Re: [Caml-list] Compiler bug?

[Dmitry Bely <dmitry.bely@gmail.com>]

On Mon, Aug 6, 2012 at 3:32 PM, Alain Frisch <alain@frisch.fr> wrote:
> On 08/06/2012 01:03 PM, Dmitry Bely wrote:
>>
>> _caml_call_gc:
>>      ; Record lowest stack address and return address
>>          mov    eax, [esp]
>>          mov     _caml_last_return_address, eax
>>          lea     eax, [esp+4]
>>          mov     _caml_bottom_of_stack, eax
>>      ; Save all regs used by the code generator
>> L105:   push    ebp
>>          push    edi
>>          push    esi
>>          push    edx
>>          push    ecx
>>          push    ebx
>>          push    eax
>>          mov     _caml_gc_regs, esp
>>      ; Call the garbage collector
>>          call   _caml_garbage_collection
>>      ; Restore all regs used by the code generator
>>         pop     eax
>>          pop     ebx
>>          pop     ecx
>>          pop     edx
>>          pop     esi
>>          pop     edi
>>          pop     ebp
>>      ; Return to caller
>>          ret
>>
>> it preserves registers and so it cannot update them
>
>
> No, this code does not preserves registers.  It stores them on the stack,
> pass a pointer to them to the C code (in caml_gc_regs), and then restore
> their values from the stack.  In between, the C code has updated the values
> on the stack, so the restored values are not the saved ones.

One more question if you don't mind. Why FPU registers are not saved
before caml_garbage_collection? What guarantees that they are not
destroyed inside?

- Dmitry Bely