Helpers for pointer tagging

Detailed Description

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

 // 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);
 }

Files
file	ptrtag.h
	Pointer Tagging Helpers.

Macros
#define	PTRTAG   __attribute__((aligned(4)))
	Pointers to data marked with this attribute will be tag-able.

Functions
static void *	ptrtag (void *ptr, uint8_t tag)
	Create a tagged pointer.
static void *	ptrtag_ptr (void *tagged_ptr)
	Extract the original pointer from a tagged pointer.
static uint8_t	ptrtag_tag (void *tagged_ptr)
	Extract the tag from a tagged pointer.

Macro Definition Documentation

PTRTAG

#define PTRTAG   __attribute__((aligned(4)))

Pointers to data marked with this attribute will be tag-able.

This will ensure a minimum alignment of 4 bytes

Definition at line 77 of file ptrtag.h.

Function Documentation

ptrtag()

static void * ptrtag ( void *  ptr, uint8_t  tag  )

inlinestatic

Create a tagged pointer.

Parameters

ptr	Pointer to tag
tag	Tag to add

Returns

Tagged pointer encoding both ptr and tag

Precondition

ptr points to data marked with PTRTAG

tag contains a two bit value (its numeric value 0, 1, 2, or 3)

Expect assertions blowing up when the preconditions are not met.

Definition at line 90 of file ptrtag.h.

ptrtag_ptr()

static void * ptrtag_ptr ( void *  tagged_ptr )

inlinestatic

Extract the original pointer from a tagged pointer.

Parameters

tagged_ptr

The tagged pointer to extract the original pointer from

Returns

The original "un-tagged" pointer encoded in tagged_ptr

Definition at line 103 of file ptrtag.h.

ptrtag_tag()

static uint8_t ptrtag_tag ( void *  tagged_ptr )

inlinestatic

Extract the tag from a tagged pointer.

Parameters

tagged_ptr

The tagged pointer to extract the original pointer from

Returns

The tag encoded into tagged_ptr

Definition at line 115 of file ptrtag.h.