On Fri, Feb 11, 2022 at 12:05 AM Martin Sebor via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > On 2/8/22 15:37, Jason Merrill wrote: > > On 2/8/22 16:59, Martin Sebor wrote: > >> Transforming a by-value arguments to by-reference as GCC does for some > >> class types can trigger -Wdangling-pointer when the argument is used > >> to store the address of a local variable. Since the stored value is > >> not accessible in the caller the warning is a false positive. > >> > >> The attached patch handles this case by excluding PARM_DECLs with > >> the DECL_BY_REFERENCE bit set from consideration. > >> > >> While testing the patch I noticed some instances of the warning are > >> uninitentionally duplicated as the pass runs more than once. To avoid > >> that, I also introduce warning suppression into the handler for this > >> instance of the warning. (There might still be others.) > > > > The second test should verify that we do warn about returning 't' from a > > function; we don't want to ignore the DECL_BY_REFERENCE RESULT_DECL. > > The indirect aggregate case isn't handled and needs more work but > since you brought it up I thought I should look into finishing it. > The attached patch #2 adds support for it. It also incorporates > Richard's suggestion to test PARM_DECL. Patch #2 applies on top > of patch #1 which is unchanged from the first revision.
patch #1 would be OK if you'd do the PARM_DECL check there - I'd rather defer #2 to stage1. Richard. > > I have retested it on x86_64-linux and by building Glibc and > Binutils + GDB. > > If now is too late for the aggregate enhancement I'm okay with > deferring it until stage 1. > > > > >> + tree var = SSA_NAME_VAR (lhs_ref.ref); > >> + if (DECL_BY_REFERENCE (var)) > >> + /* Avoid by-value arguments transformed into by-reference. */ > >> + continue; > > > > I wonder if we can we express this property of invisiref parms somewhere > > more general? I imagine optimizations would find it useful as well. > > Could pointer_query somehow treat the reference as pointing to a > > function-local object? > > I don't quite see where in the pointer_query class this would be > useful (the class also isn't used for optimization). I could add > a helper to the access_ref class to query this property in warning > code but as this is the only potential client I'm also not sure > that's quite what you had in mind. I'd need some guidance as to > what you're thinking of here. > > Martin > > > > > > I previously tried to express this by marking the reference as > > 'restrict', but that was wrong > > (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97474). > > > > Jason > >
