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Chris Lattner

2004-Jul-21 17:34 UTC

[LLVMdev] GC questions.

On Wed, 21 Jul 2004, Tobias Nurmiranta wrote:
>
> Hi, I'm thinking out loud, please give me some feedback.
>
> Regarding llvm.gcread and llvm.gcwrite, wouldn't it be nicer if they
are
> implemented as:
>
>  llvm.gcread(sbyte** object, uint offset)
>  llvm.gcwrite(sbyte* data, sbyte** object, uint offset)
>
> Where you also have the offset into the object. In this way the GC would
> know where the header of the object we are reading/writing to is. Also it
> would mean less traffic to the pointers on the stack, and in some
> languages you only want to allow pointers to the header of the objects,
> which makes object traversal easier.
Yes, this makes a tremendous amount of sense.  Do you think you could
prepare some patches to make this happen?  If you have any questions, feel
free to ask :)

-Chris

-- 
http://llvm.cs.uiuc.edu/
http://nondot.org/sabre/

Tobias Nurmiranta

2004-Jul-21 17:46 UTC

head link

[LLVMdev] GC questions.

On Wed, 21 Jul 2004, Chris Lattner wrote:>
> Yes, this makes a tremendous amount of sense.  Do you think you could
> prepare some patches to make this happen?  If you have any questions, feel
> free to ask :)
Ok, a patch[1] is attached. I didn't care to coerce the offset, since I
assume that it is an uint, but maybe I should? Hopefully I've understood
the llvm source correctly.

,	Tobias

[1] I'm kind of newbie of cvs, but I did:
 "cvs -z3 -d :pserver:anon at llvm-cvs.cs.uiuc.edu:/var/cvs/llvm diff llvm
> gcpatch"
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Index: llvm/lib/Transforms/Scalar/LowerGC.cpp
==================================================================RCS file:
/var/cvs/llvm/llvm/lib/Transforms/Scalar/LowerGC.cpp,v
retrieving revision 1.4
diff -r1.4 LowerGC.cpp
112c112
<     GCRead = M.getOrInsertFunction("llvm_gc_read", VoidPtr,
VoidPtrPtr, 0);
--->     GCRead = M.getOrInsertFunction("llvm_gc_read", VoidPtr,
VoidPtrPtr, Type::UIntTy, 0);115c115
<                                     VoidPtr, VoidPtrPtr, 0);
--->                                     VoidPtr, VoidPtrPtr, Type::UIntTy, 0);192c192
<                 CallInst *NC = new CallInst(GCRead, CI->getOperand(1),
--->                 CallInst *NC = new CallInst(GCRead, CI->getOperand(1),
CI->getOperand(2),Index: llvm/lib/VMCore/Verifier.cpp
==================================================================RCS file:
/var/cvs/llvm/llvm/lib/VMCore/Verifier.cpp,v
retrieving revision 1.115
diff -r1.115 Verifier.cpp
714,715c714,715
<   case Intrinsic::gcread:          NumArgs = 1; break;
<   case Intrinsic::gcwrite:         NumArgs = 2; break;
--->   case Intrinsic::gcread:          NumArgs = 2; break;
>   case Intrinsic::gcwrite:         NumArgs = 3; break;Index: llvm/runtime/GC/GCInterface.h
==================================================================RCS file:
/var/cvs/llvm/llvm/runtime/GC/GCInterface.h,v
retrieving revision 1.2
diff -r1.2 GCInterface.h
41c41
< void *llvm_gc_read(void **P);
---> void *llvm_gc_read(void **P, unsigned Offset);46c46
< void llvm_gc_write(void *V, void **P);
---> void llvm_gc_write(void *V, void **P, unsigned Offset);Index: llvm/runtime/GC/SemiSpace/semispace.c
==================================================================RCS file:
/var/cvs/llvm/llvm/runtime/GC/SemiSpace/semispace.c,v
retrieving revision 1.4
diff -r1.4 semispace.c
92,93c92,93
< void *llvm_gc_read(void **P) { return *P; }
< void llvm_gc_write(void *V, void **P) { *P = V; }
---> void *llvm_gc_read(void **P, unsigned Offset) { return P[Offset]; }
> void llvm_gc_write(void *V, void **P, unsigned Offset) { P[Offset] = V; }Index: llvm/test/Regression/CodeGen/Generic/GC/alloc_loop.ll
==================================================================RCS file:
/var/cvs/llvm/llvm/test/Regression/CodeGen/Generic/GC/alloc_loop.ll,v
retrieving revision 1.2
diff -r1.2 alloc_loop.ll
7c7
< declare void %llvm.gcwrite(sbyte*, sbyte**)
---> declare void %llvm.gcwrite(sbyte*, sbyte**, uint)35c35
< 	call void %llvm.gcwrite(sbyte* %A.1, sbyte** %B.1)
---> 	call void %llvm.gcwrite(sbyte* %A.1, sbyte** %B.1, uint 0)

Chris Lattner

2004-Jul-21 18:16 UTC

head link

[LLVMdev] GC questions.

On Wed, 21 Jul 2004, Tobias Nurmiranta wrote:
> [1] I'm kind of newbie of cvs, but I did:
>  "cvs -z3 -d :pserver:anon at llvm-cvs.cs.uiuc.edu:/var/cvs/llvm diff
llvm > gcpatch"
That patch is well formed.  You did exactly the right thing. :)
> Ok, a patch[1] is attached. I didn't care to coerce the offset, since I
> assume that it is an uint, but maybe I should? Hopefully I've
understood
> the llvm source correctly.
This will work, but it would be better to take two pointers in instead of
a pointer and offset.  This allows the front-end to emit target-generic
code instead of target-specific code (where it would have to know the
offset to the field).  To be more specific, llvm_gc_read should look like
this by default:

void *llvm_gc_read(void *ObjPtr, void **FieldPtr) {
  return *FieldPtr;
}

Also, please don't forget to update docs/GarbageCollection.html and
LangRef.html

Thanks for the help, sorry I didn't mention this before.  :)

-Chris

-- 
http://llvm.cs.uiuc.edu/
http://nondot.org/sabre/

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