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Re: [Help-gsl] Uninitialized variables and signaling NaNs

From: Jochen Küpper
Subject: Re: [Help-gsl] Uninitialized variables and signaling NaNs
Date: Tue, 10 Aug 2010 23:35:58 +0200


On 08.Aug.2010, at 22:04, Andrew W. Steiner wrote:

>     Thanks for your thoughts. My naive guess is
> that It would be difficult,

could be

> if not impossible for the
> optimizer to know this, as one could always
> insert a call to an external function which
> ostensibly initializes the variable but may or
> may not fail to do so. Additionally, I
> don't forsee any work on the C/C++ compilers
> which guarantees that the optimizer will
> accomplish this task, leaving it up to those
> who write the source code to take care of it.

However, the code you send is valid C and valid C++ (AFAICT).
Therefore, a correct (C or C++) compiler must create valid machine code from 
it. It's that simple.

> On Sun, Aug 8, 2010 at 3:28 PM, Jochen Küpper
> <address@hidden> wrote:
>> Isn't that an issue that should be handled by the optimizer? That's the
>> instance that knows whether this issue might occur or not, i.e., it
>> knows whether the variable is put into a register or not.
>> Therefore, the optimizer should initialize this variable if it's put into a
>> register (probably using a switch to turn off or on). Obviously, it then
>> has to determine whether it is still worth the register usage with
>> the added overhead of initialization.
>> Or would it actually be feasible to use the register (for x) without
>> moving the content of &x but instead using whatever happens to
>> be in the register?

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