migc

Small and simple library that implements conservative GC using mimalloc API.

Features

Small and tiny. libmigc.so is just 20KB when linked with mimalloc.
Has API that is needed by most users of conservative GCs in C and can replace BDWGC in simple cases (e.g when precise marking is not needed) There is: migc_add_roots and migc_delete_roots to add or delete range of pointers to scan for potential pointers, migc_register_finalizer to invoke finalizers on dead objects.
Fast allocation. When there is multiple instances of migc heap running in multiple threads this library could get much much faster than BDWGC, in single threaded situations it should be pretty fast too since mimalloc is fast in single threaded apps too.
Small overhead per allocation: 8 bytes. These 8 bytes is used to store mark,live bit and finalizer pointer (if exists).

Limitations

No threading support. migc_heap type can be used only in thread it was created in.
No API for precise marking, I could add such API in the future but I think it will increase allocation overhead by another 8 bytes.
Only Mark&Sweep is provided at the moment. There is no advanced GC techniques such as incremental or concurrent marking but sticky generational mark&sweep algorithm could be implemented in the future.

Building

git clone https://github.com/playxe/migc
cd migc
make submodules
make

This should produce libmigc.so which can be dynamically linked with your programs.

Examples

#include "migc.h"
// NOTE: current_stack_pointer is provided by migc.h

int main() {
  migc_heap heap;
  migc_heap_init(&heap,current_stack_pointer() /*stack start for conservative scanning */,1024/* GC threshold */);
  int* mptr = migc_malloc(&heap,sizeof(int)*4);
  keep_on_stack(mptr); // <- optional call, this forces compiler to put variable on stack rather than registers so GC can definitely see it
  mptr[0] = 42;
  printf("%i\n",mptr);
  migc_collect(&heap);
}

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Comments

Question about `current_stack_pointer`
Hey there! First of all: thanks for putting migc on GitHub! I've been looking into writing a conservative GC and it's super helpful to look at migc and see how you do stack scanning.

I wanted to send you an email but then figured: why not open an issue? So, this is a question and not a bug.

How is current_stack_pointer supposed to work?

https://github.com/playXE/migc/blob/fb3b807a2698a1066b3fffd6d2effce5b9c64ab7/migc.h#L303-L309

There's two things I don't understand:

If I understand it correctly it tries to take the address of an undeclared local variable (sp). Shouldn't this be something like
int dummy return &dummy

When I try to compile the example.c with
$ gcc -O3 migc.c migc.h example.c -o example -L -Wl,-rpath= -L/home/mrnugget/code/clones/migc/mimalloc/out/release -Wl,-rpath=/home/mrnugget/code/clones/migc/mimalloc/out/release -pthread -lmimalloc

I get a bunch of warnings but also this error:

/usr/bin/ld: /tmp/ccjNSGu8.o: in function `current_stack_pointer': example.c:(.text+0x80): multiple definition of `current_stack_pointer'; /tmp/ccoWCpKL.o:migc.c:(.text+0x240): first defined here collect2: error: ld returned 1 exit status make: *** [Makefile:15: example] Error 1

It seems to me like current_stack_pointer should be inlined, right?
opened by mrnugget 4