This is the mail archive of the
gcc@gcc.gnu.org
mailing list for the GCC project.
Re: Minimum target alignment for a datatype
On Jul 22, 2005, at 1:14 PM, Richard Henderson wrote:
On Fri, Jul 22, 2005 at 11:30:40AM -0700, Chris Lattner wrote:
Understood. I'm just looking for the minimum type alignment without
user alignment. It appears that this is impossible to get from the
targets, due to the way the targets can override type alignments in
(almost completely) general ways. Is this true?
For !STRICT_ALIGNMENT, there is no interesting minimum.
After talking to people on IRC about this, some have mentioned that
this may be a bug in the darwin-ppc target description. Consider
this (untested) code:
struct X {
int A; double D;
};
void test(double *P) {
// alignof(double) == 8, but the dynamic alignment of P is only 4.
memset(P, 0, sizeof(double));
}
void problem() { // This could be in another translation unit.
struct X a; // 4 byte alignment.
test(&a.D);
}
The problem in this case is that the optimizer cannot assume that a
T* is at least as aligned as TYPE_ALIGN(T) in this case. People have
pointed out that the alignment of double could be represented as 4,
and increased when necessary (instead of represented as 8 and reduced
when needed).
I have no idea whether this is right or wrong, but as Dale recently
noticed, TYPE_ALIGN leaks out to user code through __alignof__, so
changing it may not be practical.
Anyone have any thoughts or opinions? This may have impact on a
number of things, including any pointer alignment analysis.
-Chris