[Bug middle-end/39895] New: gcc-4.4 -Wstrict-aliasing and -Wstrict-aliasing=3 behaves like -Wstrict-aliasing=2 in gcc-4.3

[edwintorok at gmail dot com]

Considering this code:
void dummy(void *x);
union union_t {
    unsigned un;
    char c;
} __attribute__((packed));

unsigned foo()
{
    char x[4];
    dummy(x);
    return ((union union_t*)x)->un;
}

unsigned bar(char *x)
{
    return ((union union_t*)x)->un;
}

With gcc-4.4 -Wstrict-aliasing=1 is the only one that does *not* give a
warning, levels 2 and 3 do give warnings:
$ gcc-4.4 -Wstrict-aliasing p.c -O2 -c
p.c: In function ‘foo’:
p.c:11: warning: dereferencing type-punned pointer will break strict-aliasing 
rules
$ gcc-4.4 -Wstrict-aliasing=2 p.c -O2 -c
p.c: In function ‘foo’:
p.c:11: warning: dereferencing type-punned pointer will break strict-aliasing
rules
$ gcc-4.4 -Wstrict-aliasing=1 p.c -O2 -c

However in the case of gcc-4.3, -Wstrict-aliasing=2 is the only one that gives
warnings, levels 1 and 3 give no warning:
$ gcc-4.3 -Wstrict-aliasing p.c -O2 -c
$ gcc-4.3 -Wstrict-aliasing=2 p.c -O2 -c
p.c: In function ‘foo’:
p.c:11: warning: dereferencing type-punned pointer might break strict-aliasing
rules
$ gcc-4.3 -Wstrict-aliasing=1 p.c -O2 -c


According to the gcc manpage -Wstrict-aliasing=3 should have the fewest false
positives and false negatives, yet with gcc-4.4 -Wstrict-aliasing=3 gives a
warning that is not given at -Wstrict-aliasing=1 (the one that is supposed to
have many false positives).

This only happens if 'x' is allocated on the stack, gcc-4.4 is perfectly happy
if it is a char* argument to the function.

I've also tried the 'Casting through a union (1)' described at
http://www.cellperformance.com/mike_acton/2006/06/understanding_strict_aliasing.html
but that too gives warnings by default.

Conclusion is that with gcc-4.4 -O2 -Wall there is no way to read/store from a
stack allocated variable through a union, using a different type member of the
union without raising a warning. 

Is there another recommended way in gcc-4.4 to cast from uint8_t* to uint32_t*?


-- 
           Summary: gcc-4.4 -Wstrict-aliasing and -Wstrict-aliasing=3
                    behaves like -Wstrict-aliasing=2 in gcc-4.3
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: edwintorok at gmail dot com
 GCC build triplet: x86_64-linux-gnu
  GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39895