sys: Add helpers for pointer tagging

sys/include/ptrtag.h

@@ -0,0 +1,126 @@
+/*
+ * Copyright (C) 2020 Otto-von-Guericke-Universität Magdeburg
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser General
+ * Public License v2.1. See the file LICENSE in the top level directory for more
+ * details.
+ */
+
+/**
+ * @defgroup    sys_ptrtag          Helpers for pointer tagging
+ * @ingroup     sys
+ *
+ * # Concept
+ *
+ * Data structures are often aligned in memory to the word size. On 32 bit
+ * machines, this often results in the two least significant bits containing no
+ * information (as they have to be zero, due to this alignment). In many cases
+ * gaining two bits of information storage in RAM is all you need. In these
+ * case, pointer tagging can come in handy.
+ *
+ * The tricky part is however to get this working portable on all architectures,
+ * possibly even on 8 bit machines that have no alignment requirements. This
+ * utility provides helpers to enforce alignment requirements for all platforms,
+ * so that pointer tagging can be used everywhere.
+ *
+ * # Usage
+ *
+ *  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.c}
+ *  // Use the PTRTAG attribute to ensure that pointers to this structure can
+ *  // be tagged, even if structure would not have a suitable alignment
+ *  // otherwise
+ *  typedef struct PTRTAG {
+ *     uint32_t bar;
+ * } foo_t;
+ *
+ *  void isr_callback(void *data) {
+ *      // extract pointer
+ *      struct foo *ptr = ptrtag_ptr(data);
+ *      // extract tag
+ *      uint8_t tag = ptrtag_tag(data);
+ *      work_on_data(ptr, tag);
+ *  }
+ *
+ *  int main(void) {
+ *      foo_t data;
+ *      uint8_t tag = 3;
+ *      // pack pointer and tag into tagged pointer
+ *      void *ptr_and_tag = ptrtag(&data, tag);
+ *      init_isr(params, isr_callback, ptr_and_tag);
+ *  }
+ *
+ *  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * @{
+ *
+ * @file
+ * @brief       Pointer Tagging Helpers
+ * @author      Marian Buschsieweke <marian.buschsieweke@ovgu.de>
+ */
+
+#ifndef PTRTAG_H
+#define PTRTAG_H
+
+#include <assert.h>
+#include <inttypes.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief   Pointers to data marked with this attribute will be tag-able.
+ *
+ * @details This will ensure a minimum alignment of 4 bytes
+ */
+#define PTRTAG  __attribute__((aligned(4)))
+
+/**
+ * @brief   Create a tagged pointer
+ * @param   ptr         Pointer to tag
+ * @param   tag         Tag to add
+ * @return              Tagged pointer encoding both @p ptr and @p tag
+ *
+ * @pre     @p ptr points to data marked with @ref PTRTAG
+ * @pre     @p tag contains a two bit value (its numeric value 0, 1, 2, or 3)
+ *
+ * Expect assertions blowing up when the preconditions are not met.
+ */
+static inline void * ptrtag(void *ptr, uint8_t tag)
+{
+    uintptr_t tmp = (uintptr_t)ptr;
+    /* ensure ptr is aligned to four bytes and tag fits in two bits */
+    assert((tag < 4) && !(tmp & 0x3));
+    return (void *)(tmp | tag);
+}
+
+/**
+ * @brief   Extract the original pointer from a tagged pointer
+ * @param   tagged_ptr  The tagged pointer to extract the original pointer from
+ * @return  The original "un-tagged" pointer encoded in @p tagged_ptr
+ */
+static inline void * ptrtag_ptr(void *tagged_ptr)
+{
+    uintptr_t tagged = (uintptr_t)tagged_ptr;
+    const uintptr_t mask = 0x3;
+    return (void *)(tagged & (~mask));
+}
+
+/**
+ * @brief   Extract the tag from a tagged pointer
+ * @param   tagged_ptr  The tagged pointer to extract the original pointer from
+ * @return  The tag encoded into @p tagged_ptr
+ */
+static inline uint8_t ptrtag_tag(void *tagged_ptr)
+{
+    uintptr_t tagged = (uintptr_t)tagged_ptr;
+    return tagged & 0x3;
+}
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* PTRTAG_H */
+/** @} */