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Date: Mon, 06 Aug 2012 13:32:26 +0200
From: Alain Frisch <alain@frisch.fr>
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To: Dmitry Bely <dmitry.bely@gmail.com>
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References: <CAMT7qiTJD4hFd4Jr0tKCzhmR=ngN0yV9Pw6CCgEfRPP6a6DYvg@mail.gmail.com> <501F9855.6080709@frisch.fr> <CAMT7qiTCnxbAwbPSiXxxm3EBFVyTad98YqxxxveoPkhepKbCkA@mail.gmail.com> <501F9DB9.2050805@frisch.fr> <CAMT7qiR=7LS3211gkA2HOVwB+BQj81cs1efjy3eBESgqW03s5g@mail.gmail.com>
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Subject: Re: [Caml-list] Compiler bug?

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.

-- Alain