From fe8475c500939011b90504304aec61bf6f48ac7d Mon Sep 17 00:00:00 2001
From: Richard Biener <rguenther@suse.de>
Date: Fri, 22 Oct 2021 10:32:36 +0200
Subject: [PATCH] bootstrap/102681 - properly CSE PHIs with default def args

The PR shows that we fail to CSE PHIs containing (different)
default definitions due to the fact on how we now handle
on-demand build of VN_INFO.  The following fixes this in the
same way the PHI visitation code does.

On gcc.dg/ubsan/pr81981.c this causes one expected warning to be
elided since the uninit pass sees the change

   <bb 4> [local count: 1073741824]:
   # u$0_2 = PHI <u$0_5(D)(3), i_3(D)(5)>
-  # cstore_11 = PHI <t$0_6(D)(3), i_3(D)(5)>
   v = u$0_2;
-  return cstore_11;
+  return u$0_2;

and thus only one of the conditionally uninitialized uses (the
other became dead).  I have XFAILed the missing diagnostic,
I don't see a way to preserve that.

2021-10-22  Richard Biener  <rguenther@suse.de>

	PR bootstrap/102681
	* tree-ssa-sccvn.c (vn_phi_insert): For undefined SSA args
	record VN_TOP.
	(vn_phi_lookup): Likewise.

	* gcc.dg/tree-ssa/ssa-fre-97.c: New testcase.
	* gcc.dg/ubsan/pr81981.c: XFAIL one case.
---
 gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-97.c | 19 +++++++++++++++++++
 gcc/testsuite/gcc.dg/ubsan/pr81981.c       |  2 +-
 gcc/tree-ssa-sccvn.c                       | 14 ++++++++++++--
 3 files changed, 32 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-97.c

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-97.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-97.c
new file mode 100644
index 0000000..2f09c8b
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-97.c
@@ -0,0 +1,19 @@
+/* { dg-do compile } */
+/* ethread threading does not yet catch this but it might at some point.  */
+/* { dg-options "-O -fdump-tree-fre1-details -fno-thread-jumps" } */
+
+int foo (int b, int x)
+{
+  int i, j;
+  if (b)
+    i = x;
+  if (b)
+    j = x;
+  return j == i;
+}
+
+/* Even with different undefs we should CSE a PHI node with the
+   same controlling condition.  */
+
+/* { dg-final { scan-tree-dump "Replaced redundant PHI node" "fre1" } } */
+/* { dg-final { scan-tree-dump "return 1;" "fre1" } } */
diff --git a/gcc/testsuite/gcc.dg/ubsan/pr81981.c b/gcc/testsuite/gcc.dg/ubsan/pr81981.c
index 8a6597c..d201efb 100644
--- a/gcc/testsuite/gcc.dg/ubsan/pr81981.c
+++ b/gcc/testsuite/gcc.dg/ubsan/pr81981.c
@@ -16,6 +16,6 @@ foo (int i)
       u[0] = i;
     }
 
-  v = u[0];		/* { dg-warning "may be used uninitialized" } */
+  v = u[0];		/* { dg-warning "may be used uninitialized" "" { xfail *-*-* } } */
   return t[0];		/* { dg-warning "may be used uninitialized" } */
 }
diff --git a/gcc/tree-ssa-sccvn.c b/gcc/tree-ssa-sccvn.c
index ae0172a..893b1d0 100644
--- a/gcc/tree-ssa-sccvn.c
+++ b/gcc/tree-ssa-sccvn.c
@@ -4499,7 +4499,12 @@ vn_phi_lookup (gimple *phi, bool backedges_varying_p)
       tree def = PHI_ARG_DEF_FROM_EDGE (phi, e);
       if (TREE_CODE (def) == SSA_NAME
 	  && (!backedges_varying_p || !(e->flags & EDGE_DFS_BACK)))
-	def = SSA_VAL (def);
+	{
+	  if (ssa_undefined_value_p (def, false))
+	    def = VN_TOP;
+	  else
+	    def = SSA_VAL (def);
+	}
       vp1->phiargs[e->dest_idx] = def;
     }
   vp1->type = TREE_TYPE (gimple_phi_result (phi));
@@ -4543,7 +4548,12 @@ vn_phi_insert (gimple *phi, tree result, bool backedges_varying_p)
       tree def = PHI_ARG_DEF_FROM_EDGE (phi, e);
       if (TREE_CODE (def) == SSA_NAME
 	  && (!backedges_varying_p || !(e->flags & EDGE_DFS_BACK)))
-	def = SSA_VAL (def);
+	{
+	  if (ssa_undefined_value_p (def, false))
+	    def = VN_TOP;
+	  else
+	    def = SSA_VAL (def);
+	}
       vp1->phiargs[e->dest_idx] = def;
     }
   vp1->value_id = VN_INFO (result)->value_id;
-- 
2.7.4