Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift

[Kevin O'Connor <kevin@koconnor.net>]

On Fri, Oct 02, 2015 at 02:07:32PM +0200, Paolo Bonzini wrote:
> On 02/10/2015 13:14, Laszlo Ersek wrote:
> > On 10/02/15 10:34, Paolo Bonzini wrote:
> >> On 01/10/2015 21:17, Laszlo Ersek wrote:
> >>> - In the firmware, allocate an array of bytes, dynamically. This array
> >>>   will have no declared type.
> >>>
> >>> - Populate the array byte-wise, from fw_cfg. Because the stores happen
> >>>   through character-typed lvalues, they do not "imbue" the target
> >>>   object with any effective type, for further accesses that do not
> >>>   modify the value. (I.e., for further reads.)
> >>>
> >>> - Get a (uint8_t*) into the array somewhere, and cast it to
> >>>   (struct acpi_table_hdr *). Read fields through the cast pointer.
> >>>   Assuming no out-of-bounds situation (considering the entire
> >>>   pointed to acpi_table_hdr struct), and assuming no alignment
> >>>   violations for the fields (which is implementation-defined), these
> >>>   accesses will be fine.
> >>>
> >>> *However*. If in point 2 you populate the array with uint64_t accesses,
> >>> that *does* imbue the array elements with an effective type that is
> >>> binding for further read accesses.
> >>
> >> Then don't do it.  Use memcpy from uint64_t to the array.
> > 
> > It won't work; memcpy() propagates the effective type.
> 
> Doh.  I guess that's another "not in practice" case.  Saying "memcpy to
> {,u}int8_t doesn't propagate the effective type" would probably go to
> great lengths towards fixing this.

Just to be pedantic, uint8_t/int8_t are not the same as 'char' wrt
aliasing rules.  (The standard defines writes to a char array/pointer
as being allowed to alias with other types, but does not say that
about int8_t.)  Gcc currently treats them as the same; I actually
tried to get gcc to change that a few months ago:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66110#c13

FWIW, I think the aliasing rules allow for very useful optimizations
and I wouldn't want to turn them off for programs where performance is
important.

The test case in the bug link above (which the gcc developers
thankfully did address!) is a good example of the utility of alias
detection.  This function:

void func(struct s2 *p)
{
    p->p1->f2 = 9;
    p->p1->f2 = 10;
}

can't be optimized without -fstrict-aliasing.  Indeed, even if the
code was changed to p->p1->f3 = 11; p->p1->f4 = 12; then gcc would
still need to reload p->p1 after every store.  That's just silly.

-Kevin