[gcc r14-9165] vect: Tighten check for impossible SLP layouts [PR113205]

[gcc r14-9165] vect: Tighten check for impossible SLP layouts [PR113205]

[Richard Sandiford]

https://gcc.gnu.org/g:0394ae31e832c5303f3b4aad9c66710a30c097f0

commit r14-9165-g0394ae31e832c5303f3b4aad9c66710a30c097f0
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Sat Feb 24 11:58:22 2024 +0000

    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:
---
 gcc/testsuite/gcc.dg/torture/pr113205.c | 19 +++++++++++++++++++
 gcc/tree-vect-slp.cc                    |  4 ++++
 2 files changed, 23 insertions(+)

diff --git a/gcc/testsuite/gcc.dg/torture/pr113205.c b/gcc/testsuite/gcc.dg/torture/pr113205.c
new file mode 100644
index 000000000000..edfba7fcd0e7
--- /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 7cf9504398c9..895f4f7fb6be 100644
--- 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.  */