Re: Warn when returning the address of a temporary (middle-end) v2

[Jeff Law <law@redhat.com>]

On 06/22/14 12:20, Marc Glisse wrote:
> Hello,
>
> I followed the advice in this discussion:
> https://gcc.gnu.org/ml/gcc-patches/2014-04/msg00269.html
>
> and here is a new patch. I made an effort to isolate a path in at least
> one subcase so it doesn't look too strange that the warning is in this
> file. Computing the dominance info just to tweak the warning message may
> be a bit excessive. I kept the same option as the front-ends, I don't
> know if we want a different one, or maybe a Wmaybe-... version. There
> will be cases where we get a duplicate warning from -Wtarget-lifetime in
> fortran, but none in the testsuite, and I would rather have 2 warnings
> than miss such broken code. The uninit-G testcase is about
> initialization, not returning, so I am changing that, even if it is
> unnecessary with the current version of the patch (only activated at -O2).
>
> Bootstrap+testsuite (--enable-languages=all,obj-c++,ada,go) on
> x86_64-unknown-linux-gnu.
>
> (by the way, contrib/compare_tests is confused when I use all languages,
> it prints "comm: file 1 is not in sorted order" and tons of spurious
> differences)
>
> 2014-06-23  Marc Glisse  <marc.glisse@inria.fr>
>
>      PR c++/60517
> gcc/c/
>      * c-typeck.c (c_finish_return): Return 0 instead of the address of
>      a local variable.
> gcc/cp/
>      * typeck.c (maybe_warn_about_returning_address_of_local): Return
>      whether it is returning the address of a local variable.
>      (check_return_expr): Return 0 instead of the address of a local
>      variable.
> gcc/c-family/
>      * c.opt (-Wreturn-local-addr): Move to common.opt.
> gcc/
>      * common.opt (-Wreturn-local-addr): Moved from c.opt.
>      * gimple-ssa-isolate-paths.c: Include diagnostic-core.h.
>      (isolate_path): New argument to avoid inserting a trap.
>      (find_implicit_erroneous_behaviour): Handle returning the address
>      of a local variable.
>      (find_explicit_erroneous_behaviour): Likewise.
>      (gimple_ssa_isolate_erroneous_paths): Calculate dominance info.
> gcc/testsuite/
>      * c-c++-common/addrtmp.c: New file.
>      * c-c++-common/uninit-G.c: Adapt.
I note you don't catch return &localvar in the isolation code -- it 
looks like you just catch those which potentially flow from PHIs.

I realize you're primarily catching that in the front-ends, but can't we 
have cases which aren't caught by the front end, but after optimizations 
we're able to propagate &somelocal into the return statement?

It generally looks good and I'm ready to approve if the answer to the 
above question is "can't happen".  If it can happen, then we ought to 
handle it in the isolation code as well (ought to be relatively easy).

Jeff


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