On 2024-10-15 12:13, Morten Brørup wrote:
+void *
+rte_lcore_var_alloc(size_t size, size_t align)
+{
+ /* Having the per-lcore buffer size aligned on cache lines
+ * assures as well as having the base pointer aligned on cache
+ * size assures that aligned offsets also translate to alipgned
+ * pointers across all values.
+ */
+ RTE_BUILD_BUG_ON(RTE_MAX_LCORE_VAR % RTE_CACHE_LINE_SIZE != 0);
+ RTE_VERIFY(align <= RTE_CACHE_LINE_SIZE);
+ RTE_VERIFY(size <= RTE_MAX_LCORE_VAR);
+
+ /* '0' means asking for worst-case alignment requirements */
+ if (align == 0)
+#ifdef RTE_TOOLCHAIN_MSVC
+ /* MSVC <stddef.h> is missing the max_align_t typedef */
+ align = alignof(double);
+#else
+ align = alignof(max_align_t);
+#endif
Do we need worst-case alignment, or does automatic alignment suffice:
I think the term is "natural alignment." As I think I mentioned at some
point, I don't really have an opinion.
Worst case (max_alignt_t) alignment is the same as malloc(), so
potentially what the user may expect. On the other hand, I can't see why
natural alignment (or alignof(max_align_t), whichever is smallest) would
not always suffice. It is a bit harder to explain in the API docs what
alignment you actually get in case you don't go for worst-case alignment.
I think it doesn't matter much, because the user will very likely use
the typed macros (and get whatever alignment the compiler deems
appropriate for that type).
/* '0' means asking for automatic alignment requirements */
if (align == 0) {
#ifdef RTE_ARCH_64
align = rte_align64pow2(size);
#else
align = rte_align32pow2(size);
#endif
#ifdef RTE_TOOLCHAIN_MSVC
/* MSVC <stddef.h> is missing the max_align_t typedef */
align = RTE_MIN(align, alignof(double));
#else
align = RTE_MIN(align, alignof(max_align_t));
#endif
}
It will pack small-size lcore variables even tighter.
