[PATCH] Look through clobber stmts in uninit (PR tree-optimization/79345)

[Jakub Jelinek]

Hi!

As mentioned by Jason in the PR, we've regressed on the following testcase
since we started emitting CLOBBERs at the start of ctors (and we warn as
before with -fno-lifetime-dse -Wuninitialized).
With -fno-lifetime-dse, the vuse on the b.x read stmt is default def
and thus we warn, but if there are clobbers before that, that is not the
case and we don't warn.  The patch is quick hack to bypass the initial
clobbers as long as there aren't really many.  If we wanted to handle
all initial clobbers, I bet the first time we run into this we could
recursively walk vop uses from the default def and build say a bitmap
of vop SSA_NAMEs which are vdefs of clobbers that only have clobbers
before it as vdef stmts.

Now, the comment says:
          /* For memory the only cheap thing we can do is see if we
             have a use of the default def of the virtual operand.
             ???  Not so cheap would be to use the alias oracle via
             walk_aliased_vdefs, if we don't find any aliasing vdef
             warn as is-used-uninitialized, if we don't find an aliasing
             vdef that kills our use (stmt_kills_ref_p), warn as
             may-be-used-uninitialized.  But this walk is quadratic and
             so must be limited which means we would miss warning
             opportunities.  */
I wonder if it isn't useful to walk even limited number of vdefs this way
anyway (likely GCC8 material though), e.g. if we run into a clobber that
must (rather than just may) portion of the read ref (and of course when
seeing non-clobber stmt that could alias with the ref give up before that),
we could warn even if we are very far from the start of the function.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk (or do
you want a version with a bitmap and really ignoring all the clobbers,
rather than just 128 of them)?

2017-02-20  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/79345
	* tree-ssa-uninit.c (warn_uninitialized_vars): If vuse is not
	default def, but there are only CLOBBER stmts as vdefs, handle it like
	default def.

	* g++.dg/warn/pr79345.C: New test.

--- gcc/tree-ssa-uninit.c.jj	2017-01-01 12:45:35.000000000 +0100
+++ gcc/tree-ssa-uninit.c	2017-02-20 16:43:08.244868075 +0100
@@ -248,8 +248,7 @@ warn_uninitialized_vars (bool warn_possi
 	  use = gimple_vuse (stmt);
 	  if (use
 	      && gimple_assign_single_p (stmt)
-	      && !gimple_vdef (stmt)
-	      && SSA_NAME_IS_DEFAULT_DEF (use))
+	      && !gimple_vdef (stmt))
 	    {
 	      tree rhs = gimple_assign_rhs1 (stmt);
 	      tree base = get_base_address (rhs);
@@ -260,6 +259,23 @@ warn_uninitialized_vars (bool warn_possi
 		  || is_global_var (base))
 		continue;
 
+	      /* Look through some CLOBBER stmts.  */
+	      for (unsigned int cnt = 0; cnt < 128 && use; cnt++)
+		{
+		  if (SSA_NAME_IS_DEFAULT_DEF (use))
+		    break;
+
+		  gimple *def_stmt = SSA_NAME_DEF_STMT (use);
+		  if (!gimple_clobber_p (def_stmt))
+		    {
+		      use = NULL_TREE;
+		      break;
+		    }
+		  use = gimple_vuse (def_stmt);
+		}
+	      if (use == NULL_TREE)
+		continue;
+
 	      if (always_executed)
 		warn_uninit (OPT_Wuninitialized, use, gimple_assign_rhs1 (stmt),
 			     base, "%qE is used uninitialized in this function",
--- gcc/testsuite/g++.dg/warn/pr79345.C.jj	2017-02-20 17:19:01.138952915 +0100
+++ gcc/testsuite/g++.dg/warn/pr79345.C	2017-02-20 17:18:20.000000000 +0100
@@ -0,0 +1,22 @@
+// PR tree-optimization/79345
+// { dg-do compile { target c++11 } }
+// { dg-options "-O2 -Wuninitialized" }
+
+struct A {
+  A (int);
+};
+
+struct B : A {
+  const bool x = true;
+
+  B () : A (x ? 3 : 7) { }	// { dg-warning "is used uninitialized in this function" }
+};
+
+void foo (void*);
+
+void
+bar ()
+{
+  B b;
+  foo (&b);
+}

	Jakub