[Bug tree-optimization/88223] [8 Regression] Wrong code for intrinsic memmove

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88223

--- Comment #11 from rguenther at suse dot de <rguenther at suse dot de> ---
On Wed, 28 Nov 2018, joseph at codesourcery dot com wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88223
> 
> --- Comment #10 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
> On Wed, 28 Nov 2018, rguenth at gcc dot gnu.org wrote:
> 
> > Hmm, OTOH the C standard specifies that the store to u.b.b makes it the
> > u.b the active member and it makes the old contents undefined.  That
> > would mean when loading u.a.b after the store we cannot rely on the
> > exact value we get but at least the bits touched by u.b.b should be
> > up-to-date (which is enough to show breakage).  I thought about
> > using
> 
> In my view, storing to u.b.b should be equivalent to loading u.b (a 
> type-punning operation if u.a was previously the active member), storing 
> the new value of u.b.b in a copy of the loaded value of u.b, and storing 
> that result back in u.b (making u.b the active member of u).  (So if bits 
> set in u.a are not padding in u.b, and not part of the space occupied by 
> u.b.b, they should be preserved.)

OK, that's convenient enough to support for the compiler.  Unfortunately
that means my testcases become valid and we still miscompile them...

Now, for type-punning to be valid the union has to be visible in the
access, but this visibility can be lost in the middle-end given
the middle-end implements sth more conservative.

Still that a store is a punning access makes the following valid?

union u { struct { int i; int j; } x; struct { float f; float g; } y; };

union u A;
float foo (union u *B)
{
  u->x.i = 0;
  A.y.g = 1.0;
  return u->y.f;
}

is this valid punning of u->x.i to u->y.f (int->float) because the
actual type-punning is implicitely done via A.y.g = 1.0?  Of course the
constraint is that B points to A.

We certainly happily re-order the store and load via u.