[Caml-list] design for ocaml heap profiler

[Caml-list] design for ocaml heap profiler

[Christian Lindig]

To complete OCaml's toolbox I'd like to have a heap profiler that 
tracks allocation of memory. As a first step, I'd like to augment each 
heap-allocated block with the address of the instruction that allocated 
it. Then the GC could write out a trace that shows where all the live 
memory comes from.

My first idea was to change the the block-allocation instructions in 
the bytecode interpreter such that they allocate one additional slot  
to hold the current address' instruction:

	#define Alloc_small_hp(b,n,t)  \
     	do{Alloc_small(b,(n)+1,t);Field(b,n)=Val_int(pc);}while(0)

	Use Alloc_small_hp instead of Alloc_small in in interp.c

Only the runtime systems knows about the additional slot and I expected 
the generated code never to touch the new slot. However, it does not 
work, the compiler crashes, and I could not find out why.

A better design would reserve the new slot as part of the header; this 
requires to redefine the macros in mlvalues.h. Has anybody tried to 
enlarge the header? It would be very nice if the header could be 
enlarged by a configurable number n of slots, where n=0 for the 
production runtime system.

I appreciate any advice about how to enlarge heap blocks for run-time 
information.

-- Christian

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