C backend: while, struct tests, better undefined global handling

1. Function signatures that return a no member struct return void
2. Undefined var decls don't get a value generated for them
3. Don't generate bitcast code if the result isn't used, since
   bitcast is a pure function. Right now struct handling code
   generates some weird unused bitcast AIR, and this optimization
   side steps that issue.

1 files changed, 13 insertions, 1 deletions

diff --git a/src/codegen/c.zig b/src/codegen/c.zig
index 5935985961..29e0f1e50e 100644
--- a/src/codegen/c.zig
+++ b/src/codegen/c.zig
@@ -448,7 +448,12 @@ pub const DeclGen = struct {
                 try w.writeAll("ZIG_COLD ");
             }
         }
-        try dg.renderType(w, dg.decl.ty.fnReturnType());
+        const return_ty = dg.decl.ty.fnReturnType();
+        if (return_ty.hasCodeGenBits()) {
+            try dg.renderType(w, return_ty);
+        } else {
+            try w.writeAll("void");
+        }
         try w.writeAll(" ");
         try dg.renderDeclName(dg.decl, w);
         try w.writeAll("(");
@@ -947,6 +952,10 @@ pub fn genDecl(o: *Object) !void {
         }
         try fwd_decl_writer.writeAll(";\n");
 
+        if (variable.init.isUndef()) {
+            return;
+        }
+
         try o.indent_writer.insertNewline();
         const w = o.writer();
         try o.dg.renderType(w, o.dg.decl.ty);
@@ -1886,6 +1895,9 @@ fn airBr(f: *Function, inst: Air.Inst.Index) !CValue {
 }
 
 fn airBitcast(f: *Function, inst: Air.Inst.Index) !CValue {
+    if (f.liveness.isUnused(inst))
+        return CValue.none;
+
     const ty_op = f.air.instructions.items(.data)[inst].ty_op;
     const operand = try f.resolveInst(ty_op.operand);