On Thu, Apr 19, 2012 at 8:51 PM, Xinliang David Li <davi...@google.com> wrote:
> Compiling the attached test case with -O2 using the trunk compiler,
> the compiler will ICE. The proposed patch is also attached. The test
> is under going, but I like to have discussion on the right fix first.
The patch doesn't look correct. There needs to be a VUSE, if not then
sth else is wrong.
I'm looking into it.
Richard.
> thanks,
>
> David
>
> Analysis:
> -------------
>
> Here is what is happening:
>
> BB6 : (Successor: BB8)
> MEM_19 = phi(MEM_24, MEM_25)
> t_9 = operator new (100)@MEM_19 (--> MEM_26)
> mem_ref(pp_6, -16).x@MEM_26 = t_9 (-->MEM_27) --> (1)
>
> BB8:
> ...
> MEM_20 = PHI(MEM_27, MEM_24)
> ....
> d.2348_15 = mem_ref(pp_6, -16).x@MEM_20
>
>
> In the loop header BB3 (which dominates BB6 and BB8),
>
> BB3:
> ..
> pp_31 = phi (pp_6, 0);
> ...
> pp_6 = pp_31 + 16
>
>
>
> In PRE, ANTIC_IN(BB8) includes mem_ref(pp_31,0),x@MEM_20. After phi
> translate, ANTIC_OUT(BB6) gets mem_ref(pp_31,0).x@MEM_27. However
> this expression gets propagated into ANTIC_IN(BB6) even though the
> memory expression is killed by (1). The root cause is that the alias
> oracle reports that mem_ref(pp_6, -16) and mem_ref(pp_31, 0) are not
> aliased as their points-to set do not intersect.
>
> As as consequence of the bad aliasing, the operator new calls gets
> transformed into an gimple assignment -- this gimple assignment
> becomes the defining statement for MEM_26. In a later UD chain walk,
> the memory chain stops (and triggers the assert) at this new
> assignment statement because there is no associated vuse for it.
>
>
> Test case
> --------------
>
> The case is synthesized -- the real world examples involve lots of inlining
> etc.
>
>
> int g, *gp[100];
> struct V {
> int* x;
> int y;
> };
>
> void foo (V **p, V* end, int i)
> {
> *p = 0;
> V* pp = *p;
> int s = 100;
> for (; pp < end; )
> {
> pp++;
> (pp-1)->x = &g;
> if (g)
> {
> if (g>10)
> g++;
> int *t = (int*) operator new (100);
> (pp-1)->x = t;
> }
> else
> s--;
> gp[end-pp] = (pp-1)->x + s;
> }
> }
>
>
> Patch
> ---------
>
>
> Index: tree-ssa-structalias.c
> ===================================================================
> --- tree-ssa-structalias.c (revision 186600)
> +++ tree-ssa-structalias.c (working copy)
> @@ -6137,6 +6137,9 @@ pt_solutions_intersect_1 (struct pt_solu
> if (pt1->anything || pt2->anything)
> return true;
>
> + if (pt1->null && pt2->null)
> + return true;
> +
> /* If either points to unknown global memory and the other points to
> any global memory they alias. */
> if ((pt1->nonlocal
