libutil: Add alignUp helper function

src/libutil-tests/alignment.cc

@@ -0,0 +1,18 @@
+#include "nix/util/alignment.hh"
+
+#include <gtest/gtest.h>
+
+namespace nix {
+
+TEST(alignUp, value)
+{
+    for (uint64_t i = 1; i <= 8; ++i)
+        EXPECT_EQ(alignUp(i, 8), 8);
+}
+
+TEST(alignUp, notAPowerOf2)
+{
+    ASSERT_DEATH({ alignUp(1u, 42); }, "alignment must be a power of 2");
+}
+
+} // namespace nix

src/libutil-tests/meson.build

@@ -44,6 +44,7 @@ config_priv_h = configure_file(
 subdir('nix-meson-build-support/common')
 
 sources = files(
+  'alignment.cc',
   'archive.cc',
   'args.cc',
   'base-n.cc',

src/libutil/include/nix/util/alignment.hh

@@ -0,0 +1,23 @@
+#pragma once
+///@file
+
+#include <cassert>
+#include <type_traits>
+#include <cstdint>
+#include <bit>
+
+namespace nix {
+
+/// Aligns val upwards to be a multiple of alignment.
+///
+/// @pre alignment must be a power of 2.
+template<typename T>
+    requires std::is_unsigned_v<T>
+constexpr T alignUp(T val, unsigned alignment)
+{
+    assert(std::has_single_bit(alignment) && "alignment must be a power of 2");
+    T mask = ~(T{alignment} - 1u);
+    return (val + alignment - 1) & mask;
+}
+
+} // namespace nix

src/libutil/include/nix/util/meson.build

@@ -4,6 +4,7 @@ include_dirs = [ include_directories('../..') ]
 
 headers = files(
   'abstract-setting-to-json.hh',
+  'alignment.hh',
   'ansicolor.hh',
   'archive.hh',
   'args.hh',