[Bug c++/94905] New: Bogus warning -Werror=maybe-uninitialized
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94905
Bug ID: 94905
Summary: Bogus warning -Werror=maybe-uninitialized
Product: gcc
Version: 9.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: bug-apl at gnu dot org
Target Milestone: ---
Hi,
as of g++ 9.3.0 when building GNU APL, g++ emits bogus warnings of type
-Werror=maybe-uninitialized.
Apparently the compiler issues these warnings simply because it is lacking
enough context to really figure out what is happening (e.g. if the supposedly
missing initialization is happening in a different function).
An example is:
https://svn.savannah.gnu.org/viewvc/apl/trunk/src/Shape.hh?view=markup
Shape.hh:133:18: error: ‘shape_Z.Shape::rho[axis]’ may be used
uninitialized in this function [-Werror=maybe-uninitialized]
if (rho[r]) { volume /= rho[r]; rho[r] = sh; volume *=
rho[r]; }
~^
The line 133 is preceded by an assertion that verifies that rho[r] is
initialized. Apparently the compiler assumes that the initialization
of rho[r] on the right of = happens after the test of rho[r] on the left
of =. It completely ignores, however, the fact that rho[r] was simply
updated and was initialized already before the function (set_shape_item)
was called.
May I kindly request to move this kind of warnings away from -Wall to
-Wextra ?
According to "man g++":
-Wall
This enables all the warnings about constructions that some users
consider questionable, and that are easy to avoid (or modify to
prevent the warning), even in conjunction with macros. This also
In the example above there is no "easy to avoid" way to fix the warning.
Kind regards,
Jürgen Sauermann
Maintainer of GNU APL
[Bug c++/94905] Bogus warning -Werror=maybe-uninitialized
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94905
--- Comment #2 from Dr. Jürgen Sauermann ---
I checked different g++ versions; the problem occurs in versions 9, 10, and 11
of g++ but not in version 8.
I tried to shrink the code to the bare minimum but if I do so then the warning
is no longer emitted.
I noticed that the warning has become more verbose in g++ version 11.
Before version 11 the warning said (one example of many):
Shape.hh:133:18: error: ‘shape_Z.Shape::rho[axis]’ may be used
uninitialized in this function [-Werror=maybe-uninitialized]
if (rho[r]) { volume /= rho[r]; rho[r] = sh; volume
*= rho[r]; }
~^
In version g++ 11 it now says:
Shape.hh: In member function ‘virtual Token Bif_F12_FORMAT::eval_B(Value_P)’:
Shape.hh:133:18: error: ‘shape_Z.Shape::rho[]’ may be used
uninitialized in this function [-Werror=maybe-uninitialized]
133 | if (rho[r]) { volume /= rho[r]; rho[r] = sh; volume *=
rho[r]; }
| ~^
The in shape_Z.Shape::rho[] above is the function argument
'axis' of the function set_shape_item(axis, length) inside which the warning is
being issued.
This suggests that the warning is issued whenever the compiler is unable to
statically determine if some function argument (the vector index 'axiss' in
this case) is inside or outside the initialized region of a vector.
This seems to be overly picky; I would normally assume that the data members of
a class are always initialized. If not then the constructor and not any member
function should emit a warning.
Thanks for your attention and for gcc/g++
Jürgen Sauermann
[Bug c++/94905] Bogus warning -Werror=maybe-uninitialized
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94905 --- Comment #4 from Dr. Jürgen Sauermann --- Created attachment 48431 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48431&action=edit preprocessed apl-Bif_F12_FORMAT.cc, gzip'ed preprocessed source file apl-Bif_F12_FORMAT.cc
[Bug c++/94905] Bogus warning -Werror=maybe-uninitialized
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94905 --- Comment #5 from Dr. Jürgen Sauermann --- Hi, above is one preprocessed source file for which the warning has occurred. g++ version was 11.0.0 (latest from github). Compiled with: g++ -E -DHAVE_CONFIG_H -I. -I..-Wall -I sql -Werror -I/usr/include -I/usr/include/postgresql -rdynamic -g -O2 -MT apl-Bif_F12_FORMAT.o -MD -MP -MF .deps/apl-Bif_F12_FORMAT.Tpo -c -o apl-Bif_F12_FORMAT.cxx Interestingly, if I compile the file apl-Bif_F12_FORMAT.cxx separately (with -Wall) then the warning does not show up. Not sure how I can create a testcase file other than preprocessing or minimizing it.
