[libc-commits] [libc] [llvm] [libc][math][c23] Add rsqrtf() function (PR #159615)

[via libc-commits]

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@@ -0,0 +1,71 @@
+//===-- Implementation header for rsqrtf ----------------------*- 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___SUPPORT_MATH_RSQRTF_H
+#define LLVM_LIBC_SRC___SUPPORT_MATH_RSQRTF_H
+
+#include "src/__support/FPUtil/FEnvImpl.h"
+#include "src/__support/FPUtil/FPBits.h"
+#include "src/__support/FPUtil/ManipulationFunctions.h"
+#include "src/__support/FPUtil/cast.h"
+#include "src/__support/FPUtil/multiply_add.h"
+#include "src/__support/FPUtil/sqrt.h"
+#include "src/__support/macros/optimization.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace math {
+
+LIBC_INLINE static constexpr float rsqrtf(float x) {
+  using FPBits = fputil::FPBits<float>;
+  FPBits xbits(x);
+
+  uint32_t x_u = xbits.uintval();
+  uint32_t x_abs = x_u & 0x7fff'ffffU;
+
+  constexpr uint32_t INF_BIT = FPBits::inf().uintval();
+
+  // x is 0, inf/nan, or negative.
+  if (LIBC_UNLIKELY(x_u == 0 || x_u >= INF_BIT)) {
+    // x is NaN
+    if (x_abs > INF_BIT) {
+      if (xbits.is_signaling_nan()) {
+        fputil::raise_except_if_required(FE_INVALID);
+        return FPBits::quiet_nan().get_val();
+      }
+      return x;
+    }
+
+    // |x| = 0
+    if (x_abs == 0) {
+      fputil::raise_except_if_required(FE_DIVBYZERO);
+      fputil::set_errno_if_required(ERANGE);
+      return FPBits::inf(xbits.sign()).get_val();
+    }
+
+    // -inf <= x < 0
+    if (x_u > 0x7fff'ffffU) {
+      fputil::raise_except_if_required(FE_INVALID);
+      fputil::set_errno_if_required(EDOM);
+      return FPBits::quiet_nan().get_val();
+    }
+
+    // x = +inf => rsqrt(x) = 0
+    return FPBits::zero().get_val();
----------------
overmighty wrote:

> rsqrt(±0) returns ±∞

F.10.4.9, https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf

```suggestion
    return FPBits::zero(xbits.sign()).get_val();
```

https://github.com/llvm/llvm-project/pull/159615