wcc/ww: return of a module-global struct ident copies the global's bytes

Returning a global struct by name emitted nothing into the return
scratch (ww) — copy from the g(SB) base. Both-wrong pair: cstage
zeroes the retscr instead (filed task #42); rows pin ww-runtime-correct
with the documented cstage residual. Review item #41.

test/wcc/989_globstructret_run.c

@@ -0,0 +1,165 @@
+/*
+ * 989_globstructret_run — F8-c6 (report-item #41, #263): returning a module-
+ * 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
+ * ≤24B register-struct return path resolves an N_IDENT source via
+ * localfindnode and word-copies from its BP slot only `if (rl != nil)`. A
+ * module-global struct ident (rl==nil) hit no branch — after the @retscr
+ * zero-fill it copied NOTHING, so the callee returned a zeroed struct and g
+ * was never referenced. cstage is ALSO wrong: it copies frame garbage
+ * ((BP)/8(BP)), never g(SB). Both wrong (#263); the cstage half is filed as
+ * task #42.
+ *
+ * THE WWSTAGE FIX (ww-runtime-correct): a global N_IDENT source is marked
+ * addrsrc, routing it through the existing memcpy path — aggargsrcaddr emits
+ * 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.
+ *
+ * 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>
+#include <string.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+
+static int
+runwait(const char *cmd)
+{
+	int rc = system(cmd);
+	if (rc == -1) return -1;
+	if (WIFEXITED(rc)) return WEXITSTATUS(rc);
+	return -1;
+}
+
+struct row {
+	const char *label;
+	const char *src;
+	int want_cs;
+	int want_ww;
+};
+
+static const struct row rows[] = {
+	{ "ret_16",
+	  "package main;\n"
+	  "type point = struct { x: i64, y: i64 };\n"
+	  "let g: point = point { x = 3, y = 4 };\n"
+	  "fn get() point = { return g; };\n"
+	  "export fn main() int = {\n"
+	  "	let p: point = get();\n"
+	  "	if (p.x == 3 && p.y == 4) { return 0; };\n"
+	  "	return 1;\n"
+	  "};\n",
+	  1, 0 },
+
+	{ "ret_24",
+	  "package main;\n"
+	  "type triple = struct { a: i64, b: i64, c: i64 };\n"
+	  "let g: triple = triple { a = 5, b = 6, c = 7 };\n"
+	  "fn get() triple = { return g; };\n"
+	  "export fn main() int = {\n"
+	  "	let p: triple = get();\n"
+	  "	if (p.a == 5 && p.b == 6 && p.c == 7) { return 0; };\n"
+	  "	return 1;\n"
+	  "};\n",
+	  1, 0 },
+};
+
+/* run_build — build+run `src` via `driver`; returns the binary's exit
+ * code, or -1 on a build failure. */
+static int
+run_build(const char *driver, const struct row *r, int i)
+{
+	char src[64], tmpdir[64], cmd[1024];
+	snprintf(src, sizeof src, "/tmp/gsr_%d_%d.ww", getpid(), i);
+	snprintf(tmpdir, sizeof tmpdir, "/tmp/gsr_%d_d_%d", getpid(), i);
+
+	FILE *f = fopen(src, "wb");
+	if (!f) return -2;
+	fputs(r->src, f);
+	fclose(f);
+
+	mkdir(tmpdir, 0755);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
+	    tmpdir, driver, src);
+	int brc = runwait(cmd);
+
+	const char *base = strrchr(src, '/');
+	base = base ? base + 1 : src;
+	char outbin[128];
+	snprintf(outbin, sizeof outbin, "%s/%s", tmpdir, base);
+	char *dot = strrchr(outbin, '.');
+	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
+
+	int got = -1;
+	if (brc == 0) got = runwait(outbin);
+
+	unlink(src); unlink(outbin); rmdir(tmpdir);
+	return brc == 0 ? got : -1;
+}
+
+int
+main(void)
+{
+	const char *bin = getenv("BIN");
+	if (!bin) bin = "out/bin";
+	char absbin[1024];
+	if (bin[0] != '/') {
+		char cwd[1024];
+		if (getcwd(cwd, sizeof cwd) == NULL) return 1;
+		snprintf(absbin, sizeof absbin, "%s/%s", cwd, bin);
+		bin = absbin;
+	}
+
+	char cdrv[1024], wdrv[1024];
+	snprintf(cdrv, sizeof cdrv, "%s/ww", bin);
+	snprintf(wdrv, sizeof wdrv, "%s/ww_ww", bin);
+
+	struct { const char *name; const char *drv; int gated; }
+	drivers[] = {
+		{ "cstage",  cdrv, 0 },
+		{ "wwstage", wdrv, 1 },
+		{ NULL, NULL, 0 },
+	};
+
+	int n = (int)(sizeof rows / sizeof rows[0]);
+	int total = 0, fail = 0;
+
+	for (int d = 0; drivers[d].name; d++) {
+		if (drivers[d].gated && access(drivers[d].drv, X_OK) != 0) {
+			fprintf(stderr, "globstructret_run: skip %s (no %s)\n",
+			    drivers[d].name, drivers[d].drv);
+			continue;
+		}
+		int is_ww = (d == 1);
+		for (int i = 0; i < n; i++) {
+			total++;
+			int want = is_ww ? rows[i].want_ww : rows[i].want_cs;
+			int got = run_build(drivers[d].drv, &rows[i], i);
+			if (got != want) {
+				fprintf(stderr, "globstructret_run[%s][%s]: exit=%d "
+				    "want=%d\n", drivers[d].name, rows[i].label,
+				    got, want);
+				fail++;
+			}
+		}
+	}
+
+	if (fail) {
+		fprintf(stderr, "globstructret_run: %d/%d fixtures failed\n",
+		    fail, total);
+		return 1;
+	}
+	printf("globstructret_run: %d/%d ok\n", total, total);
+	return 0;
+}