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diff --git a/rcc-legacy/src/mid/abi.cc b/rcc-legacy/src/mid/abi.cc
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+/* Source code for RCC-Legacy.
+   Copyright (C) 2026 Richard Qin
+
+This file is part of RCC.
+
+RCC is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free
+Software Foundation; either version 3, or (at your option) any later
+version.
+
+RCC is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with RCC; see the file LICENSE.  If not see
+<http://www.gnu.org/licenses/>.  */
+
+/// @file abi.cc
+/// @brief elimsb - Eliminate ABI-level sub-word op variants (sb/ub/sh/uh for arg/par/ret).
+///
+/// See docs/abi.md for architectural notes.
+
+#include "rcc/all.h"
+
+namespace rcc {
+
+void elimsb(Fn* fn) {
+    for (Blk* b = fn->start; b; b = b->link) {
+        Ins* ie = b->ins + b->nins;
+        for (Ins* i = b->ins; i < ie; ++i) {
+            if (isargbh(i->op)) {
+                i->op = Oarg;
+            } else if (isparbh(i->op)) {
+                i->op = Opar;
+            }
+        }
+        if (isretbh(b->jmp.type)) {
+            b->jmp.type = Jretw;
+        }
+    }
+}
+
+}  // namespace rcc