[libc-commits] [libc] [libc][math][c++23] Add log2bf16 math function (PR #177275)

[via libc-commits]

================
@@ -0,0 +1,128 @@
+//===-- Double-precision log2(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___SUPPORT_MATH_LOG2BF16_H
+#define LLVM_LIBC_SRC___SUPPORT_MATH_LOG2BF16_H
+
+#include "src/__support/FPUtil/FPBits.h"
+#include "src/__support/FPUtil/bfloat16.h"
+#include "src/__support/FPUtil/cast.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/optimization.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+namespace math {
+
+// Generated by Sollya with the following commands:
+//   > display = hexadecimal;
+//   > for i from 0 to 127 do print(round(log2(1 + i * 2^-7), SG, RN));
+static constexpr float LOG2F_1_PLUS_M[128] = {
+    0x0.0p0f,       0x1.6fe50cp-7f, 0x1.6e7968p-6f, 0x1.11cd1ep-5f,
+    0x1.6bad38p-5f, 0x1.c4dfacp-5f, 0x1.0eb38ap-4f, 0x1.3aa2fep-4f,
+    0x1.663f7p-4f,  0x1.918a16p-4f, 0x1.bc8424p-4f, 0x1.e72ec2p-4f,
+    0x1.08c588p-3f, 0x1.1dcd1ap-3f, 0x1.32ae9ep-3f, 0x1.476aap-3f,
+    0x1.5c01a4p-3f, 0x1.70742ep-3f, 0x1.84c2bep-3f, 0x1.98eddp-3f,
+    0x1.acf5e2p-3f, 0x1.c0db6cp-3f, 0x1.d49ee4p-3f, 0x1.e840bep-3f,
+    0x1.fbc16cp-3f, 0x1.0790aep-2f, 0x1.11307ep-2f, 0x1.1ac05cp-2f,
+    0x1.24407ap-2f, 0x1.2db11p-2f,  0x1.37124cp-2f, 0x1.406464p-2f,
+    0x1.49a784p-2f, 0x1.52dbep-2f,  0x1.5c01a4p-2f, 0x1.6518fep-2f,
+    0x1.6e221cp-2f, 0x1.771d2cp-2f, 0x1.800a56p-2f, 0x1.88e9c8p-2f,
+    0x1.91bba8p-2f, 0x1.9a8024p-2f, 0x1.a3376p-2f,  0x1.abe188p-2f,
+    0x1.b47ecp-2f,  0x1.bd0f2ep-2f, 0x1.c592fap-2f, 0x1.ce0a4ap-2f,
+    0x1.d6753ep-2f, 0x1.ded3fep-2f, 0x1.e726aap-2f, 0x1.ef6d68p-2f,
+    0x1.f7a856p-2f, 0x1.ffd79ap-2f, 0x1.03fda8p-1f, 0x1.0809dp-1f,
+    0x1.0c105p-1f,  0x1.10113cp-1f, 0x1.140cap-1f,  0x1.18028cp-1f,
+    0x1.1bf312p-1f, 0x1.1fde3ep-1f, 0x1.23c41ep-1f, 0x1.27a4cp-1f,
+    0x1.2b8034p-1f, 0x1.2f5688p-1f, 0x1.3327c6p-1f, 0x1.36f4p-1f,
+    0x1.3abb4p-1f,  0x1.3e7d94p-1f, 0x1.423b08p-1f, 0x1.45f3aap-1f,
+    0x1.49a784p-1f, 0x1.4d56a6p-1f, 0x1.510118p-1f, 0x1.54a6e8p-1f,
+    0x1.584822p-1f, 0x1.5be4dp-1f,  0x1.5f7dp-1f,   0x1.6310b8p-1f,
+    0x1.66a008p-1f, 0x1.6a2afap-1f, 0x1.6db196p-1f, 0x1.7133eap-1f,
+    0x1.74b1fep-1f, 0x1.782bdcp-1f, 0x1.7ba19p-1f,  0x1.7f1322p-1f,
+    0x1.82809ep-1f, 0x1.85ea0cp-1f, 0x1.894f74p-1f, 0x1.8cb0e4p-1f,
+    0x1.900e62p-1f, 0x1.9367f6p-1f, 0x1.96bdaep-1f, 0x1.9a0f8ep-1f,
+    0x1.9d5dap-1f,  0x1.a0a7eep-1f, 0x1.a3ee8p-1f,  0x1.a7315ep-1f,
+    0x1.aa709p-1f,  0x1.adac1ep-1f, 0x1.b0e412p-1f, 0x1.b41874p-1f,
+    0x1.b74948p-1f, 0x1.ba769cp-1f, 0x1.bda072p-1f, 0x1.c0c6d4p-1f,
+    0x1.c3e9cap-1f, 0x1.c7095ap-1f, 0x1.ca258ep-1f, 0x1.cd3e6ap-1f,
+    0x1.d053f6p-1f, 0x1.d3663cp-1f, 0x1.d6753ep-1f, 0x1.d98106p-1f,
+    0x1.dc899ap-1f, 0x1.df8f02p-1f, 0x1.e29142p-1f, 0x1.e59064p-1f,
+    0x1.e88c6cp-1f, 0x1.eb856p-1f,  0x1.ee7b48p-1f, 0x1.f16e28p-1f,
+    0x1.f45e08p-1f, 0x1.f74afp-1f,  0x1.fa34e2p-1f, 0x1.fd1be4p-1f,
+};
+
+LIBC_INLINE static bfloat16 log2bf16(bfloat16 x) {
+  using FPBits = fputil::FPBits<bfloat16>;
+  FPBits x_bits(x);
+
+  uint16_t x_u = x_bits.uintval();
+
+  // If x <= 0, or x is 1, or x is +inf, or x is NaN.
+  if (LIBC_UNLIKELY(x_u == 0U || x_u == 0x3f80U || x_u >= 0x7f80U)) {
+    // log(NaN) = NaN
----------------
lntue wrote:

update comments from `log` to `log2` in this file.

https://github.com/llvm/llvm-project/pull/177275