[libc-commits] [libc] [libc][stdfix] Add exp function for short _Accum and _Accum types. (PR #84391)

[Michael Jones via libc-commits]

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@@ -0,0 +1,92 @@
+//===-- Implementation of exphk 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 "exphk.h"
+#include "src/__support/CPP/bit.h"
+#include "src/__support/common.h"
+#include "src/__support/fixed_point/fx_bits.h"
+
+namespace LIBC_NAMESPACE {
+
+namespace {
+
+// Look up tables for exp(hi) and exp(mid).
+// Generated with Sollya:
+// > for i from 0 to 89 do {
+//     hi = floor(i/8) - 5;
+//     m = i/8 - floor(i/8) - 0.5;
+//     e_hi = nearestint(exp(hi) * 2^7) * 2^-7;
+//     e_mid = nearestint(exp(m) * 2^7) * 2^-7;
+//     print(hi, e_hi, m, e_mid);
+//   };
+// Notice that when i = 88 and 89, e_hi will overflow short accum range.
+static constexpr short accum EXP_HI[12] = {
+    0x1.0p-7hk, 0x1.0p-6hk, 0x1.8p-5hk,  0x1.1p-3hk,  0x1.78p-2hk,  0x1.0p0hk,
+    0x1.5cp1hk, 0x1.d9p2hk, 0x1.416p4hk, 0x1.b4dp5hk, 0x1.28d4p7hk, SACCUM_MAX,
+};
+
+static constexpr short accum EXP_MID[8] = {
+    0x1.38p-1hk, 0x1.6p-1hk, 0x1.9p-1hk, 0x1.c4p-1hk,
+    0x1.0p0hk,   0x1.22p0hk, 0x1.48p0hk, 0x1.74p0hk,
+};
+
+} // anonymous namespace
+
+LLVM_LIBC_FUNCTION(short accum, exphk, (short accum x)) {
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michaelrj-google wrote:

in future it would probably be better to have a shared backing function for `exp_fixed` but given there are only two functions for the moment this seems fine.

https://github.com/llvm/llvm-project/pull/84391