gimple: Wrong -Wimplicit-fallthrough with if(1) [PR103597]

This patch fixes a wrong -Wimplicit-fallthrough warning for

    case 0:
      if (1)  // wrong may fallthrough
	return 0;
    case 1:

which in .gimple looks like

    <D.1981>: // case 0
    if (1 != 0) goto <D.1985>; else goto <D.1986>;
    <D.1985>:
    D.1987 = 0;
    // predicted unlikely by early return (on trees) predictor.
    return D.1987;
    <D.1986>:  // dead
    <D.1982>: // case 1

and the warning thinks that <D.1986>: falls through to <D.1982>:.  It
does not know that <D.1986> is effectively a dead label, only reachable
through fallthrough from previous instructions, never jumped to.  To
that effect, Jakub introduced UNUSED_LABEL_P, which is set on such dead
labels.

collect_fallthrough_labels has code to deal with cases like

    case 2:
      if (e != 10)
	i++; // this may fallthru, warn
      else
	return 44;
    case 3:

which collects labels that may fall through.  Here it sees the "goto <D.1990>;"
at the end of the then branch and so when the warning reaches

    ...
    <D.1990>: // from if-then
    <D.1984>: // case 3

it knows it should warn about the possible fallthrough.  But an UNUSED_LABEL_P
is not a label that can fallthrough like that, so it should ignore those.

However, we still want to warn about this:

    case 0:
      if (1)
	n++; // falls through
    case 1:

so collect_fallthrough_labels needs to return the "n = n + 1;" statement, rather
than the dead label.

Co-authored-by: Jakub Jelinek <jakub@redhat.com>

	PR middle-end/103597

gcc/ChangeLog:

	* gimplify.cc (collect_fallthrough_labels): Don't push UNUSED_LABEL_Ps
	into labels.  Maybe set prev to the statement preceding UNUSED_LABEL_P.
	(gimplify_cond_expr): Set UNUSED_LABEL_P.
	* tree.h (UNUSED_LABEL_P): New.

gcc/testsuite/ChangeLog:

	* c-c++-common/Wimplicit-fallthrough-39.c: New test.

1 files changed, 140 insertions, 0 deletions

diff --git a/gcc/testsuite/c-c++-common/Wimplicit-fallthrough-39.c b/gcc/testsuite/c-c++-common/Wimplicit-fallthrough-39.c
new file mode 100644
index 0000000..da4aef3
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/Wimplicit-fallthrough-39.c
@@ -0,0 +1,140 @@
+/* PR middle-end/103597 */
+/* { dg-do compile } */
+/* { dg-options "-Wimplicit-fallthrough" } */
+
+#define E(c, e) if (c) e
+
+int
+fn0 (int n)
+{
+  switch (n)
+    {
+    case 0:
+      E (1, return 0);
+    case 1:
+      return -1;
+    }
+  return 0;
+}
+
+int
+fn1 (int n)
+{
+  switch (n)
+    {
+    case 0:
+      E (1, goto out);
+    case 1:
+      return -1;
+    }
+out:
+  return 0;
+}
+
+int
+fn2 (int n)
+{
+  switch (n)
+    {
+    case 0:
+      if (1)	  /* { dg-warning "statement may fall through" "" { target c++ } } */
+       	n++;	  /* { dg-warning "statement may fall through" "" { target c } } */
+    case 1:	  /* { dg-message "here" } */
+      return -1;
+    }
+  return 0;
+}
+
+int
+fn3 (int n)
+{
+  switch (n)
+    {
+    case 0:
+      if (0)		/* { dg-warning "statement may fall through" } */
+       	return 0;
+    case 1:		/* { dg-message "here" } */
+      return -1;
+    }
+  return 0;
+}
+
+int
+fn4 (int n)
+{
+  switch (n)
+    {
+    case 0:
+      E (0, n++);
+      --n;	  /* { dg-warning "statement may fall through" } */
+    case 1:	  /* { dg-message "here" } */
+      return -1;
+    }
+  return 0;
+}
+
+int
+fn5 (int n)
+{
+  switch (n)
+    {
+    case 0:
+      if (1)
+	return 0;
+      else
+	return -1;
+    case 1:
+      return -1;
+    }
+  return 0;
+}
+
+int
+fn6 (int n)
+{
+  switch (n)
+    {
+    case 0:
+      if (1)
+	return 0;
+      else
+	{
+meow:
+	  n--;  /* { dg-warning "statement may fall through" } */
+	}
+    case 1:   /* { dg-message "here" } */
+      return -1;
+    case 2:
+      goto meow;
+    }
+  return 0;
+}
+
+int
+fn7 (int n)
+{
+  switch (n)
+    {
+    case 0:
+      if (1)
+       	return 0;
+woof:
+    case 1:
+      return -1;
+    }
+  return 0;
+}
+
+int
+fn8 (int n)
+{
+  switch (n)
+    {
+    case 0:
+      if (1) n++; /* { dg-warning "statement may fall through" } */
+woof:		  /* { dg-message "here" } */
+    case 1:
+      return -1;
+    }
+  return 0;
+}