w6c: return of a module-global struct ident copies the global's bytes

cstage zeroed the return scratch; mirror the landed wwstage emission
(copy from the g(SB) base). Flips the F8-era residual row to
cs==ww==correct. Task #42.

cmd/w6c/cgen.c

@@ -13675,12 +13675,21 @@ cgstmt(Cg *c, Node *n, Local **locals, int *frame)
 								    scr + idx * esz));
 							idx++;
 						}
-				} else if (n->lhs->kind == N_IDENT) {
+				} else if (n->lhs->kind == N_IDENT
+				    && localfind(*locals, n->lhs->str) != 0) {
 					/* N_IDENT: word-copy rhs slot into
 					 * scratch. Whole 8B words via MOVQ;
 					 * trailing partial word via MOVL/MOVB
 					 * so we read no further than the
-					 * source slot's declared size. */
+					 * source slot's declared size.
+					 *
+					 * #42 (#263): only a LOCAL ident has a
+					 * BP slot. A module-global struct ident
+					 * (localfind==0) falls to the addr-copy
+					 * else below — aggarg_srcaddr lands
+					 * g(SB) in SI. Pre-fix this read frame
+					 * garbage off 0(BP). ww half landed
+					 * F8-c6. */
 					int rhsoff = localfind(*locals,
 					    n->lhs->str);
 					int k = 0;

test/wcc/989_globstructret_run.c

@@ -3,28 +3,19 @@
  * GLOBAL struct ident by value (`return g;`) must copy g's bytes into the
  * return buffer.
  *
- * THE BUG (cat-A silent miscompile, #263 BOTH-WRONG): cgenstmt.ww cgreturn's
+ * THE BUG (cat-A silent miscompile, #263 BOTH-WRONG, now CLOSED both stages):
- * ≤24B register-struct return path resolves an N_IDENT source via
+ * the ≤24B register-struct return path word-copied an N_IDENT source from its
- * localfindnode and word-copies from its BP slot only `if (rl != nil)`. A
+ * BP slot only for a LOCAL ident. A module-global struct ident hit no branch
- * module-global struct ident (rl==nil) hit no branch — after the @retscr
+ * (wwstage zero-filled the @retscr; cstage copied frame garbage ((BP)/8(BP)),
- * zero-fill it copied NOTHING, so the callee returned a zeroed struct and g
+ * never g(SB)). The ww half landed in F8-c6; the cstage half (route a global
- * was never referenced. cstage is ALSO wrong: it copies frame garbage
+ * ident through the aggarg_srcaddr memcpy — LEAQ g(SB),SI) landed in ww-core
- * ((BP)/8(BP)), never g(SB). Both wrong (#263); the cstage half is filed as
+ * TASK #42. Both stages now copy g's bytes and the .s is byte-identical.
- * task #42.
  *
- * THE WWSTAGE FIX (ww-runtime-correct): a global N_IDENT source is marked
+ * Rows assert the field-equality check returns 0 on both stages (cs==ww).
- * addrsrc, routing it through the existing memcpy path — aggargsrcaddr emits
+ *   row        | shape (return g; then check)              | exit
- * LEAQ g(SB),SI and the loop copies rsz bytes into @retscr. Locals keep the
+ *   -----------+-------------------------------------------+-----
- * BP word-copy (byte-id preserved). cstage stays wrong → cs≠ww residual.
+ *   ret_16     | point{x=3,y=4} (rsz 16); x==3 && y==4      | 0
- *
+ *   ret_24     | triple{a=5,b=6,c=7} (rsz 24); all three    | 0
- * Rows assert ww runtime-correct (the field-equality check returns 0) AND
- * pin cstage's deterministic residual: garbage fields never satisfy the
- * equality, so the cstage binary takes the else and returns 1. (#42 closes
- * the cs side → these become cs==ww.)
- *   row        | shape (return g; then check)              | cs | ww
- *   -----------+-------------------------------------------+----+----
- *   ret_16     | point{x=3,y=4} (rsz 16); x==3 && y==4      | 1  | 0
- *   ret_24     | triple{a=5,b=6,c=7} (rsz 24); all three    | 1  | 0
  */
 #include <stdio.h>
 #include <stdlib.h>
@@ -60,7 +51,7 @@ static const struct row rows[] = {
 	  "	if (p.x == 3 && p.y == 4) { return 0; };\n"
 	  "	return 1;\n"
 	  "};\n",
-	  1, 0 },
+	  0, 0 },
 
 	{ "ret_24",
 	  "package main;\n"
@@ -72,7 +63,7 @@ static const struct row rows[] = {
 	  "	if (p.a == 5 && p.b == 6 && p.c == 7) { return 0; };\n"
 	  "	return 1;\n"
 	  "};\n",
-	  1, 0 },
+	  0, 0 },
 };
 
 /* run_build — build+run `src` via `driver`; returns the binary's exit