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Finish the rewrite of the "bit squatting".

This commit is contained in:
Sylvain Pion 2008-06-03 11:43:25 +00:00
parent 3901116641
commit dfce30f55c
1 changed files with 7 additions and 16 deletions
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23
STL_Extension/include/CGAL/Compact_container.h
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@ -453,7 +453,7 @@ private:
// ptr is composed of a pointer part and the last 2 bits. // ptr is composed of a pointer part and the last 2 bits.
// Here is the meaning of each of the 8 cases. // Here is the meaning of each of the 8 cases.
// //
// value of the last 2 bits // value of the last 2 bits as "Type"
// pointer part 0 1 2 3 // pointer part 0 1 2 3
// NULL user elt unused free_list end start/end // NULL user elt unused free_list end start/end
// != NULL user elt block boundary free elt unused // != NULL user elt block boundary free elt unused
@ -462,26 +462,16 @@ private:
enum Type { USED = 0, BLOCK_BOUNDARY = 1, FREE = 2, START_END = 3 }; enum Type { USED = 0, BLOCK_BOUNDARY = 1, FREE = 2, START_END = 3 };
// Using a union is clean and should avoid aliasing problems. // The bit squatting is implemented by casting pointers to (char *), then
union menion { // subtracting to NULL, doing bit manipulations on the resulting integer,
void * p; // and converting back.
menion(void * ptr)
: p(ptr) {}
menion(void * ptr, Type type)
: p((void*) ((clean_pointer((char *) ptr)) + (int) type))
{
CGAL_precondition(0 <= type && type < 4);
}
};
static char * clean_pointer(char * p) static char * clean_pointer(char * p)
{ {
return ((p - (char *) NULL) & ~ (std::ptrdiff_t) START_END) + (char *) NULL; return ((p - (char *) NULL) & ~ (std::ptrdiff_t) START_END) + (char *) NULL;
} }
// Returns the pointee, cleaned from the squatted bits (the last 2 bits). // Returns the pointee, cleaned up from the squatted bits.
static pointer clean_pointee(const_pointer ptr) static pointer clean_pointee(const_pointer ptr)
{ {
return (pointer) clean_pointer((char *) Traits::pointer(*ptr)); return (pointer) clean_pointer((char *) Traits::pointer(*ptr));
@ -497,7 +487,8 @@ private:
// Sets the pointer part and the type of the pointee. // Sets the pointer part and the type of the pointee.
static void set_type(pointer ptr, void * p, Type t) static void set_type(pointer ptr, void * p, Type t)
{ {
Traits::pointer(*ptr) = menion(p, t).p; CGAL_precondition(0 <= t && t < 4);
Traits::pointer(*ptr) = (void *) ((clean_pointer((char *) p)) + (int) t);
} }
// We store a vector of pointers to all allocated blocks and their sizes. // We store a vector of pointers to all allocated blocks and their sizes.