[gcc r13-2926] tree-optimization/105646 - re-interpret always executed in uninit diag

[Richard Biener]

https://gcc.gnu.org/g:a1cd4d52d6ef90b977fb2d80c1cf17f3efa5b01d

commit r13-2926-ga1cd4d52d6ef90b977fb2d80c1cf17f3efa5b01d
Author: Richard Biener <rguenther@suse.de>
Date:   Fri Aug 19 15:11:14 2022 +0200

    tree-optimization/105646 - re-interpret always executed in uninit diag
    
    The following fixes PR105646, not diagnosing
    
    int f1();
    int f3(){
        auto const & a = f1();
        bool v3{v3};
        return a;
    }
    
    with optimization because the early uninit diagnostic pass only
    diagnoses always executed cases.  The patch does this by
    re-interpreting what always executed means and choosing to
    ignore exceptional and abnormal control flow for this.  At the
    same time it improves things as suggested in a comment - when
    the value-numbering run done without optimizing figures there's
    a fallthru path, consider blocks on it as always executed.
    
            PR tree-optimization/105646
            * tree-ssa-uninit.cc (warn_uninitialized_vars): Pre-compute
            the set of fallthru reachable blocks from function entry
            and use that to determine wlims.always_executed.
    
            * g++.dg/uninit-pr105646.C: New testcase.

Diff:
---
 gcc/testsuite/g++.dg/uninit-pr105646.C | 17 +++++++++++++++++
 gcc/tree-ssa-uninit.cc                 | 10 ++++++++--
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/gcc/testsuite/g++.dg/uninit-pr105646.C b/gcc/testsuite/g++.dg/uninit-pr105646.C
new file mode 100644
index 00000000000..48ceb986ec6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/uninit-pr105646.C
@@ -0,0 +1,17 @@
+// { dg-do compile }
+// { dg-require-effective-target c++11 }
+// { dg-options "-O2 -Wuninitialized" }
+
+int f1();
+int f2(){
+    bool v2{v2}; // { dg-warning "is used uninitialized" }
+    auto const & a = f1();
+    return a;
+}
+int f3(){
+    auto const & a = f1();
+    // Diagnose the following when optimizing and as unconditional
+    // uninitialized use despite f1 possibly throwing
+    bool v3{v3}; // { dg-warning "is used uninitialized" }
+    return a;
+}
diff --git a/gcc/tree-ssa-uninit.cc b/gcc/tree-ssa-uninit.cc
index eae29f88f9d..bde23997a0a 100644
--- a/gcc/tree-ssa-uninit.cc
+++ b/gcc/tree-ssa-uninit.cc
@@ -991,14 +991,20 @@ warn_uninitialized_vars (bool wmaybe_uninit)
   auto_bb_flag ft_reachable (cfun);
 
   /* Mark blocks that are always executed when we ignore provably
-     not executed edges.  */
+     not executed and EH and abnormal edges.  */
   basic_block bb = single_succ (ENTRY_BLOCK_PTR_FOR_FN (cfun));
   while (!(bb->flags & ft_reachable))
     {
       bb->flags |= ft_reachable;
+      edge e = find_fallthru_edge (bb->succs);
+      if (e && e->flags & EDGE_EXECUTABLE)
+	{
+	  bb = e->dest;
+	  continue;
+	}
       /* Find a single executable edge.  */
       edge_iterator ei;
-      edge e, ee = NULL;
+      edge ee = NULL;
       FOR_EACH_EDGE (e, ei, bb->succs)
 	if (e->flags & EDGE_EXECUTABLE)
 	  {