From 1b55ca18ef4d55b3152b107e56358b2b4022a2be Mon Sep 17 00:00:00 2001
From: Matthew Lugg <mlugg@mlugg.co.uk>
Date: Wed, 22 Apr 2026 08:46:10 +0100
Subject: [PATCH] cbe: add missing cast of overflow arithmetic out pointer

---
 src/codegen/c.zig | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/src/codegen/c.zig b/src/codegen/c.zig
index ee9e90ce80..885ca0aa1c 100644
--- a/src/codegen/c.zig
+++ b/src/codegen/c.zig
@@ -3497,9 +3497,27 @@ fn airOverflow(f: *Function, inst: Air.Inst.Index, operation: []const u8, info:
     try w.writeAll(operation);
     try w.writeAll("o_");
     try f.dg.renderTypeForBuiltinFnName(w, scalar_ty);
-    try w.writeAll("(&");
+    try w.writeByte('(');
+
+    // '&dest', possibly preceded by a cast
+    switch (zcu.intern_pool.indexToKey(scalar_ty.toIntern())) {
+        .int_type => {}, // we already have a '[u]intX_t *'
+        .simple_type => {
+            // '&dest' will be something like a 'uintptr_t *', which might be a different C type to
+            // the equivalent sized integer (e.g. 'uint64_t *'), so we need a cast. We don't need a
+            // cast on the *operands* because they are passed by value (except for big integers,
+            // where this issue doesn't exist because no "simple" int type needs bigint repr).
+            try w.print("({s}int{d}_t *)", .{
+                if (scalar_ty.isUnsignedInt(zcu)) "u" else "",
+                scalar_ty.abiSize(zcu) * 8,
+            });
+        },
+        else => unreachable,
+    }
+    try w.writeByte('&');
     try f.writeCValueMember(w, local, .{ .field = 0 });
     try v.elem(f, w);
+
     try w.writeAll(", ");
     if (ref_arg) try w.writeByte('&');
     try f.writeCValue(w, lhs, .other);