On Mon, Sep 28, 2020 at 03:05:55PM -0400, Jason Merrill via Gcc-patches wrote:
> On 9/28/20 12:30 PM, Marek Polacek wrote:
> > On Sat, Sep 26, 2020 at 01:22:41AM -0400, Jason Merrill wrote:
> > > > +bool
> > > > +ref_conv_binds_directly_p (tree type, tree expr)
> > > > +{
> > > > + gcc_assert (TYPE_REF_P (type));
> > > > + conversion *conv = implicit_conversion (type, TREE_TYPE (expr), expr,
> > > > + /*c_cast_p=*/false,
> > > > + LOOKUP_IMPLICIT, tf_none);
> > > > + return conv && !conv_binds_ref_to_prvalue (conv);
> > >
> > > You probably want to free any allocated conversions, like in
> > > can_convert_arg.
> >
> > I ought to free them, indeed. Apologies for missing that. Fixed:
> >
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> >
> > -- >8 --
> > This new warning can be used to prevent expensive copies inside range-based
> > for-loops, for instance:
> >
> > struct S { char arr[128]; };
> > void fn () {
> > S arr[5];
> > for (const auto x : arr) { }
> > }
> >
> > where auto deduces to S and then we copy the big S in every iteration.
> > Using "const auto &x" would not incur such a copy. With this patch the
> > compiler will warn:
> >
> > q.C:4:19: warning: loop variable 'x' creates a copy from type 'const S'
> > [-Wrange-loop-construct]
> > 4 | for (const auto x : arr) { }
> > | ^
> > q.C:4:19: note: use reference type 'const S&' to prevent copying
> > 4 | for (const auto x : arr) { }
>
> It's unfortunate that we seem to suggest the unnecessary change from auto to
> S here. Maybe just say "reference type" without printing the type?
Yeah, I wish there was a way to avoid it. But I don't think we have
a TREE_TYPE bit that says that a type was deduced from auto/decltype(auto).
I'll just avoid printing the type...
> > + auto_diagnostic_group d;
> > + if (warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wrange_loop_construct,
>
> Why not use 'loc' here?
Fixed.
> OK however you want to resolve these comments.
Pushed, thanks.
Marek
