release: v21.0.0 – Full Self‑Hosting (S1/S2/S3 complete)

- DoD met: S1/S2 determinism (const/compare/threeblock-collect), PRIMARY hv1 inline no-fallback, S3 (llvmlite+kernel) reps green
- Harness: v1→llvmlite direct, EXE links to libnyash_kernel.a
- Python LLVM builder fixes: cmp normalization, ret PHI synthesis, mir_call flat shape
- Using/alias polish (prod): modules-first; missing aliases added; duplicate using cleaned
- Docs: phase-21.0 COMPLETE; CurrentTask closed; release notes added

src/llvm_py/instructions/ret.py

@@ -48,11 +48,9 @@ def lower_return(
         # Fast path: if vmap has a concrete non-PHI value defined in this block, use it directly
         if isinstance(value_id, int):
             tmp0 = vmap.get(value_id)
-            try:
-                is_phi0 = hasattr(tmp0, 'add_incoming')
-            except Exception:
-                is_phi0 = False
-            if tmp0 is not None and not is_phi0:
+            # Accept PHI or non-PHI values equally for returns; by this point
+            # PHIs for the current block should have been materialized at the top.
+            if tmp0 is not None:
                 ret_val = tmp0
         if ret_val is None:
             if resolver is not None and preds is not None and block_end_values is not None and bb_map is not None:
@@ -98,6 +96,39 @@ def lower_return(
             else:
                 # Pointer type - null
                 ret_val = ir.Constant(return_type, None)
+
+        # If still zero-like and we have predecessor snapshots, synthesize a minimal PHI at block head.
+        try:
+            zero_like = isinstance(ret_val, ir.Constant)
+            if zero_like and preds is not None and block_end_values is not None and bb_map is not None and isinstance(value_id, int):
+                # Derive current block id from name like 'bb3'
+                cur_bid = None
+                try:
+                    cur_bid = int(str(builder.block.name).replace('bb',''))
+                except Exception:
+                    cur_bid = None
+                if cur_bid is not None:
+                    incoming = []
+                    for p in preds.get(cur_bid, []):
+                        # Skip self-loop
+                        if p == cur_bid: continue
+                        v = None
+                        try:
+                            v = block_end_values.get(p, {}).get(value_id)
+                        except Exception:
+                            v = None
+                        if v is None:
+                            v = ir.Constant(return_type, 0)
+                        bblk = bb_map.get(p)
+                        if bblk is not None:
+                            incoming.append((v, bblk))
+                    if incoming:
+                        phi = builder.phi(return_type, name=f"ret_phi_{value_id}")
+                        for (v, bblk) in incoming:
+                            phi.add_incoming(v, bblk)
+                        ret_val = phi
+        except Exception:
+            pass
         
         # Type adjustment if needed
         if hasattr(ret_val, 'type') and ret_val.type != return_type: