https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66500
Bug ID: 66500
Summary: C/C++ inconsistency in diagnostic context involving
macros
Product: gcc
Version: 5.1.0
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
The following test case shows that gcc and g++ emit different and sometimes
misleading context in diagnostic messages involving macros:
1) In C mode, gcc points at the offending operator, while in C++ mode it points
at an operand. Since in this case the type of the operand the C++ diagnostic
points to is (before conversion) int, the diagnostic is misleading.
2) In C mode, gcc includes a note showing the context from which the macro was
invoked when the operand is the result of macro expansion, while in C++ mode it
does the same for the operand.
For consistency's sake it would be useful to harmonize the two. It seems to me
that the C output makes more sense than the output produced in C++ mode since
the issue the warning points out is not specific to one or the other operand
but rather to the operator for operators of the (converted) type.
$ (set -x && cat u.c && gcc -S -Wfloat-equal -o/dev/null u.c && gcc -S
-Wfloat-equal -o/dev/null -xc++ u.c)
+ cat u.c
int f0 (double x, int y) {
return x == y;
}
#define A(A) A
int f1 (double x, int y) {
return A (x) == A (y);
}
#define EQL ==
int f2 (double x, int y) {
return x EQL y;
}
#define EQUAL(A, B) A == B
int f3 (double x, int y) {
return EQUAL (x, y);
}
+ gcc -S -Wfloat-equal -o/dev/null u.c
u.c: In function ‘f0’:
u.c:2:14: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
return x == y;
^
u.c: In function ‘f1’:
u.c:7:18: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
return A (x) == A (y);
^
u.c: In function ‘f2’:
u.c:10:13: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
#define EQL ==
^
u.c:12:14: note: in expansion of macro ‘EQL’
return x EQL y;
^
u.c: In function ‘f3’:
u.c:15:23: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
#define EQUAL(A, B) A == B
^
u.c:17:12: note: in expansion of macro ‘EQUAL’
return EQUAL (x, y);
^
+ gcc -S -Wfloat-equal -o/dev/null -xc++ u.c
u.c: In function ‘int f0(double, int)’:
u.c:2:17: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
return x == y;
^
u.c: In function ‘int f1(double, int)’:
u.c:7:24: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
return A (x) == A (y);
^
u.c:5:14: note: in definition of macro ‘A’
#define A(A) A
^
u.c: In function ‘int f2(double, int)’:
u.c:12:18: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
return x EQL y;
^
u.c: In function ‘int f3(double, int)’:
u.c:17:22: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
return EQUAL (x, y);
^
u.c:15:26: note: in definition of macro ‘EQUAL’
#define EQUAL(A, B) A == B
^
In contrast, Clang output is consistent and clear in both modes without notes
that include the context of macro expansion where it isn't necessary:
+ /build/llvm-3.6.0/bin/clang -S -Wfloat-equal -o/dev/null u.c
u.c:2:14: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
return x == y;
~ ^ ~
u.c:7:18: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
return A (x) == A (y);
~ ^ ~
u.c:12:14: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
return x EQL y;
~ ^ ~
u.c:10:13: note: expanded from macro 'EQL'
#define EQL ==
^
u.c:17:12: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
return EQUAL (x, y);
^ ~ ~
u.c:15:23: note: expanded from macro 'EQUAL'
#define EQUAL(A, B) A == B
^
4 warnings generated.
+ /build/llvm-3.6.0/bin/clang++ -S -Wfloat-equal -o/dev/null -xc++ u.c
u.c:2:14: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
return x == y;
~ ^ ~
u.c:7:18: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
return A (x) == A (y);
~ ^ ~
u.c:12:14: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
return x EQL y;
~ ^ ~
u.c:10:13: note: expanded from macro 'EQL'
#define EQL ==
^
u.c:17:12: warning: comparing floating point with == or != is unsafe
[-Wfloat-equal]
return EQUAL (x, y);
^ ~ ~
u.c:15:23: note: expanded from macro 'EQUAL'
#define EQUAL(A, B) A == B
^
4 warnings generated.
