tree-optimization/102920 - fix PHI VN with undefined args

This fixes a latent issue exposed by now allowing VN_TOP in PHI
arguments.  We may only use optimistic equality when merging values on
different edges, not when merging values on the same edge - in particular
we may not choose the undef value on any edge when there's a not undef
value as well.

2021-10-25  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/102920
	* tree-ssa-sccvn.h (expressions_equal_p): Add argument
	controlling VN_TOP matching behavior.
	* tree-ssa-sccvn.c (expressions_equal_p): Likewise.
	(vn_phi_eq): Do not optimistically match VN_TOP.

	* gcc.dg/torture/pr102920.c: New testcase.

3 files changed, 40 insertions, 8 deletions

diff --git a/gcc/testsuite/gcc.dg/torture/pr102920.c b/gcc/testsuite/gcc.dg/torture/pr102920.c
new file mode 100644
index 0000000..aa27ac5
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr102920.c
@@ -0,0 +1,25 @@
+/* { dg-do run } */
+/* { dg-additional-options "-funswitch-loops" } */
+
+unsigned short a = 42;
+unsigned short b = 1;
+long int c = 1;
+unsigned char var_120;
+unsigned char var_123;
+
+void __attribute__((noipa)) test(unsigned short a, unsigned short b, long c)
+{
+  for (char i = 0; i < (char)c; i += 5)
+    if (!b)
+      var_120 = a;
+    else
+      var_123 = a;
+}
+
+int main()
+{
+  test(a, b, c);
+  if (var_123 != 42)
+    __builtin_abort ();
+  return 0;
+}
diff --git a/gcc/tree-ssa-sccvn.c b/gcc/tree-ssa-sccvn.c
index 893b1d0..d524259 100644
--- a/gcc/tree-ssa-sccvn.c
+++ b/gcc/tree-ssa-sccvn.c
@@ -4441,11 +4441,15 @@ vn_phi_eq (const_vn_phi_t const vp1, const_vn_phi_t const vp2)
 	    if (inverted_p)
 	      std::swap (te2, fe2);
 
-	    /* ???  Handle VN_TOP specially.  */
+	    /* Since we do not know which edge will be executed we have
+	       to be careful when matching VN_TOP.  Be conservative and
+	       only match VN_TOP == VN_TOP for now, we could allow
+	       VN_TOP on the not prevailing PHI though.  See for example
+	       PR102920.  */
 	    if (! expressions_equal_p (vp1->phiargs[te1->dest_idx],
-				       vp2->phiargs[te2->dest_idx])
+				       vp2->phiargs[te2->dest_idx], false)
 		|| ! expressions_equal_p (vp1->phiargs[fe1->dest_idx],
-					  vp2->phiargs[fe2->dest_idx]))
+					  vp2->phiargs[fe2->dest_idx], false))
 	      return false;
 
 	    return true;
@@ -4470,7 +4474,7 @@ vn_phi_eq (const_vn_phi_t const vp1, const_vn_phi_t const vp2)
       tree phi2op = vp2->phiargs[i];
       if (phi1op == phi2op)
 	continue;
-      if (!expressions_equal_p (phi1op, phi2op))
+      if (!expressions_equal_p (phi1op, phi2op, false))
 	return false;
     }
 
@@ -5816,17 +5820,20 @@ get_next_constant_value_id (void)
 }
 
 
-/* Compare two expressions E1 and E2 and return true if they are equal.  */
+/* Compare two expressions E1 and E2 and return true if they are equal.
+   If match_vn_top_optimistically is true then VN_TOP is equal to anything,
+   otherwise VN_TOP only matches VN_TOP.  */
 
 bool
-expressions_equal_p (tree e1, tree e2)
+expressions_equal_p (tree e1, tree e2, bool match_vn_top_optimistically)
 {
   /* The obvious case.  */
   if (e1 == e2)
     return true;
 
   /* If either one is VN_TOP consider them equal.  */
-  if (e1 == VN_TOP || e2 == VN_TOP)
+  if (match_vn_top_optimistically
+      && (e1 == VN_TOP || e2 == VN_TOP))
     return true;
 
   /* SSA_NAME compare pointer equal.  */
diff --git a/gcc/tree-ssa-sccvn.h b/gcc/tree-ssa-sccvn.h
index 8a1b649..7d53ab5 100644
--- a/gcc/tree-ssa-sccvn.h
+++ b/gcc/tree-ssa-sccvn.h
@@ -22,7 +22,7 @@
 #define TREE_SSA_SCCVN_H
 
 /* In tree-ssa-sccvn.c  */
-bool expressions_equal_p (tree, tree);
+bool expressions_equal_p (tree, tree, bool = true);
 
 
 /* TOP of the VN lattice.  */