[LLVMdev] getting started with IR needing GC

Gordon Henriksen gordonhenriksen at mac.com
Sun Apr 27 19:34:56 PDT 2008

On 2008-04-27, at 21:29, Lane Schwartz wrote:

> Hi guys,

Hi Lane!

This is a lot of questions. I'm not going to answer each individually,  
but will instead give general guidance to help you avoid the pain  

> I somehow need to inform the garbage collection runtime (my  
> copycollector.c) about my variables - specifically about gc roots.  
> So, after I get new memory using llvm_gc_initialize, I think I  
> should generate an @llvm.gcroot intrinsic.

This is correct.

Think of the llvm.gcroot intrinsic call as an annotation on a stack  
variable (an alloca). Like noalias, it doesn't generate any code  
itself. Rather, it instructs the code generator to keep track of the  
location of the variable in the stack frame. You must store your  
allocated pointer into this variable. Since you are using a copying  
collector, you must also reload the value of this variable before each  
use. This is to guard against the possibility that the collector has  
been invoked, updating the contents of the alloca.

As for the compiler plugin interface, I suggest you ignore it  
initially and use the provided shadow-stack option for the time being.  
The shadow stack generates significantly suboptimal code, but will let  
you avoid writing some platform-specific code. Instead, simply copy  
the llvm_cg_walk_gcroots routine from the semispace example. Call it  
from your collection routine in order to visit each gcroot on the  
runtime stack.

The shadow stack lets you find roots by automatically emitting a  
function prologue and an epilogue to push and pop gcroot entries on a  
stack as the program runs. This avoids the need to traverse the native  
call stack. If your language becomes sufficiently performance  
sensitive, you can later investigate implementing a custom Collector  

A few additional notes:

> When my frontend generates LLVM IR code for a program, it should also
> generate a call (early in the IR code) to llvm_gc_initialize. This
> function uses the system calloc to allocate two big blocks of memory,
> then stores pointers to that memory in static variables.
> [...]
> There is also a function called llvm_gc_allocate. Now, instead of
> using alloca or malloc, my frontend generates a call to
> llvm_gc_allocate.

These function names are entirely optional. Your runtime can use any  
names and prototypes it likes to provide this functionality. A bump- 
ptr allocator might easily inline part of its allocation routine.

> Since this is a simple copying collector, the functions llvm_gc_read
> and llvm_gc_write won't really do much:
> void *llvm_gc_read(void *ObjPtr, void **FieldPtr) { return  
> *FieldPtr; }
> void llvm_gc_write(void *V, void *ObjPtr, void **FieldPtr)  
> { *FieldPtr = V; }

You can just emit loads and stores directly if your read/write  
barriers do nothing. Also, there's nothing special about the  
llvm_gc_read or llvm_gc_write functions any more; they will not be  
called unless you call them yourself.

— Gordon

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