c: check actual reachability for non-void func may not return value

3 files changed, 28 insertions, 2 deletions

diff --git a/c/c.c b/c/c.c
index c2c0537..d609cd0 100644
--- a/c/c.c
+++ b/c/c.c
@@ -4201,11 +4201,15 @@ function(struct comp *cm, struct function *fn, const char **pnames, const struct
    }
    if (fn->curblk) {
       if (!strcmp(fn->name, "main") && fn->retty.t == TYINT) {
-         /* implicit return 0 for main function */
+         /* implicit return 0 for main function (ISO C standard behavior) */
          putreturn(fn, ZEROREF, NOREF);
       } else {
          if (fn->retty.t != TYVOID && !nerror) {
-            warn(&cm->fnblkspan, "non-void function may not return a value");
+            /* it may not actually be reachable after constant-folding
+             * peephole optimizations (from code like assert(0 && "x")) */
+            if (blkreachable(fn, fn->curblk)) {
+               warn(&cm->fnblkspan, "non-void function may not return a value");
+            }
          }
          putreturn(fn, NOREF, NOREF);
       }
diff --git a/ir/ir.c b/ir/ir.c
index 412d0f9..4a092b3 100644
--- a/ir/ir.c
+++ b/ir/ir.c
@@ -451,6 +451,27 @@ numberinstrs(struct function *fn)
    } while ((blk = blk->lnext) != fn->entry);
 }
 
+static bool
+reachablerec(struct function *fn, struct block *blk)
+{
+   if (blk == fn->entry) return 1;
+   markvisited(blk);
+   if (blk->npred == 1 && !wasvisited(blkpred(blk, 0)))
+      return reachablerec(fn, blkpred(blk, 0));
+   else for (int i = 0; i < blk->npred; ++i) {
+      struct block *p = blkpred(blk, i);
+      if (!wasvisited(p) && reachablerec(fn, p)) return 1;
+   }
+   return 0;
+}
+
+bool
+blkreachable(struct function *fn, struct block *blk)
+{
+   startbbvisit();
+   return reachablerec(fn, blk);
+}
+
 /* require use */
 void
 replcuses(union ref from, union ref to)
diff --git a/ir/ir.h b/ir/ir.h
index 77f25d4..9047233 100644
--- a/ir/ir.h
+++ b/ir/ir.h
@@ -277,6 +277,7 @@ void fillblkids(struct function *);
 #define wasvisited(blk) ((blk)->visit == visitmark)
 #define markvisited(blk) ((blk)->visit = visitmark)
 void numberinstrs(struct function *);
+bool blkreachable(struct function *fn, struct block *blk);
 
 /** builder.c **/
 union ref irbinop(struct function *, enum op, enum irclass, union ref lhs, union ref rhs);