[libc-commits] [libc] [libc] [search] implement hcreate(_r)/hsearch(_r)/hdestroy(_r) (PR #73469)

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+//===-- Portable string hash function ---------------------------*- 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_HASH_H
+#define LLVM_LIBC_SRC___SUPPORT_HASH_H
+
+#include "src/__support/UInt128.h" // UInt128
+#include "src/__support/macros/attributes.h"
+#include <stdint.h> // For uint64_t
+
+namespace LIBC_NAMESPACE {
+namespace internal {
+
+// Folded multiplication.
+// This function multiplies two 64-bit integers and xor the high and
+// low 64-bit parts of the result.
+LIBC_INLINE static uint64_t folded_multiply(uint64_t x, uint64_t y) {
+  UInt128 mask = static_cast<UInt128>(0xffffffffffffffff);
+  UInt128 p = static_cast<UInt128>(x) * static_cast<UInt128>(y);
+  uint64_t low = static_cast<uint64_t>(p & mask);
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lntue wrote:

I don't think you need the lower 64-bit mask here.  It should be well-defined in the same way as `static_cast<uint32_t>` from a `uint64_t`.

https://github.com/llvm/llvm-project/pull/73469