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<TITLE>Conservative GC Porting Directions</TITLE> | |
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<H1>Conservative GC Porting Directions</h1> | |
The collector is designed to be relatively easy to port, but is not | |
portable code per se. The collector inherently has to perform operations, | |
such as scanning the stack(s), that are not possible in portable C code. | |
<P> | |
All of the following assumes that the collector is being ported to a | |
byte-addressable 32- or 64-bit machine. Currently all successful ports | |
to 64-bit machines involve LP64 targets. The code base includes some | |
provisions for P64 targets (notably win64), but that has not been tested. | |
You are hereby discouraged from attempting a port to non-byte-addressable, | |
or 8-bit, or 16-bit machines. | |
<P> | |
The difficulty of porting the collector varies greatly depending on the needed | |
functionality. In the simplest case, only some small additions are needed | |
for the <TT>include/private/gcconfig.h</tt> file. This is described in the | |
following section. Later sections discuss some of the optional features, | |
which typically involve more porting effort. | |
<P> | |
Note that the collector makes heavy use of <TT>ifdef</tt>s. Unlike | |
some other software projects, we have concluded repeatedly that this is preferable | |
to system dependent files, with code duplicated between the files. | |
However, to keep this manageable, we do strongly believe in indenting | |
<TT>ifdef</tt>s correctly (for historical reasons usually without the leading | |
sharp sign). (Separate source files are of course fine if they don't result in | |
code duplication.) | |
<H2>Adding Platforms to <TT>gcconfig.h</tt></h2> | |
If neither thread support, nor tracing of dynamic library data is required, | |
these are often the only changes you will need to make. | |
<P> | |
The <TT>gcconfig.h</tt> file consists of three sections: | |
<OL> | |
<LI> A section that defines GC-internal macros | |
that identify the architecture (e.g. <TT>IA64</tt> or <TT>I386</tt>) | |
and operating system (e.g. <TT>LINUX</tt> or <TT>MSWIN32</tt>). | |
This is usually done by testing predefined macros. By defining | |
our own macros instead of using the predefined ones directly, we can | |
impose a bit more consistency, and somewhat isolate ourselves from | |
compiler differences. | |
<P> | |
It is relatively straightforward to add a new entry here. But please try | |
to be consistent with the existing code. In particular, 64-bit variants | |
of 32-bit architectures general are <I>not</i> treated as a new architecture. | |
Instead we explicitly test for 64-bit-ness in the few places in which it | |
matters. (The notable exception here is <TT>I386</tt> and <TT>X86_64</tt>. | |
This is partially historical, and partially justified by the fact that there | |
are arguably more substantial architecture and ABI differences here than | |
for RISC variants.) | |
<P> | |
on GNU-based systems, <TT>cpp -dM empty_source_file.c</tt> seems to generate | |
a set of predefined macros. On some other systems, the "verbose" | |
compiler option may do so, or the manual page may list them. | |
<LI> | |
A section that defines a small number of platform-specific macros, which are | |
then used directly by the collector. For simple ports, this is where most of | |
the effort is required. We describe the macros below. | |
<P> | |
This section contains a subsection for each architecture (enclosed in a | |
suitable <TT>ifdef</tt>. Each subsection usually contains some | |
architecture-dependent defines, followed by several sets of OS-dependent | |
defines, again enclosed in <TT>ifdef</tt>s. | |
<LI> | |
A section that fills in defaults for some macros left undefined in the preceding | |
section, and defines some other macros that rarely need adjustment for | |
new platforms. You will typically not have to touch these. | |
If you are porting to an OS that | |
was previously completely unsupported, it is likely that you will | |
need to add another clause to the definition of <TT>GET_MEM</tt>. | |
</ol> | |
The following macros must be defined correctly for each architecture and operating | |
system: | |
<DL> | |
<DT><TT>MACH_TYPE</tt> | |
<DD> | |
Defined to a string that represents the machine architecture. Usually | |
just the macro name used to identify the architecture, but enclosed in quotes. | |
<DT><TT>OS_TYPE</tt> | |
<DD> | |
Defined to a string that represents the operating system name. Usually | |
just the macro name used to identify the operating system, but enclosed in quotes. | |
<DT><TT>CPP_WORDSZ</tt> | |
<DD> | |
The word size in bits as a constant suitable for preprocessor tests, | |
i.e. without casts or sizeof expressions. Currently always defined as | |
either 64 or 32. For platforms supporting both 32- and 64-bit ABIs, | |
this should be conditionally defined depending on the current ABI. | |
There is a default of 32. | |
<DT><TT>ALIGNMENT</tt> | |
<DD> | |
Defined to be the largest <TT>N</tt>, such that | |
all pointer are guaranteed to be aligned on <TT>N</tt>-byte boundaries. | |
defining it to be 1 will always work, but perform poorly. | |
For all modern 32-bit platforms, this is 4. For all modern 64-bit | |
platforms, this is 8. Whether or not X86 qualifies as a modern | |
architecture here is compiler- and OS-dependent. | |
<DT><TT>DATASTART</tt> | |
<DD> | |
The beginning of the main data segment. The collector will trace all | |
memory between <TT>DATASTART</tt> and <TT>DATAEND</tt> for root pointers. | |
On some platforms,this can be defined to a constant address, | |
though experience has shown that to be risky. Ideally the linker will | |
define a symbol (e.g. <TT>_data</tt> whose address is the beginning | |
of the data segment. Sometimes the value can be computed using | |
the <TT>GC_SysVGetDataStart</tt> function. Not used if either | |
the next macro is defined, or if dynamic loading is supported, and the | |
dynamic loading support defines a function | |
<TT>GC_register_main_static_data()</tt> which returns false. | |
<DT><TT>SEARCH_FOR_DATA_START</tt> | |
<DD> | |
If this is defined <TT>DATASTART</tt> will be defined to a dynamically | |
computed value which is obtained by starting with the address of | |
<TT>_end</tt> and walking backwards until non-addressable memory is found. | |
This often works on Posix-like platforms. It makes it harder to debug | |
client programs, since startup involves generating and catching a | |
segmentation fault, which tends to confuse users. | |
<DT><TT>DATAEND</tt> | |
<DD> | |
Set to the end of the main data segment. Defaults to <TT>end</tt>, | |
where that is declared as an array. This works in some cases, since | |
the linker introduces a suitable symbol. | |
<DT><TT>DATASTART2, DATAEND2</tt> | |
<DD> | |
Some platforms have two discontiguous main data segments, e.g. | |
for initialized and uninitialized data. If so, these two macros | |
should be defined to the limits of the second main data segment. | |
<DT><TT>STACK_GROWS_UP</tt> | |
<DD> | |
Should be defined if the stack (or thread stacks) grow towards higher | |
addresses. (This appears to be true only on PA-RISC. If your architecture | |
has more than one stack per thread, and is not already supported, you will | |
need to do more work. Grep for "IA64" in the source for an example.) | |
<DT><TT>STACKBOTTOM</tt> | |
<DD> | |
Defined to be the cool end of the stack, which is usually the | |
highest address in the stack. It must bound the region of the | |
stack that contains pointers into the GC heap. With thread support, | |
this must be the cold end of the main stack, which typically | |
cannot be found in the same way as the other thread stacks. | |
If this is not defined and none of the following three macros | |
is defined, client code must explicitly set | |
<TT>GC_stackbottom</tt> to an appropriate value before calling | |
<TT>GC_INIT()</tt> or any other <TT>GC_</tt> routine. | |
<DT><TT>LINUX_STACKBOTTOM</tt> | |
<DD> | |
May be defined instead of <TT>STACKBOTTOM</tt>. | |
If defined, then the cold end of the stack will be determined | |
Currently we usually read it from /proc. | |
<DT><TT>HEURISTIC1</tt> | |
<DD> | |
May be defined instead of <TT>STACKBOTTOM</tt>. | |
<TT>STACK_GRAN</tt> should generally also be undefined and defined. | |
The cold end of the stack is determined by taking an address inside | |
<TT>GC_init's frame</tt>, and rounding it up to | |
the next multiple of <TT>STACK_GRAN</tt>. This works well if the stack base is | |
always aligned to a large power of two. | |
(<TT>STACK_GRAN</tt> is predefined to 0x1000000, which is | |
rarely optimal.) | |
<DT><TT>HEURISTIC2</tt> | |
<DD> | |
May be defined instead of <TT>STACKBOTTOM</tt>. | |
The cold end of the stack is determined by taking an address inside | |
GC_init's frame, incrementing it repeatedly | |
in small steps (decrement if <TT>STACK_GROWS_UP</tt>), and reading the value | |
at each location. We remember the value when the first | |
Segmentation violation or Bus error is signalled, round that | |
to the nearest plausible page boundary, and use that as the | |
stack base. | |
<DT><TT>DYNAMIC_LOADING</tt> | |
<DD> | |
Should be defined if <TT>dyn_load.c</tt> has been updated for this | |
platform and tracing of dynamic library roots is supported. | |
<DT><TT>MPROTECT_VDB, PROC_VDB</tt> | |
<DD> | |
May be defined if the corresponding "virtual dirty bit" | |
implementation in os_dep.c is usable on this platform. This | |
allows incremental/generational garbage collection. | |
<TT>MPROTECT_VDB</tt> identifies modified pages by | |
write protecting the heap and catching faults. | |
<TT>PROC_VDB</tt> uses the /proc primitives to read dirty bits. | |
<DT><TT>PREFETCH, PREFETCH_FOR_WRITE</tt> | |
<DD> | |
The collector uses <TT>PREFETCH</tt>(<I>x</i>) to preload the cache | |
with *<I>x</i>. | |
This defaults to a no-op. | |
<DT><TT>CLEAR_DOUBLE</tt> | |
<DD> | |
If <TT>CLEAR_DOUBLE</tt> is defined, then | |
<TT>CLEAR_DOUBLE</tt>(x) is used as a fast way to | |
clear the two words at GC_malloc-aligned address x. By default, | |
word stores of 0 are used instead. | |
<DT><TT>HEAP_START</tt> | |
<DD> | |
<TT>HEAP_START</tt> may be defined as the initial address hint for mmap-based | |
allocation. | |
<DT><TT>ALIGN_DOUBLE</tt> | |
<DD> | |
Should be defined if the architecture requires double-word alignment | |
of <TT>GC_malloc</tt>ed memory, e.g. 8-byte alignment with a | |
32-bit ABI. Most modern machines are likely to require this. | |
This is no longer needed for GC7 and later. | |
</dl> | |
<H2>Additional requirements for a basic port</h2> | |
In some cases, you may have to add additional platform-specific code | |
to other files. A likely candidate is the implementation of | |
<TT>GC_with_callee_saves_pushed</tt> in </tt>mach_dep.c</tt>. | |
This ensure that register contents that the collector must trace | |
from are copied to the stack. Typically this can be done portably, | |
but on some platforms it may require assembly code, or just | |
tweaking of conditional compilation tests. | |
<P> | |
For GC7, if your platform supports <TT>getcontext()</tt>, then definining | |
the macro <TT>UNIX_LIKE</tt> for your OS in <TT>gcconfig.h</tt> | |
(if it isn't defined there already) is likely to solve the problem. | |
otherwise, if you are using gcc, <TT>_builtin_unwind_init()</tt> | |
will be used, and should work fine. If that is not applicable either, | |
the implementation will try to use <TT>setjmp()</tt>. This will work if your | |
<TT>setjmp</tt> implementation saves all possibly pointer-valued registers | |
into the buffer, as opposed to trying to unwind the stack at | |
<TT>longjmp</tt> time. The <TT>setjmp_test</tt> test tries to determine this, | |
but often doesn't get it right. | |
<P> | |
In GC6.x versions of the collector, tracing of registers | |
was more commonly handled | |
with assembly code. In GC7, this is generally to be avoided. | |
<P> | |
Most commonly <TT>os_dep.c</tt> will not require attention, but see below. | |
<H2>Thread support</h2> | |
Supporting threads requires that the collector be able to find and suspend | |
all threads potentially accessing the garbage-collected heap, and locate | |
any state associated with each thread that must be traced. | |
<P> | |
The functionality needed for thread support is generally implemented | |
in one or more files specific to the particular thread interface. | |
For example, somewhat portable pthread support is implemented | |
in <TT>pthread_support.c</tt> and <TT>pthread_stop_world.c</tt>. | |
The essential functionality consists of | |
<DL> | |
<DT><TT>GC_stop_world()</tt> | |
<DD> | |
Stops all threads which may access the garbage collected heap, other | |
than the caller. | |
<DT><TT>GC_start_world()</tt> | |
<DD> | |
Restart other threads. | |
<DT><TT>GC_push_all_stacks()</tt> | |
<DD> | |
Push the contents of all thread stacks (or at least of pointer-containing | |
regions in the thread stacks) onto the mark stack. | |
</dl> | |
These very often require that the garbage collector maintain its | |
own data structures to track active threads. | |
<P> | |
In addition, <TT>LOCK</tt> and <TT>UNLOCK</tt> must be implemented | |
in <TT>gc_locks.h</tt> | |
<P> | |
The easiest case is probably a new pthreads platform | |
on which threads can be stopped | |
with signals. In this case, the changes involve: | |
<OL> | |
<LI>Introducing a suitable <TT>GC_</tt><I>X</i><TT>_THREADS</tt> macro, which should | |
be automatically defined by <TT>gc_config_macros.h</tt> in the right cases. | |
It should also result in a definition of <TT>GC_PTHREADS</tt>, as for the | |
existing cases. | |
<LI>For GC7+, ensuring that the <TT>atomic_ops</tt> package at least | |
minimally supports the platform. | |
If incremental GC is needed, or if pthread locks don't | |
perform adequately as the allocation lock, you will probably need to | |
ensure that a sufficient <TT>atomic_ops</tt> port | |
exists for the platform to provided an atomic test and set | |
operation. (Current GC7 versions require more<TT>atomic_ops</tt> | |
asupport than necessary. This is a bug.) For earlier versions define | |
<TT>GC_test_and_set</tt> in <TT>gc_locks.h</tt>. | |
<LI>Making any needed adjustments to <TT>pthread_stop_world.c</tt> and | |
<TT>pthread_support.c</tt>. Ideally none should be needed. In fact, | |
not all of this is as well standardized as one would like, and outright | |
bugs requiring workarounds are common. | |
</ol> | |
Non-preemptive threads packages will probably require further work. Similarly | |
thread-local allocation and parallel marking requires further work | |
in <TT>pthread_support.c</tt>, and may require better <TT>atomic_ops</tt> | |
support. | |
<H2>Dynamic library support</h2> | |
So long as <TT>DATASTART</tt> and <TT>DATAEND</tt> are defined correctly, | |
the collector will trace memory reachable from file scope or <TT>static</tt> | |
variables defined as part of the main executable. This is sufficient | |
if either the program is statically linked, or if pointers to the | |
garbage-collected heap are never stored in non-stack variables | |
defined in dynamic libraries. | |
<P> | |
If dynamic library data sections must also be traced, then | |
<UL> | |
<LI><TT>DYNAMIC_LOADING</tt> must be defined in the appropriate section | |
of <TT>gcconfig.h</tt>. | |
<LI>An appropriate versions of the functions | |
<TT>GC_register_dynamic_libraries()</tt> should be defined in | |
<TT>dyn_load.c</tt>. This function should invoke | |
<TT>GC_cond_add_roots(</tt><I>region_start, region_end</i><TT>, TRUE)</tt> | |
on each dynamic library data section. | |
</ul> | |
<P> | |
Implementations that scan for writable data segments are error prone, particularly | |
in the presence of threads. They frequently result in race conditions | |
when threads exit and stacks disappear. They may also accidentally trace | |
large regions of graphics memory, or mapped files. On at least | |
one occasion they have been known to try to trace device memory that | |
could not safely be read in the manner the GC wanted to read it. | |
<P> | |
It is usually safer to walk the dynamic linker data structure, especially | |
if the linker exports an interface to do so. But beware of poorly documented | |
locking behavior in this case. | |
<H2>Incremental GC support</h2> | |
For incremental and generational collection to work, <TT>os_dep.c</tt> | |
must contain a suitable "virtual dirty bit" implementation, which | |
allows the collector to track which heap pages (assumed to be | |
a multiple of the collectors block size) have been written during | |
a certain time interval. The collector provides several | |
implementations, which might be adapted. The default | |
(<TT>DEFAULT_VDB</tt>) is a placeholder which treats all pages | |
as having been written. This ensures correctness, but renders | |
incremental and generational collection essentially useless. | |
<H2>Stack traces for debug support</h2> | |
If stack traces in objects are need for debug support, | |
<TT>GC_dave_callers</tt> and <TT>GC_print_callers</tt> must be | |
implemented. | |
<H2>Disclaimer</h2> | |
This is an initial pass at porting guidelines. Some things | |
have no doubt been overlooked. | |
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