warning: comparing floating point with == or != is unsafe [-Wfloat-equal]

[Lubomir Rintel]

We like to build network-manager-applet's libnm with many compiler warnings enabled. Now that Gtk 4.0 drags in graphene, we now get some extra ones:

[lkundrak@belphegor linux]$ cat test.c 
#include <gtk/gtk.h>
[lkundrak@belphegor linux]$ cc $(pkg-config --cflags gtk+-4.0) -Wfloat-equal -c test.c
In file included from /usr/include/graphene-1.0/graphene.h:35,
                 from /usr/include/gtk-4.0/gsk/gsktypes.h:25,
                 from /usr/include/gtk-4.0/gsk/gskrenderer.h:26,
                 from /usr/include/gtk-4.0/gsk/gsk.h:24,
                 from /usr/include/gtk-4.0/gtk/gtkwidget.h:33,
                 from /usr/include/gtk-4.0/gtk/gtkapplication.h:27,
                 from /usr/include/gtk-4.0/gtk/gtkwindow.h:33,
                 from /usr/include/gtk-4.0/gtk/gtkdialog.h:32,
                 from /usr/include/gtk-4.0/gtk/gtkaboutdialog.h:30,
                 from /usr/include/gtk-4.0/gtk/gtk.h:31,
                 from test.c:1:
/usr/include/graphene-1.0/graphene-simd4f.h: In function ‘graphene_simd4f_is_zero3’:
/usr/include/graphene-1.0/graphene-simd4f.h:1730:36: warning: comparing floating point with == or != is unsafe [-Wfloat-equal]
 1730 |   return graphene_simd4f_get_x (v) == 0.f &&
      |                                    ^~
/usr/include/graphene-1.0/graphene-simd4f.h:1731:36: warning: comparing floating point with == or != is unsafe [-Wfloat-equal]
 1731 |          graphene_simd4f_get_y (v) == 0.f &&
      |                                    ^~
/usr/include/graphene-1.0/graphene-simd4f.h:1732:36: warning: comparing floating point with == or != is unsafe [-Wfloat-equal]
 1732 |          graphene_simd4f_get_z (v) == 0.f;
      |                                    ^~
/usr/include/graphene-1.0/graphene-simd4f.h: In function ‘graphene_simd4f_is_zero2’:
/usr/include/graphene-1.0/graphene-simd4f.h:1749:36: warning: comparing floating point with == or != is unsafe [-Wfloat-equal]
 1749 |   return graphene_simd4f_get_x (v) == 0.f &&
      |                                    ^~
/usr/include/graphene-1.0/graphene-simd4f.h:1750:36: warning: comparing floating point with == or != is unsafe [-Wfloat-equal]
 1750 |          graphene_simd4f_get_y (v) == 0.f;
      |                                    ^~
In file included from /usr/include/graphene-1.0/graphene.h:36,
                 from /usr/include/gtk-4.0/gsk/gsktypes.h:25,
                 from /usr/include/gtk-4.0/gsk/gskrenderer.h:26,
                 from /usr/include/gtk-4.0/gsk/gsk.h:24,
                 from /usr/include/gtk-4.0/gtk/gtkwidget.h:33,
                 from /usr/include/gtk-4.0/gtk/gtkapplication.h:27,
                 from /usr/include/gtk-4.0/gtk/gtkwindow.h:33,
                 from /usr/include/gtk-4.0/gtk/gtkdialog.h:32,
                 from /usr/include/gtk-4.0/gtk/gtkaboutdialog.h:30,
                 from /usr/include/gtk-4.0/gtk/gtk.h:31,
                 from test.c:1:
/usr/include/graphene-1.0/graphene-simd4x4f.h: In function ‘graphene_simd4x4f_inverse’:
/usr/include/graphene-1.0/graphene-simd4x4f.h:900:35: warning: comparing floating point with == or != is unsafe [-Wfloat-equal]
  900 |   if (graphene_simd4f_get_x (det) != 0.f)
      |                                   ^~
[lkundrak@belphegor linux]$ 

I have no idea how to avoid the warning, or even whether comparing a float to zero is actually a bad idea, but we'd be super thankful if it could be avoided.

Thanks
Lubo

[Emmanuele Bassi]

The way we use float comparisons is, in theory, safe because we're only using 0. Of course, GCC doesn't know that, so the warning is avoidable by using -Wno-float-equal.

It would be better to avoid that, but while it's easy to fix Graphene itself—and I have a work in progress branch that does that—we use g_assert_cmpfloat() a lot in the test suite, and that will need some more work to move to fuzzy comparisons.