forwprop: Handle already true/false branchs in optimize_unreachable [PR122588]

When optimize_unreachable was moved from fab to forwprop, I missed that due to
the integrated copy prop, we might end up with an already true branch leading
to a __builtin_unreachable block. optimize_unreachable would switch around
the if and things go down hill from there since the other edge was already
marked as non-executable, forwprop didn't process those blocks and didn't
do copy prop into that block and the original assignment statement was removed.

This fixes the problem by having optimize_unreachable not touch the if
statement was already changed to true/false.

Note I placed the testcase in gcc.c-torture/compile as gcc.dg/torture
is NOT currently testing -Og (see PR 122450 for that).

Changes since v1:
 * v2: Add gimple testcase.

Bootstrapped and tested on x86_64-linux-gnu.

	PR tree-optimization/122588

gcc/ChangeLog:

	* tree-ssa-forwprop.cc (optimize_unreachable): Don't touch
	if the condition was already true or false.

gcc/testsuite/ChangeLog:

	* gcc.c-torture/compile/pr122588-1.c: New test.
	* gcc.dg/tree-ssa/pr122588-1.c: New test.

Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>

3 files changed, 61 insertions, 1 deletions

diff --git a/gcc/testsuite/gcc.c-torture/compile/pr122588-1.c b/gcc/testsuite/gcc.c-torture/compile/pr122588-1.c
new file mode 100644
index 0000000..43ec621
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/compile/pr122588-1.c
@@ -0,0 +1,25 @@
+/* Disable warnings about __sync_nand_and_fetch. */
+/* { dg-options "-w" } */
+/* PR tree-optimization/122588 */
+
+int i;
+char c;
+
+static inline __attribute__((__always_inline__))
+void foo0 (int a)
+{
+l5:
+  __sync_nand_and_fetch (&i, 0);
+  int x = __builtin_memcmp_eq (&a, 0, 4);
+  if (__builtin_iseqsig (x, 0.))
+    goto l5;
+  if (a)
+    __builtin_unreachable ();
+  c = a;
+}
+
+int
+main ()
+{
+  foo0 (1);
+}
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr122588-1.c b/gcc/testsuite/gcc.dg/tree-ssa/pr122588-1.c
new file mode 100644
index 0000000..2c214c9
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr122588-1.c
@@ -0,0 +1,30 @@
+/* { dg-do compile } */
+/* { dg-options "-O1 -fgimple" } */
+/* PR tree-optimization/122588 */
+
+/* The removal of unreachable blocks should not
+   change blocks which have already become true/false.
+   The function below was is an example of that. And
+   forwprop does not go into non-executable blocks
+   so the statement `t = _1;` was still holding the
+   old reference.  */
+
+int t;
+
+__GIMPLE(ssa,startwith("forwprop4")) void g(void)
+{
+  int _1;
+  __BB(2):
+  _1 = 1;
+  if (_1 != 0)
+    goto __BB3;
+  else
+    goto __BB4;
+
+  __BB(3):
+  __builtin_unreachable ();
+
+  __BB(4):
+  t = _1;
+  return;
+}
diff --git a/gcc/tree-ssa-forwprop.cc b/gcc/tree-ssa-forwprop.cc
index ae7f0e7..9f8d4ad 100644
--- a/gcc/tree-ssa-forwprop.cc
+++ b/gcc/tree-ssa-forwprop.cc
@@ -5080,7 +5080,12 @@ optimize_unreachable (basic_block bb)
       stmt = gsi_stmt (gsi);
       if (gcond *cond_stmt = dyn_cast <gcond *> (stmt))
 	{
-	  if (e->flags & EDGE_TRUE_VALUE)
+	  /* If the condition is already true/false
+	     ignore it. This can happen during copy prop of forwprop. */
+	  if (gimple_cond_true_p (cond_stmt)
+	      || gimple_cond_false_p (cond_stmt))
+	    continue;
+	  else if (e->flags & EDGE_TRUE_VALUE)
 	    gimple_cond_make_false (cond_stmt);
 	  else if (e->flags & EDGE_FALSE_VALUE)
 	    gimple_cond_make_true (cond_stmt);