[libc-commits] [libc] [libc][math] Implement powf function correctly rounded to all rounding modes. (PR #71188)

[via libc-commits]

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@@ -0,0 +1,840 @@
+//===-- Single-precision x^y 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/powf.h"
+#include "common_constants.h" // Lookup tables EXP_M1 and EXP_M2.
+#include "src/__support/CPP/bit.h"
+#include "src/__support/CPP/optional.h"
+#include "src/__support/FPUtil/FPBits.h"
+#include "src/__support/FPUtil/PolyEval.h"
+#include "src/__support/FPUtil/double_double.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/FPUtil/sqrt.h"
+#include "src/__support/builtin_wrappers.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/optimization.h" // LIBC_UNLIKELY
+#include "src/math/exp10f.h"
+#include "src/math/exp2f.h"
+
+#include <errno.h>
+
+namespace LIBC_NAMESPACE {
+
+using fputil::DoubleDouble;
+using fputil::TripleDouble;
+
+namespace {
+
+#ifdef LIBC_TARGET_CPU_HAS_FMA
+constexpr uint64_t ERR = 64;
+#else
+constexpr uint64_t ERR = 128;
+#endif // LIBC_TARGET_CPU_HAS_FMA
+
+// We choose the precision of the high part to be 53 - 24 - 8, so that when
+//   y * (e_x + LOG2_R_DD[i].hi) is exact.
+// Generated by Sollya with:
+// > for i from 0 to 127 do {
+//     r = 2^-8 * ceil(2^8 * (1 - 2^-8) / (1 + i * 2^-7) );
+//     a = -log2(r);
+//     b = round(1 + a, 53 - 24 - 8, RN) - 1;
+//     c = round(a - b, D, RN);
+//     d = round(a - b - c, D, RN);
+//     print("{", d, ",", c, ", ", b, "},");
+//    };
+static constexpr TripleDouble LOG2_R_TD[128] = {
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michaelrj-google wrote:

should these tables be in `common_constants.h`?

https://github.com/llvm/llvm-project/pull/71188