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

[Marc Glisse <marc dot glisse at inria dot fr>]

On Wed, 16 Jul 2014, Jeff Law wrote:

> 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 thought I was handling it in the find_explicit_erroneous_behaviour part 
of the patch (as opposed to the implicit part which deals with PHIs). 
Function f1 in the testcase addrtmp.c has no PHI. Am I missing something?

> 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?

We can, and it was my original motivation. I only added PHI handling when 
you asked for it.

> 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).

Just to be clear, the approval would include the PARM_DECL tweak in
https://gcc.gnu.org/ml/gcc-patches/2014-06/msg02327.html
?

Thanks,

--
Marc Glisse