[gcc r12-2334] analyzer: handle self-referential phis

[gcc r12-2334] analyzer: handle self-referential phis

[David Malcolm]

https://gcc.gnu.org/g:a9241df96e1950c630550ada9371c0b4a03496cf

commit r12-2334-ga9241df96e1950c630550ada9371c0b4a03496cf
Author: David Malcolm <dmalcolm@redhat.com>
Date:   Thu Jul 15 15:01:57 2021 -0400

    analyzer: handle self-referential phis
    
    gcc/analyzer/ChangeLog:
            * state-purge.cc (self_referential_phi_p): New.
            (state_purge_per_ssa_name::process_point): Don't purge an SSA name
            at its def-stmt if the def-stmt is self-referential.
    
    gcc/testsuite/ChangeLog:
            * gcc.dg/analyzer/phi-1.c: New test.
    
    Signed-off-by: David Malcolm <dmalcolm@redhat.com>

Diff:
---
 gcc/analyzer/state-purge.cc           | 37 ++++++++++++++++++++++++++++++++---
 gcc/testsuite/gcc.dg/analyzer/phi-1.c | 24 +++++++++++++++++++++++
 2 files changed, 58 insertions(+), 3 deletions(-)

diff --git a/gcc/analyzer/state-purge.cc b/gcc/analyzer/state-purge.cc
index 70a09ed581f..e82ea87e735 100644
--- a/gcc/analyzer/state-purge.cc
+++ b/gcc/analyzer/state-purge.cc
@@ -288,6 +288,20 @@ state_purge_per_ssa_name::add_to_worklist (const function_point &point,
     }
 }
 
+/* Does this phi depend on itself?
+   e.g. in:
+     added_2 = PHI <added_6(2), added_2(3), added_11(4)>
+   the middle defn (from edge 3) requires added_2 itself.  */
+
+static bool
+self_referential_phi_p (const gphi *phi)
+{
+  for (unsigned i = 0; i < gimple_phi_num_args (phi); i++)
+    if (gimple_phi_arg_def (phi, i) == gimple_phi_result (phi))
+      return true;
+  return false;
+}
+
 /* Process POINT, popped from WORKLIST.
    Iterate over predecessors of POINT, adding to WORKLIST.  */
 
@@ -326,11 +340,28 @@ state_purge_per_ssa_name::process_point (const function_point &point,
 	     !gsi_end_p (gpi); gsi_next (&gpi))
 	  {
 	    gphi *phi = gpi.phi ();
+	    /* Are we at the def-stmt for m_name?  */
 	    if (phi == def_stmt)
 	      {
-		if (logger)
-		  logger->log ("def stmt within phis; terminating");
-		return;
+		/* Does this phi depend on itself?
+		   e.g. in:
+		     added_2 = PHI <added_6(2), added_2(3), added_11(4)>
+		   the middle defn (from edge 3) requires added_2 itself
+		   so we can't purge it here.  */
+		if (self_referential_phi_p (phi))
+		  {
+		    if (logger)
+		      logger->log ("self-referential def stmt within phis;"
+				   " continuing");
+		  }
+		else
+		  {
+		    /* Otherwise, we can stop here, so that m_name
+		       can be purged.  */
+		    if (logger)
+		      logger->log ("def stmt within phis; terminating");
+		    return;
+		  }
 	      }
 	  }
 
diff --git a/gcc/testsuite/gcc.dg/analyzer/phi-1.c b/gcc/testsuite/gcc.dg/analyzer/phi-1.c
new file mode 100644
index 00000000000..09260033fef
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/analyzer/phi-1.c
@@ -0,0 +1,24 @@
+/* { dg-do "compile" } */
+
+typedef __SIZE_TYPE__ size_t;
+#define NULL ((void *) 0)
+
+extern const char *foo (void);
+extern size_t bar (void);
+
+void
+_nl_expand_alias (const char *locale_alias_path)
+{
+  size_t added;
+  do
+    {
+      added = 0;
+      while (added == 0 && locale_alias_path[0] != '\0')
+	{
+	  const char *start = foo ();
+	  if (start < locale_alias_path)
+	    added = bar ();
+	}
+    }
+  while (added != 0);
+}