llvm dev - Dec 2012 - [LLVMdev] Use rand_r() instead of non-reentrant thread-unsafe rand() in GetRandomNumber()

Dear all,

In our LLVM-based compiler pipeline a major part of code generation is
taken into application runtime. One side-effect of this organization is a
need to be very careful about using code that might diverge application
state. And we found that simple generation of temporary files over LLVM
APIs introduces random noise into the program result. There reason is that
LLVM's GetRandomNumber call uses rand(), that clashes with rand() used in
application. Taking in account that many client applications developers may
be not aware of this at all [and just complain to us, that our compiler is
bad because their program is now wrong], we propose switching to self-
state maintaining rand() on LLVM side. Attached patch simply replaces the
calls to srand() and rand() with a call to rand_r(), which uses static
unsigned x value to keep its state locally.

Best,
- D.
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If we're keeping the state locally now, perhaps we should store it in a
per-thread variable.  I know rand() isn't thread safe to begin with, but it
seems like rand_r() can be since it should keep no external state.


On Sat, Dec 1, 2012 at 11:17 AM, Dmitry Mikushin <dmitry at
kernelgen.org>wrote:
> Dear all,
>
> In our LLVM-based compiler pipeline a major part of code generation is
> taken into application runtime. One side-effect of this organization is a
> need to be very careful about using code that might diverge application
> state. And we found that simple generation of temporary files over LLVM
> APIs introduces random noise into the program result. There reason is that
> LLVM's GetRandomNumber call uses rand(), that clashes with rand() used
in
> application. Taking in account that many client applications developers may
> be not aware of this at all [and just complain to us, that our compiler is
> bad because their program is now wrong], we propose switching to self-
> state maintaining rand() on LLVM side. Attached patch simply replaces the
> calls to srand() and rand() with a call to rand_r(), which uses static
> unsigned x value to keep its state locally.
>
> Best,
> - D.
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>

-- 

Thanks,

Justin Holewinski
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Agreed, done.

One thing I'm not sure about is this statement in docs:

POSIX.1-2008 marks *rand_r*() as obsolete.

- And... what is the replacement?

2012/12/1 Justin Holewinski <justin.holewinski at gmail.com>
> If we're keeping the state locally now, perhaps we should store it in a
> per-thread variable.  I know rand() isn't thread safe to begin with,
but it
> seems like rand_r() can be since it should keep no external state.
>
>
> On Sat, Dec 1, 2012 at 11:17 AM, Dmitry Mikushin <dmitry at
kernelgen.org>wrote:
>
>> Dear all,
>>
>> In our LLVM-based compiler pipeline a major part of code generation is
>> taken into application runtime. One side-effect of this organization is
a
>> need to be very careful about using code that might diverge application
>> state. And we found that simple generation of temporary files over LLVM
>> APIs introduces random noise into the program result. There reason is
that
>> LLVM's GetRandomNumber call uses rand(), that clashes with rand()
used in
>> application. Taking in account that many client applications developers
may
>> be not aware of this at all [and just complain to us, that our compiler
is
>> bad because their program is now wrong], we propose switching to self-
>> state maintaining rand() on LLVM side. Attached patch simply replaces
the
>> calls to srand() and rand() with a call to rand_r(), which uses static
>> unsigned x value to keep its state locally.
>>
>> Best,
>> - D.
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>>
>
>
> --
>
> Thanks,
>
> Justin Holewinski
>
>
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