10. Roots

Roots tell the garbage collector where to start tracing. The garbage collector determines which blocks are reachable from the roots, and (in automatically managed pools) reclaims the unreachable blocks. This is quite efficient and can be a very good approximation to liveness.

It is therefore important that all references that the client program can directly access are registered as roots, otherwise the garbage collector might recycle an object that would be used in the future. Some collectors, for example Boehm’s, assume that all references stored in static data are roots; the Memory Pool System is more flexible, but requires the client program to declare which references are roots.

10.1. Registering roots

You can register a root at any time by calling one of the mps_root_create functions. Roots may not be registered twice, and no two roots may overlap (that is, each reference is fixed by at most one root). Roots may be:

1. on the program’s control stack;
2. in the program’s static data;
3. in heap not managed by the MPS (provided that you destroy the root before freeing it; see the Scheme interpreter’s global symbol table for an example);
4. in manually managed pools (provided that you remove the root before freeing it).

Roots must not be in memory that is subject to garbage collection (and so roots must not be in automatically managed pools).

When you register a root you describe to the MPS how to scan it for references, providing your own scanning function in the cases of mps_root_create() and mps_root_create_fmt(). Such a root scanning function must follow the Scanning protocol.

All the references in a root are of the same rank (just as in a formatted object). So they are all exact, ambiguous or weak.

Note

If the rank of the root is exact, or weak, the references in the root must always be valid while the root is registered: that is, they must be references to actual objects or null pointers. This could be immediately after the root is registered, so the root must be valid before it is registered.

Note

As with scanning in general, it’s safe to fix references that point to memory not managed by the MPS. These will be ignored.

Roots can be deregistered at any time by calling mps_root_destroy(). All roots registered in an arena must be deregistered before the arena is destroyed.

There are five ways to register a root, depending on how you need to scan it for references:

1. mps_root_create() if you need a custom root scanning function (of type mps_root_scan_t);
2. mps_root_create_fmt() if the root consists of a block of objects belonging to an object format, which can be scanned by the format’s scan method (of type mps_fmt_scan_t);
3. mps_root_create_table() if the root consists of a table of references;
4. mps_root_create_table_masked() if the root consists of a table of tagged references;
5. mps_root_create_reg() if the root consists of the registers and control stack of a thread. See Thread roots below.

10.2. Cautions

Creating a root and then registering is similar to reserving a block and then committing it (in the Allocation point protocol), and similar cautions apply. Before registering a root:

1. The root must be valid (that is, the appropriate root scanning function can scan it).
2. All exact references in the root (references that are fixed by the root scanning function) must contain valid references or null pointers.
3. You must not store a reference in the root to a block in an automatically managed pool (such a reference is hidden from the MPS until you register the root, and may become invalid).

So the typical sequence of operations when creating a root is:

1. Initialize references in the root with null pointers or other safe values.
2. Register the root.
3. Fill in the references in the root.

10.3. Thread roots

Every thread’s registers and control stack potentially contain references to allocated objects, so should be registered as a root by calling mps_root_create_reg(). It’s not easy to write a scanner for the registers and the stack: it depends on the operating system, the processor architecture, and in some cases on the compiler. For this reason, the MPS provides mps_stack_scan_ambig() (and in fact, this is the only supported stack scanner).

A stack scanner needs to know how to find the bottom of the part of the stack to scan. The bottom of the relevant part of stack can be found by taking the address of a local variable in the function that calls the main work function of your thread. You should take care to ensure that the work function is not inlined so that the address is definitely in the stack frame below any potential roots.

For example, here’s the code from the toy Scheme interpreter that registers a thread root and then calls the program:

mps_thr_t thread;
mps_root_t reg_root;
int exit_code;
void *marker = ▮

res = mps_thread_reg(&thread, arena);
if (res != MPS_RES_OK) error("Couldn't register thread");

res = mps_root_create_reg(&reg_root,
                          arena,
                          mps_rank_ambig(),
                          0,
                          thread,
                          mps_stack_scan_ambig,
                          marker,
                          0);
if (res != MPS_RES_OK) error("Couldn't create root");

exit_code = start(argc, argv);

mps_root_destroy(reg_root);
mps_thread_dereg(thread);

10.4. Ranks

mps_rank_t

The type of ranks. It is a transparent alias for unsigned int, provided for convenience and clarity.

mps_rank_t mps_rank_ambig(void)

Return the rank of ambiguous roots.

mps_rank_t mps_rank_exact(void)

Return the rank of exact roots.

mps_rank_t mps_rank_weak(void)

Return the rank of weak roots.

10.5. Root modes

The root mode provides a way for the client to declare various facts about a root that allow the MPS to make optimizations. Roots that are declared to be constant need not be re-scanned, and roots that are declared to be protectable may have barriers placed on them, allowing the MPS to detect whether they have changed.

Note

The MPS does not currently perform either of these optimizations, so root modes have no effect. These features may be added in a future release.

mps_rm_t

The type of root modes.

It should be zero (meaning neither constant or protectable), or the sum of some subset of MPS_RM_CONST and MPS_RM_PROT.

MPS_RM_CONST

Deprecated

starting with version 1.111.

This was introduced in the hope of being able to maintain a remembered set for the root without needing a write barrier, but it can’t work as described, since you can’t reliably create a valid registered constant root that contains any references. (If you add the references before registering the root, they may have become invalid; but you can’t add them afterwards because the root is supposed to be constant.)

The root mode for constant roots. This tells the MPS that the client program will not change the root after it is registered: that is, scanning the root will produce the same set of references every time. Furthermore, for roots registered by mps_root_create_fmt() and mps_root_create_table(), the client program will not write to the root at all.

MPS_RM_PROT

The root mode for protectable roots. This tells the MPS that it may place a barrier⁽¹⁾ on any page which any part of the root covers. No format method or scan method (except for the one for this root) may write data in this root. They may read it.

Note

You must not specify MPS_RM_PROT on a root allocated by the MPS.

No page may contain parts of two or more protectable roots. You mustn’t specify MPS_RM_PROT if the client program or anything other than (this instance of) the MPS is going to protect or unprotect the relevant pages.

This mode may not be suitable if the client program wants the operating system to be able to access the root. Many operating systems can’t cope with writing to protected pages.

10.6. Root interface

mps_root_t

The type of root descriptions.

The arena uses root descriptions to find references within the client program’s roots.

mps_res_t mps_root_create(mps_root_t *root_o, mps_arena_t arena, mps_rank_t rank, mps_rm_t rm, mps_root_scan_t root_scan, void *p, size_t s)

Register a root that consists of the references fixed by a scanning function.

root_o points to a location that will hold the address of the new root description.

arena is the arena.

rank is the rank of references in the root.

rm is the root mode.

root_scan is the root scanning function. See mps_root_scan_t.

p and s are arguments that will be passed to root_scan each time it is called. This is intended to make it easy to pass, for example, an array and its size as parameters.

Returns MPS_RES_OK if the root was registered successfully, MPS_RES_MEMORY if the new root description could not be allocated, or another result code if there was another error.

The registered root description persists until it is destroyed by calling mps_root_destroy().

mps_res_t (*mps_root_scan_t)(mps_ss_t ss, void *p, size_t s)

The type of root scanning functions for mps_root_create().

ss is the scan state. It must be passed to MPS_SCAN_BEGIN() and MPS_SCAN_END() to delimit a sequence of fix operations, and to the functions MPS_FIX1() and MPS_FIX2() when fixing a reference.

p and s are the corresponding values that were passed to mps_root_create().

Returns a result code. If a fix function returns a value other than MPS_RES_OK, the scan method must return that value, and may return without fixing any further references. Generally, it is better if it returns as soon as possible. If the scanning is completed successfully, the function should return MPS_RES_OK.

mps_res_t mps_root_create_fmt(mps_root_t *root_o, mps_arena_t arena, mps_rank_t rank, mps_rm_t rm, mps_fmt_scan_t fmt_scan, mps_addr_t base, mps_addr_t limit)

Register a root that consists of the references fixed by a scanning function in a block of formatted objects.

root_o points to a location that will hold the address of the new root description.

arena is the arena.

rank is the rank of references in the root.

rm is the root mode.

fmt_scan is a scanning function. See mps_fmt_scan_t.

base is the address of the base of the block of formatted objects.

limit is the address just beyond the end of the block of formatted objects.

Returns MPS_RES_OK if the root was registered successfully, MPS_RES_MEMORY if the new root description could not be allocated, or another result code if there was another error.

The registered root description persists until it is destroyed by calling mps_root_destroy().

mps_res_t mps_root_create_reg(mps_root_t *root_o, mps_arena_t arena, mps_rank_t rank, mps_rm_t rm, mps_thr_t thr, mps_reg_scan_t reg_scan, void *p, size_t s)

Register a root that consists of the references fixed in a thread’s stack by a scanning function.

root_o points to a location that will hold the address of the new root description.

arena is the arena.

rank is the rank of references in the root.

rm is the root mode.

thr is the thread.

reg_scan is a scanning function. See mps_reg_scan_t.

p and s are arguments that will be passed to reg_scan each time it is called. This is intended to make it easy to pass, for example, an array and its size as parameters.

Returns MPS_RES_OK if the root was registered successfully, MPS_RES_MEMORY if the new root description could not be allocated, or another result code if there was another error.

The registered root description persists until it is destroyed by calling mps_root_destroy().

Note

It is not supported for client programs to pass their own scanning functions to this function. The built-in MPS function mps_stack_scan_ambig() must be used.

This function is intended as a hook should we ever need to allow client-specific extension or customization of stack and register scanning. If you’re in a position where you need this, for example, if you’re writing a compiler and have control over what goes in the registers, contact us.

mps_res_t (*mps_reg_scan_t)(mps_ss_t ss, mps_thr_t thr, void *p, size_t s)

The type of a root scanning function for roots created with mps_root_create_reg().

ss is the scan state. It must be passed to MPS_SCAN_BEGIN() and MPS_SCAN_END() to delimit a sequence of fix operations, and to the functions MPS_FIX1() and MPS_FIX2() when fixing a reference.

thr is the thread.

p and s are the corresponding values that were passed to mps_root_create_reg().

Returns a result code. If a fix function returns a value other than MPS_RES_OK, the scan method must return that value, and may return without fixing any further references. Generally, it is better if it returns as soon as possible. If the scanning is completed successfully, the function should return MPS_RES_OK.

A root scan method is called whenever the MPS needs to scan the root. It must then indicate references within the root by calling MPS_FIX1() and MPS_FIX2().

See also

Scanning.

Note

Client programs are not expected to write scanning functions of this type. The built-in MPS function mps_stack_scan_ambig() must be used.

mps_reg_scan_t mps_stack_scan_ambig()

A root scanning function for ambiguous scanning of threads, suitable for passing to mps_root_create_reg().

It scans all integer registers and everything on the stack of the thread given, and can therefore only be used with ambiguous roots. It only scans locations that are at, or higher on the stack (that is, more recently added), the stack bottom that was passed to mps_thread_reg(). References are assumed to be represented as machine words, and are required to be 4-byte-aligned; unaligned values are ignored.

Note

The MPS provides this function because it’s hard to write: it depends on the operating system, the processor architecture, and in some cases on the compiler.

mps_res_t mps_root_create_table(mps_root_t *root_o, mps_arena_t arena, mps_rank_t rank, mps_rm_t rm, mps_addr_t *base, size_t count)

Register a root that consists of a vector of references.

root_o points to a location that will hold the address of the new root description.

arena is the arena.

rank is the rank of references in the root.

rm is the root mode.

base points to a vector of references.

count is the number of references in the vector.

Returns MPS_RES_OK if the root was registered successfully, MPS_RES_MEMORY if the new root description could not be allocated, or another result code if there was another error.

The registered root description persists until it is destroyed by calling mps_root_destroy().

mps_res_t mps_root_create_table_masked(mps_root_t *root_o, mps_arena_t arena, mps_rank_t rank, mps_rm_t rm, mps_addr_t *base, size_t count, mps_word_t mask)

Register a root that consists of a vector of tagged references.

root_o points to a location that will hold the address of the new root description.

arena is the arena.

rank is the rank of references in the root.

rm is the root mode.

base points to a vector of tagged references.

count is the number of tagged references in the vector.

mask is a bitmask whose set bits specify the location of the tag. References are assumed to have a tag of zero: any value in the vector with a non-zero tag is ignored.

Returns MPS_RES_OK if the root was registered successfully, MPS_RES_MEMORY if the new root description could not be allocated, or another result code if there was another error.

The registered root description persists until it is destroyed by calling mps_root_destroy().

For example:

#define TAG_MASK 0x3            /* bottom two bits */

/* Global symbol table. */
size_t symtab_size;
struct {
    obj_t symbol;
    obj_t value;
} *symtab;

mps_res_t res;
mps_root_t root;
res = mps_root_create_table_masked(&root, arena,
                                   mps_rank_exact(),
                                   (mps_rm_t)0,
                                   symtab, symtab_size * 2,
                                   (mps_word_t)TAG_MASK);
if (res != MPS_RES_OK) errror("can't create symtab root");

void mps_root_destroy(mps_root_t root)

Deregister a root and destroy its description.

root is the root.

10.7. Root introspection

void mps_arena_roots_walk(mps_arena_t arena, mps_roots_stepper_t f, void *p, size_t s)

Deprecated

starting with version 1.111.

If you think you need this, there’s probably a better way to achieve what you’re trying to do. Contact us.

Visit references in registered roots in an arena.

arena is the arena whose roots you want to visit.

f is a function that will be called for each reference to an object in an automatically managed pool class that was found in a registered root belonging to the arena. It takes four arguments: ref is the address of a reference to an object in the arena, root is the root in which ref was found, and p and s are the corresponding arguments that were passed to mps_arena_roots_walk().

p and s are arguments that will be passed to f each time it is called. This is intended to make it easy to pass, for example, an array and its size as parameters.

This function may only be called when the arena is in the parked state.

See also

Arenas.

Note

If a root is ambiguous then the reference might not be to the start of an object; the client program should handle this case. There is no guarantee that the reference corresponds to the actual location that holds the pointer to the object (since this might be a register, for example), but the actual location will be passed if possible. This may aid analysis of roots via a debugger.

void (*mps_roots_stepper_t)(mps_addr_t *ref, mps_root_t root, void *p, size_t s)

The type of a root stepper function.

A function of this type can be passed to mps_arena_roots_walk(), in which case it will be called for each reference into the arena from a root registered with the arena. It receives four arguments:

ref points to a reference in a root. The reference points to something in the arena. If the root is exact then the reference points to the start of an allocated block, but if the root is ambiguous it might point to somewhere in the middle of an allocated block.

root is the description of the root which contains ref.

p and s are the corresponding values that were passed to mps_arena_roots_walk().