
+  }
+}
diff --git a/libc/test/src/math/performance_testing/CMakeLists.txt b/libc/test/src/math/performance_testing/CMakeLists.txt
@@ -164,6 +164,17 @@ add_perf_binary(
     -fno-builtin
 )
 
+add_perf_binary(
+  exp2f16_perf
+  SRCS
+    exp2f16_perf.cpp
+  DEPENDS
+    .single_input_single_output_diff
+    libc.src.math.exp2f16
+  COMPILE_OPTIONS
+    -fno-builtin
+)
+
 add_perf_binary(
   expf_perf
   SRCS

diff --git a/libc/test/src/math/performance_testing/exp2f16_perf.cpp b/libc/test/src/math/performance_testing/exp2f16_perf.cpp
@@ -0,0 +1,22 @@
+//===-- Performance test for exp2f16 --------------------------------------===//
+//
+// 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 "SingleInputSingleOutputPerf.h"
+
+#include "src/math/exp2f16.h"
+
+// LLVM libc might be the only libc implementation with support for float16 math
+// functions currently. We can't compare our float16 functions against the
+// system libc, so we compare them against this placeholder function.
+static float16 placeholderf16(float16 x) { return x; }
+
+int main() {
+  SINGLE_INPUT_SINGLE_OUTPUT_PERF_EX(float16, LIBC_NAMESPACE::exp2f16,
+                                     ::placeholderf16, 20'000,
+                                     "exp2f16_perf.log")
+}
diff --git a/libc/test/src/math/smoke/CMakeLists.txt b/libc/test/src/math/smoke/CMakeLists.txt
@@ -1029,6 +1029,18 @@ add_fp_unittest(
     libc.src.math.expf16
 )
 
+add_fp_unittest(
+  exp2_test
+  SUITE
+    libc-math-smoke-tests
+  SRCS
+    exp2_test.cpp
+  DEPENDS
+    libc.src.errno.errno
+    libc.src.math.exp2
+    libc.src.__support.FPUtil.fp_bits
+)
+
 add_fp_unittest(
   exp2f_test
   SUITE
@@ -1042,15 +1054,15 @@ add_fp_unittest(
 )
 
 add_fp_unittest(
- exp2_test
- SUITE
-   libc-math-smoke-tests
- SRCS
-   exp2_test.cpp
- DEPENDS
-   libc.src.errno.errno
-   libc.src.math.exp2
-   libc.src.__support.FPUtil.fp_bits
+  exp2f16_test
+  SUITE
+    libc-math-smoke-tests
+  SRCS
+    exp2f16_test.cpp
+  DEPENDS
+    libc.hdr.fenv_macros
+    libc.src.errno.errno
+    libc.src.math.exp2f16
 )
 
 add_fp_unittest(