Re: Warn when returning the address of a temporary (middle-end)

[Jeff Law <law at redhat dot com>]

On 04/05/14 07:52, Marc Glisse wrote:
> Hello,
>
> we have front-end warnings about returning the address of a local
> variable. However, quite often in C++, people don't directly return the
> address of a temporary, it goes through a few functions which hide that
> fact. After some inlining, the fact that we are returning the address of
> a local variable can become obvious to the compiler, to the point where
> I have used, for debugging purposes, grep 'return &' on the optimized
> dump produced with -O3 -fkeep-inline-functions (I then had to sort
> through the global/local variables).
>
> fold_stmt looks like a good place for this, but it could go almost
> anywhere. It has to happen after enough inlining / copy propagation to
> make it useful though.
I was wondering if this would be better implemented as a propagation 
problem so that cases where some, but not all paths to the return 
statement have &local which reaches the return.  ie

...
if (foo)
  x = &local
else
  x = &global

return x;

ISTM it ought to be a standard propagation problem and that the 
problematical ADDR_EXPRs are all going to be in PHI nodes.  So I think 
you just search for problematical ADDR_EXPRs in PHI nodes as your seeds, 
then forward propagate through the CFG.  Ultimately looking for any 
cases where those ADDR_EXPRs ultimately reach the return statement.

Thoughts?

jeff