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diff --git a/boehm_gc/doc/porting.html b/boehm_gc/doc/porting.html new file mode 100644 index 0000000..5a06c22 --- /dev/null +++ b/boehm_gc/doc/porting.html @@ -0,0 +1,333 @@ +<HTML> +<HEAD> + <TITLE>Conservative GC Porting Directions</TITLE> +</HEAD> +<BODY> +<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. +</body> +</html> |