Re: C bug of the day

[brangdon@cix.co.uk (Dave Harris)]

ron@sensor.com (Ron Natalie) wrote (abridged):
>      int i;                // initialize to 0
>      stream >> i;
> 
>      auto int i;        // recycle auto to mean uninitialized.
>      stream >> i;
> 
> In the above case existing code that used the unqualified name
> would work in old and new compilers (with a slight performance
> hit in the new complier), those in the second case would work
> the same way in the old and new compiler (the same way it used
> to).

A good optimiser could emit the same code in both cases. The value
zero is never read, so never needs to be written.

The compiler could figure this out either from the source (eg if
stream is from a template) or from knowing the semantics of the
std library (if stream is from the std). Cases where zero-
initialisation cannot be optimised away are fairly rare - although 
sometimes important when they do arise.

If we actually need zeros, it may be more efficient to let the
compiler provide them. For example, with an instance of a deep
class hierarchy, the compiler can zero-initialise all of the
instance variables with a single memset(), instead of every
class needing its own set of assignments.

Indeed, it can make sense to push the nulling even further back,
and have a "nulled heap". Then the system can do the work of
writing nulls during its idle time.

-- Dave Harris, Nottingham, UK

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