vect: Tighten check for impossible SLP layouts [PR113205]

During its forward pass, the SLP layout code tries to calculate
the cost of a layout change on an incoming edge.  This is taken
as the minimum of two costs: one in which the source partition
keeps its current layout (chosen earlier during the pass) and
one in which the source partition switches to the new layout.
The latter can sometimes be arranged by the backward pass.

If only one of the costs is valid, the other cost was ignored.
But the PR shows that this is not safe.  If the source partition
has layout 0 (the normal layout), we have to be prepared to handle
the case in which that ends up being the only valid layout.

Other code already accounts for this restriction, e.g. see
the code starting with:

    /* Reject the layout if it would make layout 0 impossible
       for later partitions.  This amounts to testing that the
       target supports reversing the layout change on edges
       to later partitions.

gcc/
	PR tree-optimization/113205
	* tree-vect-slp.cc (vect_optimize_slp_pass::forward_cost): Reject
	the proposed layout if it does not allow a source partition with
	layout 2 to keep that layout.

gcc/testsuite/
	PR tree-optimization/113205
	* gcc.dg/torture/pr113205.c: New test.

diff --git a/gcc/testsuite/gcc.dg/torture/pr113205.c b/gcc/testsuite/gcc.dg/torture/pr113205.c
new file mode 100644 (file)
index 0000000..edfba7f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr113205.c
@@ -0,0 +1,19 @@
+char a;
+char *b, *c;
+int d, e, f, g, h;
+int *i;
+
+void
+foo (void)
+{
+  unsigned p;
+  d = i[0];
+  e = i[1];
+  f = i[2];
+  g = i[3];
+  p = d * b[0];
+  p += f * c[h];
+  p += e * b[h];
+  p += g * c[h];
+  a = (p + 8000) >> (__SIZEOF_INT__ * __CHAR_BIT__ / 2);
+}

diff --git a/gcc/tree-vect-slp.cc b/gcc/tree-vect-slp.cc
index 7cf9504398c98cccfdba3b91c3e995f73f8b5d50..895f4f7fb6bea28203a74d1ac461a454f2da6133 100644 (file)
--- a/gcc/tree-vect-slp.cc
+++ b/gcc/tree-vect-slp.cc
@@ -5034,6 +5034,10 @@ vect_optimize_slp_pass::forward_cost (graph_edge *ud, unsigned int from_node_i,
       cost.split (from_partition.out_degree);
       cost.add_serial_cost (edge_cost);
     }
+  else if (from_partition.layout == 0)
+    /* We must allow the source partition to have layout 0 as a fallback,
+       in case all other options turn out to be impossible.  */
+    return cost;
 
   /* Take the minimum of that cost and the cost that applies if
      FROM_PARTITION instead switches to TO_LAYOUT_I.  */