[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 +1,141 @@
+//===-- Single-precision 10^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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_MATH_GENERIC_EXP10F_IMPL_H
+#define LLVM_LIBC_SRC_MATH_GENERIC_EXP10F_IMPL_H
+
+#include "explogxf.h"
+#include "src/__support/FPUtil/BasicOperations.h"
+#include "src/__support/FPUtil/FEnvImpl.h"
+#include "src/__support/FPUtil/FPBits.h"
+#include "src/__support/FPUtil/PolyEval.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/optimization.h" // LIBC_UNLIKELY
+#include "src/math/exp10f.h"
+
+#include <errno.h>
+
+namespace LIBC_NAMESPACE::generic {
+
+LIBC_INLINE float exp10f(float x) {
+  using FPBits = typename fputil::FPBits<float>;
+  FPBits xbits(x);
+
+  uint32_t x_u = xbits.uintval();
+  uint32_t x_abs = x_u & 0x7fff'ffffU;
+
+  // When |x| >= log10(2^128), or x is nan
+  if (LIBC_UNLIKELY(x_abs >= 0x421a'209bU)) {
+    // When x < log10(2^-150) or nan
+    if (x_u > 0xc234'9e35U) {
+      // exp(-Inf) = 0
+      if (xbits.is_inf())
+        return 0.0f;
+      // exp(nan) = nan
+      if (xbits.is_nan())
+        return x;
+      if (fputil::fenv_is_round_up())
+        return static_cast<float>(FPBits(FPBits::MIN_SUBNORMAL));
+      fputil::set_errno_if_required(ERANGE);
+      fputil::raise_except_if_required(FE_UNDERFLOW);
+      return 0.0f;
+    }
+    // x >= log10(2^128) or nan
+    if (!xbits.get_sign() && (x_u >= 0x421a'209bU)) {
+      // x is finite
+      if (x_u < 0x7f80'0000U) {
+        int rounding = fputil::quick_get_round();
+        if (rounding == FE_DOWNWARD || rounding == FE_TOWARDZERO)
+          return static_cast<float>(FPBits(FPBits::MAX_NORMAL));
+
+        fputil::set_errno_if_required(ERANGE);
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michaelrj-google wrote:

The usual way we've handled needing to set `errno` based on internal functions is by returning it in a struct (e.g. `str_to_num_result`). That would let you make the internal implementations object libraries while also sharing the complicated logic.

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