Re: [Caml-list] Compiler bug?

[Fabrice Le Fessant <fabrice@lefessant.net>]

On Wed, Aug 8, 2012 at 8:40 PM, Dmitry Bely <dmitry.bely@gmail.com> wrote:
>>> I'm not sure, someone else would have to reply!  I guess that these
>>> registers are supposed to be preserved by the callee in the x64 ABI (and
>>> obviously, they don't hold pointers to OCaml values, so they don't have to
>>> be tracked by the GC).
>
> Yes, they should not be tracked by GC, but they may hold some FP
> values (can they across GC call?). Actually for x64 FPU registers are
> saved; see amd64.S/amd64nt.asm. But not for x86. I'm curious why.

MMX registers are not used by OCaml on x86 (except in the SSE2
branch), the backend still uses the x87 stack. I have no x86 computer
at hand to check, but I suppose that all temporary floating point
values are stored on the C stack by the generated code between
floating point computations, for example when calling C functions
(including the garbage collector).

>> Also, the GC itself will not be using Floating Point code at all, so
>> we can save a lot by not saving/restoring these values on the stack.
>
> This is not entirely true; GC also triggers pending signals processing
> that can modify FPU state and calls C library fuctions (e.g. malloc)
> that can do anything inside.

The kernel is supposed to save all the context before calling signal
handlers, otherwise, you would have to disable signals handling during
all floating point computations, no ?

--Fabrice