opt: Start optimizing dead code a bit

If a condition is always zero/non-zero we can omit the
then or else code.  This is complicated a bit by having to
deal with labels that might make such code reachable without
us yet knowing during parsing.

2 files changed, 140 insertions, 0 deletions

diff --git a/tests/tests2/87_dead_code.c b/tests/tests2/87_dead_code.c
new file mode 100644
index 0000000..92983f5
--- /dev/null
+++ b/tests/tests2/87_dead_code.c
@@ -0,0 +1,122 @@
+/* This checks various ways of dead code inside if statements
+   where there are non-obvious ways of how the code is actually
+   not dead due to reachable by labels.  */
+extern int printf (const char *, ...);
+static void kb_wait_1(void)
+{
+  unsigned long timeout = 2;
+  do {
+      /* Here the else arm is a statement expression that's supposed
+         to be suppressed.  The label inside the while would unsuppress
+	 code generation again if not handled correctly.  And that
+	 would wreak havok to the cond-expression because there's no
+	 jump-around emitted, the whole statement expression really
+	 needs to not generate code (perhaps except useless forward jumps).  */
+      (1 ? 
+       printf("timeout=%ld\n", timeout) :
+       ({
+	int i = 1;
+	while (1)
+	  while (i--)
+	    some_label:
+	      printf("error\n");
+	goto some_label;
+	})
+      );
+      timeout--;
+  } while (timeout);
+}
+int main (void)
+{
+  int i = 1;
+  kb_wait_1();
+
+  /* Simple test of dead code at first sight which isn't actually dead. */
+  if (0) {
+yeah:
+      printf ("yeah\n");
+  } else {
+      printf ("boo\n");
+  }
+  if (i--)
+    goto yeah;
+
+  /* Some more non-obvious uses where the problems are loops, so that even
+     the first loop statements aren't actually dead.  */
+  i = 1;
+  if (0) {
+      while (i--) {
+	  printf ("once\n");
+enterloop:
+	  printf ("twice\n");
+      }
+  }
+  if (i >= 0)
+    goto enterloop;
+
+  /* The same with statement expressions.  One might be tempted to
+     handle them specially by counting if inside statement exprs and
+     not unsuppressing code at loops at all then.
+     See kb_wait_1 for the other side of the medal where that wouldn't work.  */
+  i = ({
+      int j = 1;
+      if (0) {
+	  while (j--) {
+	      printf ("SEonce\n");
+    enterexprloop:
+	      printf ("SEtwice\n");
+	  }
+      }
+      if (j >= 0)
+	goto enterexprloop;
+      j; });
+
+  /* The other two loop forms: */
+  i = 1;
+  if (0) {
+      for (i = 1; i--;) {
+	  printf ("once2\n");
+enterloop2:
+	  printf ("twice2\n");
+      }
+  }
+  if (i > 0)
+    goto enterloop2;
+
+  i = 1;
+  if (0) {
+      do {
+	  printf ("once3\n");
+enterloop3:
+	  printf ("twice3\n");
+      } while (i--);
+  }
+  if (i > 0)
+    goto enterloop3;
+
+  /* And check that case and default labels have the same effect
+     of disabling code suppression.  */
+  i = 41;
+  switch (i) {
+      if (0) {
+	  printf ("error\n");
+      case 42:
+	  printf ("error2\n");
+      case 41:
+	  printf ("caseok\n");
+      }
+  }
+
+  i = 41;
+  switch (i) {
+      if (0) {
+	  printf ("error3\n");
+      default:
+	  printf ("caseok2\n");
+	  break;
+      case 42:
+	  printf ("error4\n");
+      }
+  }
+  return 0;
+}
diff --git a/tests/tests2/87_dead_code.expect b/tests/tests2/87_dead_code.expect
new file mode 100644
index 0000000..0b3ec1d
--- /dev/null
+++ b/tests/tests2/87_dead_code.expect
@@ -0,0 +1,18 @@
+timeout=2
+timeout=1
+boo
+yeah
+twice
+once
+twice
+SEtwice
+SEonce
+SEtwice
+twice2
+once2
+twice2
+twice3
+once3
+twice3
+caseok
+caseok2