[gcc r15-8050] tree-optimization/119274 - improve VN optimistic dominance query

[Richard Biener via Gcc-cvs]

https://gcc.gnu.org/g:6470b4d749a0b9896b985858ee6eae095cd8a37a

commit r15-8050-g6470b4d749a0b9896b985858ee6eae095cd8a37a
Author: Richard Biener <rguent...@suse.de>
Date:   Fri Mar 14 10:01:20 2025 +0100

    tree-optimization/119274 - improve VN optimistic dominance query
    
    The following improves how VN performs its dominance queries to
    determine availability, exploiting edges considered unreachable.
    The function already contains code to handle the leader block
    forking the CFG, but that looks like a situation that won't
    help the dominance query ever.  The following adds handling
    of the more useful case where this block forwards to a CFG
    merge with the forwarder being the only executable entry.
    
    This helps optimizing the code so the spurious array diagnostic
    does no longer appear.
    
            PR tree-optimization/119274
            * tree-ssa-sccvn.cc (dominated_by_p_w_unex): Handle the
            top block being the only executable forwarder to a CFG
            merge.
    
            * g++.dg/opt/pr119274.C: New testcase.

Diff:
---
 gcc/testsuite/g++.dg/opt/pr119274.C | 20 ++++++++++++++++++++
 gcc/tree-ssa-sccvn.cc               | 28 ++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/gcc/testsuite/g++.dg/opt/pr119274.C 
b/gcc/testsuite/g++.dg/opt/pr119274.C
new file mode 100644
index 000000000000..79b406162db9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/pr119274.C
@@ -0,0 +1,20 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-O2 -Wall" }
+
+#include <cstdlib>
+#include <vector>
+
+typedef std::vector<int> v1;
+
+static
+void drop_inplace(v1 & c, size_t len)
+{
+  if (len <= c.size())
+    c[len-1] = 0;  /* { dg-bogus "outside array bounds" } */
+}
+
+void func()
+{
+  v1 vec1{1,2,3,4,5,6};
+  drop_inplace(vec1, 10);
+}
diff --git a/gcc/tree-ssa-sccvn.cc b/gcc/tree-ssa-sccvn.cc
index 5a8c7c3aa10b..40c38fa020a6 100644
--- a/gcc/tree-ssa-sccvn.cc
+++ b/gcc/tree-ssa-sccvn.cc
@@ -5169,6 +5169,34 @@ dominated_by_p_w_unex (basic_block bb1, basic_block bb2, 
bool allow_back)
            }
        }
     }
+  /* Iterate to the single successor of bb2 with only a single executable
+     incoming edge.  */
+  else if (EDGE_COUNT (bb2->succs) == 1
+          && EDGE_COUNT (single_succ (bb2)->preds) > 1)
+    {
+      edge prede = NULL;
+      FOR_EACH_EDGE (e, ei, single_succ (bb2)->preds)
+       if ((e->flags & EDGE_EXECUTABLE)
+           || (!allow_back && (e->flags & EDGE_DFS_BACK)))
+         {
+           if (prede)
+             {
+               prede = NULL;
+               break;
+             }
+           prede = e;
+         }
+      /* We might actually get to a query with BB2 not visited yet when
+        we're querying for a predicated value.  */
+      if (prede && prede->src == bb2)
+       {
+         bb2 = prede->dest;
+
+         /* Re-do the dominance check with changed bb2.  */
+         if (dominated_by_p (CDI_DOMINATORS, bb1, bb2))
+           return true;
+       }
+    }
 
   /* We could now iterate updating bb1 / bb2.  */
   return false;