Currently, the middle end seems to use the same rules for handling
constant overflow of integer arithmetic and conversion between integer
types: set TREE_OVERFLOW on the INTEGER_CST if the type is signed and
the value doesn't fit in the target type. But this doesn't seem to
match the C/C++ standards.
C99 says,
6.3.1.3 Signed and unsigned integers
1 When a value with integer type is converted to another integer type
other than _Bool, if the value can be represented by the new type, it is
unchanged.
2 Otherwise, if the new type is unsigned, the value is converted by
repeatedly adding or subtracting one more than the maximum value that
can be represented in the new type until the value is in the range of
the new type.49)
3 Otherwise, the new type is signed and the value cannot be represented
in it; either the result is implementation-defined or an
implementation-defined signal is raised.
6.5 Expressions
5 If an exceptional condition occurs during the evaluation of an
expression (that is, if the result is not mathematically defined or not
in the range of representable values for its type), the behavior is
undefined.
Note the difference. When converting an integer value to another
integer type that it doesn't fit into, the behavior is either
well-defined or implementation-defined. When arithmetic produces a
value that doesn't fit into the type in which the arithmetic is done,
the behavior is undefined even if the type is unsigned.
So we're setting TREE_OVERFLOW inappropriately for conversion to signed
integer types (though the front ends unset it again in cast context),
and, more problematically, failing to set it for unsigned arithmetic
overflow:
#include <limits.h>
enum E {
A = (unsigned char)-1, // OK
B = (signed char)UCHAR_MAX, // implementation-defined
C = INT_MAX+1, // undefined (C)/ill-formed (C++)
D = UINT_MAX+1 // undefined (C)/ill-formed (C++)
};
Am I missing something?
This is more of a problem for C++, which says that arithmetic overflow
in a context that requires a constant expression is ill-formed; in C
it's merely undefined.
Jason
