[gcc r11-9800] tree-optimization/105198 - wrong code with predictive commoning

[Richard Biener]

https://gcc.gnu.org/g:542c30dc4d220f6d2138e55d5fb8e1529339badf

commit r11-9800-g542c30dc4d220f6d2138e55d5fb8e1529339badf
Author: Richard Biener <rguenther@suse.de>
Date:   Fri Apr 8 13:13:29 2022 +0200

    tree-optimization/105198 - wrong code with predictive commoning
    
    When predictive commoning looks for a looparound PHI it tries
    to match the entry value definition (a load) up with the appropriate
    member of the chain.  But it fails to consider stmts clobbering
    the very same memory location inbetween the load and loop entry.
    
    In theory we could be more clever on must aliases that would be
    also picked up from a load (so not exactly stmt_kills_ref_p) and
    use the stored value from that if it is an exact match.  But we
    currently have no way to propagate this information inside predcom.
    
    2022-04-08  Richard Biener  <rguenther@suse.de>
    
            PR tree-optimization/105198
            * tree-predcom.c (find_looparound_phi): Check whether
            the found memory location of the entry value is clobbered
            inbetween the value we want to use and loop entry.
    
            * gcc.dg/torture/pr105198.c: New testcase.
    
    (cherry picked from commit e5453bcc217ea4ac53a4ac277661d6ef0ccd425b)

Diff:
---
 gcc/testsuite/gcc.dg/torture/pr105198.c | 26 ++++++++++++++++++++++
 gcc/tree-predcom.c                      | 38 ++++++++++++++++++++++++++++-----
 2 files changed, 59 insertions(+), 5 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/torture/pr105198.c b/gcc/testsuite/gcc.dg/torture/pr105198.c
new file mode 100644
index 00000000000..91f92afc163
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr105198.c
@@ -0,0 +1,26 @@
+/* { dg-do run } */
+/* { dg-additional-options "-fno-tree-pre -fpredictive-commoning" } */
+
+static  __attribute__ ((noipa)) void
+next_set(int *x, int n, int k)
+{
+  int j = k - 1;
+  int tmp = x[j]++;
+  while (j > 0)
+    {
+      if (x[j] < n - (k - 1 -j))
+        break;
+      j--;
+      x[j]++;
+      tmp = x[j];
+    }
+  if (tmp != 2 || j != 1 || x[0] != 0 || x[1] != 2 || x[2] != 5)
+    __builtin_abort ();
+}
+
+int main()
+{
+  int x[3] = {0, 1, 4};
+  next_set(x, 5, 3);
+  return 0;
+}
diff --git a/gcc/tree-predcom.c b/gcc/tree-predcom.c
index 5482f50198a..0a7c1a936ba 100644
--- a/gcc/tree-predcom.c
+++ b/gcc/tree-predcom.c
@@ -1253,7 +1253,6 @@ static gphi *
 find_looparound_phi (class loop *loop, dref ref, dref root)
 {
   tree name, init, init_ref;
-  gphi *phi = NULL;
   gimple *init_stmt;
   edge latch = loop_latch_edge (loop);
   struct data_reference init_dr;
@@ -1271,14 +1270,19 @@ find_looparound_phi (class loop *loop, dref ref, dref root)
   if (!name)
     return NULL;
 
+  tree entry_vuse = NULL_TREE;
+  gphi *phi = NULL;
   for (psi = gsi_start_phis (loop->header); !gsi_end_p (psi); gsi_next (&psi))
     {
-      phi = psi.phi ();
-      if (PHI_ARG_DEF_FROM_EDGE (phi, latch) == name)
+      gphi *p = psi.phi ();
+      if (PHI_ARG_DEF_FROM_EDGE (p, latch) == name)
+	phi = p;
+      else if (virtual_operand_p (gimple_phi_result (p)))
+	entry_vuse = PHI_ARG_DEF_FROM_EDGE (p, loop_preheader_edge (loop));
+      if (phi && entry_vuse)
 	break;
     }
-
-  if (gsi_end_p (psi))
+  if (!phi || !entry_vuse)
     return NULL;
 
   init = PHI_ARG_DEF_FROM_EDGE (phi, loop_preheader_edge (loop));
@@ -1306,6 +1310,30 @@ find_looparound_phi (class loop *loop, dref ref, dref root)
   if (!valid_initializer_p (&init_dr, ref->distance + 1, root->ref))
     return NULL;
 
+  /* Make sure nothing clobbers the location we re-use the initial value
+     from.  */
+  if (entry_vuse != gimple_vuse (init_stmt))
+    {
+      ao_ref ref;
+      ao_ref_init (&ref, init_ref);
+      unsigned limit = param_sccvn_max_alias_queries_per_access;
+      tree vdef = entry_vuse;
+      do
+	{
+	  gimple *def = SSA_NAME_DEF_STMT (vdef);
+	  if (limit-- == 0 || gimple_code (def) == GIMPLE_PHI)
+	    return NULL;
+	  if (stmt_may_clobber_ref_p_1 (def, &ref))
+	    /* When the stmt is an assign to init_ref we could in theory
+	       use its RHS for the initial value of the looparound PHI
+	       we replace in prepare_initializers_chain, but we have
+	       no convenient place to store this info at the moment.  */
+	    return NULL;
+	  vdef = gimple_vuse (def);
+	}
+      while (vdef != gimple_vuse (init_stmt));
+    }
+
   return phi;
 }