[libc][math] Add place-holder implementation for pow function.

[lntue]

Summary

Add place-holder implementation for pow function to unblock libc demo
examples.

[michaelrj]

LGTM

libc/config/linux/aarch64/entrypoints.txt

set(TARGET_LIBM_ENTRYPOINTS
     libc.src.math.modff
     libc.src.math.modfl
     libc.src.math.nearbyint
     libc.src.math.nearbyintf
     libc.src.math.nearbyintl
     libc.src.math.nextafter
     libc.src.math.nextafterf
     libc.src.math.nextafterl
+    libc.src.math.pow
     libc.src.math.remainderf
     libc.src.math.remainder
     libc.src.math.remainderl
     libc.src.math.remquof
     libc.src.math.remquo
     libc.src.math.remquol
     libc.src.math.rint
     libc.src.math.rintf

libc/config/linux/x86_64/entrypoints.txt

set(TARGET_LIBM_ENTRYPOINTS
     libc.src.math.modff
     libc.src.math.modfl
     libc.src.math.nearbyint
     libc.src.math.nearbyintf
     libc.src.math.nearbyintl
     libc.src.math.nextafter
     libc.src.math.nextafterf
     libc.src.math.nextafterl
+    libc.src.math.pow
     libc.src.math.remainderf
     libc.src.math.remainder
     libc.src.math.remainderl
     libc.src.math.remquof
     libc.src.math.remquo
     libc.src.math.remquol
     libc.src.math.rint
     libc.src.math.rintf

libc/config/windows/entrypoints.txt

set(TARGET_LIBM_ENTRYPOINTS
     libc.src.math.modff
     libc.src.math.modfl
     libc.src.math.nearbyint
     libc.src.math.nearbyintf
     libc.src.math.nearbyintl
     libc.src.math.nextafter
     libc.src.math.nextafterf
     libc.src.math.nextafterl
+    libc.src.math.pow
     libc.src.math.remainderf
     libc.src.math.remainder
     libc.src.math.remainderl
     libc.src.math.remquof
     libc.src.math.remquo
     libc.src.math.remquol
     libc.src.math.rint
     libc.src.math.rintf

libc/spec/stdc.td

HeaderSpec Math = HeaderSpec<
           FunctionSpec<"nearbyint", RetValSpec<DoubleType>, [ArgSpec<DoubleType>]>,
           FunctionSpec<"nearbyintf", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,
           FunctionSpec<"nearbyintl", RetValSpec<LongDoubleType>, [ArgSpec<LongDoubleType>]>,
 
           FunctionSpec<"nextafterf", RetValSpec<FloatType>, [ArgSpec<FloatType>, ArgSpec<FloatType>]>,
           FunctionSpec<"nextafter", RetValSpec<DoubleType>, [ArgSpec<DoubleType>, ArgSpec<DoubleType>]>,
           FunctionSpec<"nextafterl", RetValSpec<LongDoubleType>, [ArgSpec<LongDoubleType>, ArgSpec<LongDoubleType>]>,
 
+          FunctionSpec<"pow", RetValSpec<DoubleType>, [ArgSpec<DoubleType>, ArgSpec<DoubleType>]>,
+
           FunctionSpec<"coshf", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,
           FunctionSpec<"sinhf", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,
           FunctionSpec<"tanhf", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,
 
           FunctionSpec<"acosf", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,
           FunctionSpec<"asinf", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,
           FunctionSpec<"asin", RetValSpec<DoubleType>, [ArgSpec<DoubleType>]>,
           FunctionSpec<"atanf", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,

libc/src/math/CMakeLists.txt

 add_math_entrypoint_object(nearbyint)
 add_math_entrypoint_object(nearbyintf)
 add_math_entrypoint_object(nearbyintl)
 
 add_math_entrypoint_object(nextafter)
 add_math_entrypoint_object(nextafterf)
 add_math_entrypoint_object(nextafterl)
 
+add_math_entrypoint_object(pow)
+
 add_math_entrypoint_object(remainder)
 add_math_entrypoint_object(remainderf)
 add_math_entrypoint_object(remainderl)
 
 add_math_entrypoint_object(remquo)
 add_math_entrypoint_object(remquof)
 add_math_entrypoint_object(remquol)
 

libc/src/math/generic/CMakeLists.txt

add_entrypoint_object(
   DEPENDS
     .inv_trigf_utils
     .math_utils
     libc.src.__support.FPUtil.fp_bits
   COMPILE_OPTIONS
     -O3
 )
 
+add_entrypoint_object(
+  pow
+  SRCS
+    pow.cpp
+  HDRS
+    ../pow.h
+  DEPENDS
+    .expf
+    .logf
+  COMPILE_OPTIONS
+    -O3
+)

libc/src/math/generic/pow.cpp

+//===-- Implementation of pow 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/pow.h"
+#include "src/math/expf.h"
+#include "src/math/logf.h"
+
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+LLVM_LIBC_FUNCTION(double, pow, (double x, double y)) {
+  // Place-holder implementation for double precision pow function.
+  // TODO: Implement correctly rounded pow function for double precision.
+  return static_cast<double>(
+      expf(static_cast<float>(y) * logf(static_cast<float>(x))));
+}
+
+} // namespace __llvm_libc

libc/src/math/pow.h

+//===-- Implementation header for pow ---------------------------*- 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_POW_H
+#define LLVM_LIBC_SRC_MATH_POW_H
+
+namespace __llvm_libc {
+
+double pow(double x, double y);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_MATH_POW_H

libc/test/src/math/CMakeLists.txt

HDRS
     in_float_range_test_helper.h
   SRCS
     inv_trigf_utils_test.cpp
   DEPENDS
     libc.src.math.generic.inv_trigf_utils
     libc.src.__support.FPUtil.fp_bits
 )
 
+add_fp_unittest(
+  pow_test
+  NEED_MPFR
+  SUITE
+    libc_math_unittests
+  SRCS
+    pow_test.cpp
+  DEPENDS
+  libc.include.errno
+  libc.src.errno.errno
+  libc.src.math.pow
+)
+
 add_subdirectory(generic)
 add_subdirectory(exhaustive)
 add_subdirectory(differential_testing)

libc/test/src/math/pow_test.cpp

+//===-- Unittests for pow -------------------------------------------------===//
+//
+// 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/pow.h"
+#include "utils/MPFRWrapper/MPFRUtils.h"
+#include "utils/UnitTest/FPMatcher.h"
+#include "utils/UnitTest/Test.h"
+#include <math.h>
+
+#include <errno.h>
+#include <stdint.h>
+
+using FPBits = __llvm_libc::fputil::FPBits<double>;
+
+namespace mpfr = __llvm_libc::testing::mpfr;
+
+DECLARE_SPECIAL_CONSTANTS(double)
+
+TEST(LlvmLibcAsinTest, SpecialNumbers) {
+  errno = 0;
+
+  EXPECT_FP_EQ(aNaN, __llvm_libc::pow(aNaN, aNaN));
+  EXPECT_MATH_ERRNO(0);
+
+  EXPECT_FP_EQ(1.0, __llvm_libc::pow(1.0, 1.0));
+  EXPECT_MATH_ERRNO(0);
+
+  EXPECT_FP_EQ(1.0, __llvm_libc::pow(1.0, 2.0));
+  EXPECT_MATH_ERRNO(0);
+
+  EXPECT_FP_EQ(inf, __llvm_libc::pow(2.0, inf));
+}