[Bug c++/27464] New: unsuitable aliasing rule warning

["thor at math dot tu-berlin dot de" <gcc-bugzilla at gcc dot gnu dot org>]

Enter the following program and compile with -Wall -fstrict-aliasing

int main(int argc,char **argv)
{
  float f = 0.5;

  return reinterpret_cast<int &>(f);
}

g++ will return the following warning message:

warning: dereferencing type-punned pointer will break strict-aliasing rules

However, this warning message is confusing at best since no pointer handling
is involved here. And then, this program cannot exhibit pointer-aliasing
breakage since the variable "f" and the return code cannot interpret the same
variable as two different types - the re-interpretation of the floating point
value as integer-bit pattern is copied before any aliasing problems could
possibly enter the scene.

Interestingly, g++ explicitly allows the re-interpretation of bit patterns of a
specific type thru the "union trick", i.e. 

union {
 float i;
 int o;
} u;

then assigning the input value to u.i and reading the bit-pattern back from
u.o. 

However, the former "reinterpret_cast" is only implementation-defined (as the
implementation defines the bit-pattern transition from float and its re-inter-
pretation as int) whereas the latter "union cast" is undefined behaivour: 
A compiler is free to re-organize the access patterns by moving the assignment
to u.i *behind* the read-back from u.o. Thus, the first method is clearly
preferable when this kind of type-reinterpretation is apropriate at all, and
thus should be allowed to pass without a warning. The latter is only defined
because g++ took the freedom to allow it and to avoid the instruction
re-ordering.


-- 
           Summary: unsuitable aliasing rule warning
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: thor at math dot tu-berlin dot de
 GCC build triplet: i486-pc-linux-gnu
  GCC host triplet: i486-pc-linux-gnu
GCC target triplet: i486-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27464