[gcc r14-8524] tree-optimization/113659 - early exit vectorization and missing VUSE

[Richard Biener <rguenth@gcc.gnu.org>]

https://gcc.gnu.org/g:4c2169d2f4061e72e1e61e9a175d16f7ff50f5c0

commit r14-8524-g4c2169d2f4061e72e1e61e9a175d16f7ff50f5c0
Author: Richard Biener <rguenther@suse.de>
Date:   Tue Jan 30 09:42:08 2024 +0100

    tree-optimization/113659 - early exit vectorization and missing VUSE
    
    The following handles the case of the main exit going to a path without
    virtual use and handles it similar to the alternate exit handling.
    
            PR tree-optimization/113659
            * tree-vect-loop-manip.cc (slpeel_tree_duplicate_loop_to_edge_cfg):
            Handle main exit without virtual use.
    
            * gcc.dg/pr113659.c: New testcase.

Diff:
---
 gcc/testsuite/gcc.dg/pr113659.c | 14 ++++++++++++++
 gcc/tree-vect-loop-manip.cc     | 20 +++++++++++++++++---
 2 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/pr113659.c b/gcc/testsuite/gcc.dg/pr113659.c
new file mode 100644
index 000000000000..dc1091b647d2
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr113659.c
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-O3 -fno-tree-sra" } */
+/* { dg-additional-options "-msse4.1" { target { x86_64-*-* i?86-*-* } } } */
+
+struct Foo {
+  int *ptr;
+};
+int Baz(struct Foo first)
+{
+  while (first.ptr)
+    if (*first.ptr++)
+      return 0;
+  __builtin_unreachable ();
+}
diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
index 873a28d7c560..3f974d6d839e 100644
--- a/gcc/tree-vect-loop-manip.cc
+++ b/gcc/tree-vect-loop-manip.cc
@@ -1708,9 +1708,23 @@ slpeel_tree_duplicate_loop_to_edge_cfg (class loop *loop, edge loop_exit,
 		     LC PHI node to feed the merge PHI.  */
 		  tree *res;
 		  if (virtual_operand_p (new_arg))
-		    /* Use the existing virtual LC SSA from exit block.  */
-		    new_arg = gimple_phi_result
-				(get_virtual_phi (main_loop_exit_block));
+		    {
+		      /* Use the existing virtual LC SSA from exit block.  */
+		      gphi *vphi = get_virtual_phi (main_loop_exit_block);
+		      /* ???  When the exit yields to a path without
+			 any virtual use we can miss a LC PHI for the
+			 live virtual operand.  Simply choosing the
+			 one live at the start of the loop header isn't
+			 correct, but we should get here only with
+			 early-exit vectorization which will move all
+			 defs after the main exit, so leave a temporarily
+			 wrong virtual operand in place.  This happens
+			 for gcc.dg/pr113659.c.  */
+		      if (vphi)
+			new_arg = gimple_phi_result (vphi);
+		      else
+			new_arg = gimple_phi_result (from_phi);
+		    }
 		  else if ((res = new_phi_args.get (new_arg)))
 		    new_arg = *res;
 		  else