Fix detail::alignment_for(size)

It must always be divisible by the size.

Author: foonathan

src/detail/align.cpp

@@ -12,10 +12,11 @@ bool foonathan::memory::detail::is_aligned(void* ptr, std::size_t alignment) noe
 {
     FOONATHAN_MEMORY_ASSERT(is_valid_alignment(alignment));
     auto address = reinterpret_cast<std::uintptr_t>(ptr);
-    return address % alignment == 0u;
+    return (address & (alignment - 1)) == 0u;
 }
 
 std::size_t foonathan::memory::detail::alignment_for(std::size_t size) noexcept
 {
-    return size >= max_alignment ? max_alignment : (std::size_t(1) << ilog2(size));
+    auto largest_possible_alignment = size & -size;
+    return largest_possible_alignment > max_alignment ? max_alignment : largest_possible_alignment;
 }

test/detail/align.cpp

@@ -54,12 +54,18 @@ TEST_CASE("detail::alignment_for")
     static_assert(max_alignment >= 8, "test case not working");
     REQUIRE(alignment_for(1) == 1);
     REQUIRE(alignment_for(2) == 2);
-    REQUIRE(alignment_for(3) == 2);
+    REQUIRE(alignment_for(3) == 1);
     REQUIRE(alignment_for(4) == 4);
-    REQUIRE(alignment_for(5) == 4);
-    REQUIRE(alignment_for(6) == 4);
-    REQUIRE(alignment_for(7) == 4);
+    REQUIRE(alignment_for(5) == 1);
+    REQUIRE(alignment_for(6) == 2);
+    REQUIRE(alignment_for(7) == 1);
     REQUIRE(alignment_for(8) == 8);
-    REQUIRE(alignment_for(9) == 8);
-    REQUIRE(alignment_for(100) == max_alignment);
+    REQUIRE(alignment_for(9) == 1);
+    REQUIRE(alignment_for(10) == 2);
+    REQUIRE(alignment_for(12) == 4);
+    REQUIRE(alignment_for(13) == 1);
+    REQUIRE(alignment_for(14) == 2);
+    REQUIRE(alignment_for(15) == 1);
+    REQUIRE(alignment_for(16) == max_alignment);
+    REQUIRE(alignment_for(1024) == max_alignment);
 }