spawn/ResourceLimits: optimized hash function for MakeId()

The new loads a 64 bit integer at a time instead of calculating the
hash with each byte.

src/spawn/ResourceLimits.cxx

@@ -5,10 +5,9 @@
 #include "ResourceLimits.hxx"
 #include "lib/fmt/SystemError.hxx"
 #include "util/Base32.hxx"
-#include "util/djb_hash.hxx"
 #include "util/CharUtil.hxx"
+#include "util/IntHash.hxx"
 #include "util/Sanitizer.hxx"
-#include "util/SpanCast.hxx"
 
 #include <cassert>
 #include <charconv>
@@ -72,7 +71,7 @@ ResourceLimits::IsEmpty() const noexcept
 inline std::size_t
 ResourceLimits::GetHash() const noexcept
 {
-	return djb_hash(ReferenceAsBytes(*this));
+	return IntHashT(*this);
 }
 
 char *

src/util/IntHash.hxx

@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: BSD-2-Clause
+// Copyright CM4all GmbH
+// author: Max Kellermann <mk@cm4all.com>
+
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+#include <span>
+
+static constexpr std::size_t INT_HASH_INIT = 17;
+
+/**
+ * A very simple/naive and fast hash function for integers of
+ * arbitrary size.
+ */
+template<std::integral T>
+[[nodiscard]] [[gnu::always_inline]] [[gnu::hot]]
+constexpr std::size_t
+IntHashUpdate(T src, std::size_t hash) noexcept
+{
+	return (hash * 19) + static_cast<std::size_t>(src);
+}
+
+template<std::integral T, std::size_t extent=std::dynamic_extent>
+[[nodiscard]] [[gnu::hot]]
+constexpr std::size_t
+IntHash(std::span<const T, extent> src, std::size_t hash=INT_HASH_INIT) noexcept
+{
+	for (const T i : src)
+		hash = IntHashUpdate(i, hash);
+	return hash;
+}
+
+template<typename T, typename I>
+requires(std::has_unique_object_representations_v<T> && sizeof(T) % sizeof(I) == 0 && alignof(T) % sizeof(I) == 0)
+[[nodiscard]] [[gnu::hot]]
+constexpr std::size_t
+_IntHashT(const T &src, std::size_t hash=INT_HASH_INIT) noexcept
+{
+	constexpr std::size_t n = sizeof(src) / sizeof(I);
+	const std::span<const I, n> span{reinterpret_cast<const I *>(&src), n};
+	return IntHash(span, hash);
+}
+
+/**
+ * Calculate the hash of an arbitrary (trivial) object, using the
+ * largest integer according to the object's alignment at compile
+ * time.
+ */
+template<typename T>
+requires std::has_unique_object_representations_v<T>
+[[nodiscard]] [[gnu::hot]]
+constexpr std::size_t
+IntHashT(const T &src, std::size_t hash=INT_HASH_INIT) noexcept
+{
+	if constexpr (sizeof(T) % 8 == 0)
+	return _IntHashT<T, uint64_t>(src, hash);
+	else if constexpr (sizeof(T) % 4 == 0)
+	return _IntHashT<T, uint32_t>(src, hash);
+	else if constexpr (sizeof(T) % 2 == 0)
+	return _IntHashT<T, uint16_t>(src, hash);
+	else
+	return _IntHashT<T, uint8_t>(src, hash);
+}