wcc/ww: match on a global value-struct tagged field reads g(SB) (#29)

A match whose scrutinee is a tagged field of a GLOBAL value-struct read
the tag/payload from the BP region (saved-BP + return-addr) instead of
g(SB) and returned garbage. Both stages were identical-wrong, so the
byte-id gate could not see it -- a gate-blind regression introduced by
M1 (#25): M1's in-place N_DOT match arm uses localfind(base), which
returns the 0 not-found sentinel for a global base, so 0+field.offset
landed in the frame.

Gate the in-place arm on a confirmed-local base -- `localfind(base)==0
&& let_islet/isletvar(base)`, verbatim from cstage's own global test at
cgen.c:2000 (both stages, same spelling). A global base now falls
through to the existing spill path, which cgexprs the scrutinee and
resolves g(SB). M1's local-field in-place ($32) path is untouched.

Regenerates the w6c and wwdump combined.ww. Table-driven 841 test
(global int/reassign/str-payload + a local-field M1 regression row),
runtime-discriminating: pre-fix returns garbage, post-fix 42 on both
stages; rob's direct-global-field spill caveat confirmed at runtime.

Makefile

@@ -343,6 +343,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_tagged_widen_f64 \
 	  $(BIN)/test_match_spill_pointer_payload \
 	  $(BIN)/test_match_field_inplace \
+	  $(BIN)/test_match_global_field \
 	  $(BIN)/test_struct_byval_param \
 	  $(BIN)/test_struct_multi_return_scratch \
 	  $(BIN)/test_signed_data_emit \
@@ -2125,6 +2126,12 @@ $(BIN)/test_match_field_inplace: test/wcc/831_match_field_inplace.c \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_match_global_field: test/wcc/841_match_global_field.c \
+		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_struct_byval_param: test/wcc/717_struct_byval_param.c \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \

cmd/w6c/cgen.c

@@ -10243,7 +10243,11 @@ cgexpr(Cg *c, Node *n, Local *locals)
 			/* `match (p.field)` — point sl_off at the field's slot
 			 * inside the parent struct. The slot layout (tag at +0,
 			 * value words at +8/+16) is contiguous within the struct,
-			 * so no spill is needed. */
+			 * so no spill is needed. #29: gated below on a
+			 * CONFIRMED-LOCAL base — a global base (`match (g.field)`)
+			 * has localfind==0, so 0+field.offset would land in the
+			 * saved-BP/return-addr region; it falls through to the
+			 * spill `else`, which resolves g(SB). */
 			Type *bt = s->lhs->type;
 			Type *bu = type_chase_named(bt);
 			Tfield *f = NULL;
@@ -10254,8 +10258,8 @@ cgexpr(Cg *c, Node *n, Local *locals)
 					}
 				}
 			}
-			if (f) {
+			int boff = f ? localfind(locals, s->lhs->str) : 0;
-				int boff = localfind(locals, s->lhs->str);
+			if (f && !(boff == 0 && let_islet(s->lhs->str))) {
 				sl_off = boff + (int)f->offset;
 			} else {
 				/* fall back to spill — `match (h.e)` where

selfhost/cmd/w6c/main.combined.ww

@@ -26307,11 +26307,17 @@ fn cgmatch(c: *cgen, n: *node) void = {
 			// type chases to TY_STRUCT and whose field resolves by name.
 			// A *ptr-field base (`match (h.e)`, h:*struct) chases to
 			// TY_PTR, the by-name scan misses, and it falls to the spill
-			// arm below — exactly as cstage, no separate deref guard. A
+			// arm below — exactly as cstage, no separate deref guard.
-			// global VALUE-struct base mis-resolves here in BOTH stages
+			// #29: the in-place arm is also gated on a CONFIRMED-LOCAL
-			// (shared latent #29), matched identically — no ww-only
+			// base (bglobal below). A global VALUE-struct base
-			// global guard. align-DOWN to cstage (rule 10): both stages
+			// (`match (g.field)`) has localfind==0, so the M1 base.off
-			// emit TEXT $32 on the local-field case.
+			// would land in the saved-BP/return-addr region — both
+			// stages emitted `MOVQ (BP),AX` (gate-blind both-wrong). The
+			// global-base predicate `localfind==0 && isletvar` (cstage
+			// twin: let_islet, cgen.c:2000) routes it through to the
+			// spill `else`, which resolves g(SB). align-DOWN to cstage
+			// (rule 10): both stages emit TEXT $32 on the local-field
+			// case and the same g(SB) spill on the global-field case.
 			let mfld: *tfield = nil;
 			if (scrut.kind == nkind.N_DOT && scrut.lhs != nil
 			    && scrut.lhs.kind == nkind.N_IDENT
@@ -26328,7 +26334,12 @@ fn cgmatch(c: *cgen, n: *node) void = {
 					};
 				};
 			};
+			let bglobal: bool = false;
 			if (mfld != nil) {
+				bglobal = (localfind(c, scrut.lhs.str) == 0)
+				    && isletvar(c, scrut.lhs.str);
+			};
+			if (mfld != nil && !bglobal) {
 				let foff: i32 = mfld.offset: i32;
 				scrutoff = localfind(c, scrut.lhs.str) + foff;
 				scrutt = matchscrutt(c, scrut);

selfhost/cmd/wcc/cgenexpr.ww

@@ -2961,11 +2961,17 @@ fn cgmatch(c: *cgen, n: *node) void = {
 			// type chases to TY_STRUCT and whose field resolves by name.
 			// A *ptr-field base (`match (h.e)`, h:*struct) chases to
 			// TY_PTR, the by-name scan misses, and it falls to the spill
-			// arm below — exactly as cstage, no separate deref guard. A
+			// arm below — exactly as cstage, no separate deref guard.
-			// global VALUE-struct base mis-resolves here in BOTH stages
+			// #29: the in-place arm is also gated on a CONFIRMED-LOCAL
-			// (shared latent #29), matched identically — no ww-only
+			// base (bglobal below). A global VALUE-struct base
-			// global guard. align-DOWN to cstage (rule 10): both stages
+			// (`match (g.field)`) has localfind==0, so the M1 base.off
-			// emit TEXT $32 on the local-field case.
+			// would land in the saved-BP/return-addr region — both
+			// stages emitted `MOVQ (BP),AX` (gate-blind both-wrong). The
+			// global-base predicate `localfind==0 && isletvar` (cstage
+			// twin: let_islet, cgen.c:2000) routes it through to the
+			// spill `else`, which resolves g(SB). align-DOWN to cstage
+			// (rule 10): both stages emit TEXT $32 on the local-field
+			// case and the same g(SB) spill on the global-field case.
 			let mfld: *tfield = nil;
 			if (scrut.kind == nkind.N_DOT && scrut.lhs != nil
 			    && scrut.lhs.kind == nkind.N_IDENT
@@ -2982,7 +2988,12 @@ fn cgmatch(c: *cgen, n: *node) void = {
 					};
 				};
 			};
+			let bglobal: bool = false;
 			if (mfld != nil) {
+				bglobal = (localfind(c, scrut.lhs.str) == 0)
+				    && isletvar(c, scrut.lhs.str);
+			};
+			if (mfld != nil && !bglobal) {
 				let foff: i32 = mfld.offset: i32;
 				scrutoff = localfind(c, scrut.lhs.str) + foff;
 				scrutt = matchscrutt(c, scrut);

selfhost/cmd/wwdump/main.combined.ww

@@ -26307,11 +26307,17 @@ fn cgmatch(c: *cgen, n: *node) void = {
 			// type chases to TY_STRUCT and whose field resolves by name.
 			// A *ptr-field base (`match (h.e)`, h:*struct) chases to
 			// TY_PTR, the by-name scan misses, and it falls to the spill
-			// arm below — exactly as cstage, no separate deref guard. A
+			// arm below — exactly as cstage, no separate deref guard.
-			// global VALUE-struct base mis-resolves here in BOTH stages
+			// #29: the in-place arm is also gated on a CONFIRMED-LOCAL
-			// (shared latent #29), matched identically — no ww-only
+			// base (bglobal below). A global VALUE-struct base
-			// global guard. align-DOWN to cstage (rule 10): both stages
+			// (`match (g.field)`) has localfind==0, so the M1 base.off
-			// emit TEXT $32 on the local-field case.
+			// would land in the saved-BP/return-addr region — both
+			// stages emitted `MOVQ (BP),AX` (gate-blind both-wrong). The
+			// global-base predicate `localfind==0 && isletvar` (cstage
+			// twin: let_islet, cgen.c:2000) routes it through to the
+			// spill `else`, which resolves g(SB). align-DOWN to cstage
+			// (rule 10): both stages emit TEXT $32 on the local-field
+			// case and the same g(SB) spill on the global-field case.
 			let mfld: *tfield = nil;
 			if (scrut.kind == nkind.N_DOT && scrut.lhs != nil
 			    && scrut.lhs.kind == nkind.N_IDENT
@@ -26328,7 +26334,12 @@ fn cgmatch(c: *cgen, n: *node) void = {
 					};
 				};
 			};
+			let bglobal: bool = false;
 			if (mfld != nil) {
+				bglobal = (localfind(c, scrut.lhs.str) == 0)
+				    && isletvar(c, scrut.lhs.str);
+			};
+			if (mfld != nil && !bglobal) {
 				let foff: i32 = mfld.offset: i32;
 				scrutoff = localfind(c, scrut.lhs.str) + foff;
 				scrutt = matchscrutt(c, scrut);

test/wcc/841_match_global_field.c

@@ -0,0 +1,266 @@
+/*
+ * 841_match_global_field — match on a tagged-union struct-FIELD of a
+ * GLOBAL (module-level) VALUE struct: `match (g.t)`, g a `let g: box`.
+ * Task #29.
+ *
+ * GATE-BLIND both-wrong regression from M1 (#25). M1 added an in-place
+ * N_DOT match arm that points the dispatch slot at base.off +
+ * field.offset, computing base.off via localfind. For a LOCAL base that
+ * is the field's true frame slot ($32 in-place, correct). For a GLOBAL
+ * base localfind returns its 0/not-found sentinel, so 0 + field.offset
+ * lands in the saved-BP / return-addr region (BP+0/BP+8): both stages
+ * emit `MOVQ (BP),AX` and read garbage. Both stages were identical-wrong
+ * (byte-id GREEN), so only the RUNTIME oracle catches it — pre-fix
+ * match(g.t) returns 213 instead of 42.
+ *
+ * Fix (both stages, rule 10): gate the in-place arm on a confirmed-LOCAL
+ * base. A base is GLOBAL iff localfind(base)==0 && let_islet(base)
+ * (cstage) / isletvar(c, base) (wwstage) — the same idiom cstage uses at
+ * cgen.c:2000 (also 4707/4730/5393). A global base falls THROUGH to the
+ * existing spill `else`, which cgexprs the scrutinee and resolves g(SB)
+ * correctly. M1's LOCAL in-place path is unchanged (row (d) pins it).
+ *
+ * Rows pin the RUNTIME exit code on BOTH driver stages (cstage +
+ * wwstage) — the discriminator is the value, not byte-id, since both
+ * stages were identical-wrong. A cstage-vs-wwstage asm byte-identity
+ * check is kept per row so the symmetric (rule-10) property stays pinned
+ * after the guard lands in both stages.
+ *   (a) global VALUE-struct int-variant field      [spill, g(SB)]
+ *   (b) global field bool->int payload remap        [1-word variant]
+ *   (c) global field str-payload (24B box)          [3-word variant]
+ *   (d) local VALUE-struct field, in place          [M1 regression guard]
+ */
+#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; };
+
+static const struct row rows[] = {
+	/* (a) GLOBAL value struct, field set to the int variant in main,
+	 * matched. Pre-fix the in-place arm read BP+0 (213); post-fix the
+	 * spill path resolves g(SB) and returns 42. */
+	{ "global_field_int",
+	  "package main;\n"
+	  "type box = struct { t: (int | bool), n: int };\n"
+	  "let g: box = box { t = 0, n = 5 };\n"
+	  "fn main() i32 = {\n"
+	  "    g.t = 42;\n"
+	  "    match (g.t) {\n"
+	  "    case let n: int => return n: i32;\n"
+	  "    case bool => return 1;\n"
+	  "    };\n"
+	  "};\n",
+	  42 },
+	/* (b) GLOBAL field reassigned, then matched; the int arm also reads
+	 * the sibling field g.n to prove the surrounding global is
+	 * undisturbed. Returns 7 + 5 = 12. The static initializer uses the
+	 * ZERO payload (t = 0): a nonzero tagged-field payload in a static
+	 * struct initializer trips a SEPARATE, deferred static-init DATA
+	 * divergence (task #19/#30 — wwstage drops the payload word to 0,
+	 * cstage emits it) that is orthogonal to the #29 match-cgen path
+	 * exercised here; a zero payload is byte-identical across stages. */
+	{ "global_field_reassign_remap",
+	  "package main;\n"
+	  "type box = struct { t: (int | bool), n: int };\n"
+	  "let g: box = box { t = 0, n = 5 };\n"
+	  "fn main() i32 = {\n"
+	  "    g.t = 7;\n"
+	  "    match (g.t) {\n"
+	  "    case let n: int => return (n + g.n): i32;\n"
+	  "    case bool => return 1;\n"
+	  "    };\n"
+	  "};\n",
+	  12 },
+	/* (c) GLOBAL field, 24B str-payload variant (tag@+0, .ptr@+8,
+	 * .len@+16). Returns len("hello") = 5. */
+	{ "global_field_str_payload_24B",
+	  "package main;\n"
+	  "type box = struct { t: (str | int), n: int };\n"
+	  "let g: box = box { t = 0, n = 5 };\n"
+	  "fn main() i32 = {\n"
+	  "    g.t = \"hello\";\n"
+	  "    match (g.t) {\n"
+	  "    case let s: str => return len(s): i32;\n"
+	  "    case int => return 99;\n"
+	  "    };\n"
+	  "};\n",
+	  5 },
+	/* (d) regression guard: a LOCAL value-struct field still reads in
+	 * place (M1's $32 path stays). Returns 42. */
+	{ "local_field_inplace_guard",
+	  "package main;\n"
+	  "type box = struct { t: (int | bool), n: int };\n"
+	  "fn main() i32 = {\n"
+	  "    let b: box = box { t = 0, n = 5 };\n"
+	  "    b.t = 42;\n"
+	  "    match (b.t) {\n"
+	  "    case let n: int => return n: i32;\n"
+	  "    case bool => return 1;\n"
+	  "    };\n"
+	  "};\n",
+	  42 },
+};
+
+static int
+run_driver(const char *driver, const struct row *r, int i)
+{
+	char src[64], tmpdir[64], cmd[1024];
+	snprintf(src, sizeof src, "/tmp/wcmgf_%d_%d.ww", getpid(), i);
+	snprintf(tmpdir, sizeof tmpdir, "/tmp/wcmgf_%d_d_%d", getpid(), i);
+
+	FILE *f = fopen(src, "wb");
+	if (!f) return -1;
+	fputs(r->src, f);
+	fclose(f);
+
+	mkdir(tmpdir, 0755);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s",
+	    tmpdir, driver, src);
+	if (runwait(cmd) != 0) {
+		fprintf(stderr, "row[%s]: build via %s failed\n",
+		    r->label, driver);
+		unlink(src); rmdir(tmpdir);
+		return -1;
+	}
+
+	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 = runwait(outbin);
+
+	unlink(src); unlink(outbin); rmdir(tmpdir);
+	return got;
+}
+
+/* asm_byte_identical — the guard lands in BOTH stages, so cstage vs
+ * wwstage text output must stay byte-for-byte identical on every row. */
+static int
+asm_byte_identical(const char *bin, const struct row *r, int i)
+{
+	char src[64], cs[64], ws[64], cmd[1024];
+	snprintf(src, sizeof src, "/tmp/wcmgf_asm_%d_%d.ww", getpid(), i);
+	snprintf(cs,  sizeof cs,  "/tmp/wcmgf_asm_%d_%d_c.s", getpid(), i);
+	snprintf(ws,  sizeof ws,  "/tmp/wcmgf_asm_%d_%d_w.s", getpid(), i);
+
+	FILE *f = fopen(src, "wb");
+	if (!f) return -1;
+	fputs(r->src, f);
+	fclose(f);
+
+	snprintf(cmd, sizeof cmd, "%s/w6c -o %s %s 2>/dev/null", bin, cs, src);
+	if (runwait(cmd) != 0) {
+		fprintf(stderr, "row[%s]: w6c errored\n", r->label);
+		unlink(src);
+		return -1;
+	}
+	snprintf(cmd, sizeof cmd, "%s/w6c_ww -o %s %s 2>/dev/null",
+	    bin, ws, src);
+	if (runwait(cmd) != 0) {
+		fprintf(stderr, "row[%s]: w6c_ww errored\n", r->label);
+		unlink(src); unlink(cs);
+		return -1;
+	}
+
+	FILE *fc = fopen(cs, "rb");
+	FILE *fw = fopen(ws, "rb");
+	int rc = 0;
+	if (!fc || !fw) {
+		rc = -1;
+	} else {
+		for (;;) {
+			int a = fgetc(fc);
+			int b = fgetc(fw);
+			if (a != b) { rc = -1; break; }
+			if (a == EOF) break;
+		}
+	}
+	if (fc) fclose(fc);
+	if (fw) fclose(fw);
+	if (rc != 0)
+		fprintf(stderr, "row[%s]: cstage vs wwstage asm differs\n",
+		    r->label);
+	unlink(src); unlink(cs); unlink(ws);
+	return rc;
+}
+
+int
+main(void)
+{
+	const char *bin = getenv("BIN");
+	if (!bin) bin = "out/bin";
+	char absbin[512];
+	if (bin[0] != '/') {
+		char cwd[256];
+		if (getcwd(cwd, sizeof cwd) == NULL) return 1;
+		snprintf(absbin, sizeof absbin, "%s/%s", cwd, bin);
+		bin = absbin;
+	}
+
+	char cdrv[640];
+	snprintf(cdrv, sizeof cdrv, "%s/ww", bin);
+	char wdrv[640];
+	snprintf(wdrv, sizeof wdrv, "%s/ww_ww", bin);
+
+	struct { const char *name; const char *path; int gated_on_existence; }
+	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_on_existence
+		    && access(drivers[d].path, X_OK) != 0) {
+			fprintf(stderr, "match_global_field: skip %s (no %s)\n",
+			    drivers[d].name, drivers[d].path);
+			continue;
+		}
+		for (int i = 0; i < n; i++) {
+			int got = run_driver(drivers[d].path, &rows[i], i);
+			total++;
+			if (got != rows[i].want) {
+				fprintf(stderr,
+				    "match_global_field[%s][%s]: exit=%d want=%d\n",
+				    drivers[d].name, rows[i].label,
+				    got, rows[i].want);
+				fail++;
+			}
+		}
+	}
+
+	if (access(wdrv, X_OK) == 0) {
+		for (int i = 0; i < n; i++) {
+			total++;
+			if (asm_byte_identical(bin, &rows[i], i) != 0)
+				fail++;
+		}
+	}
+
+	if (fail) {
+		fprintf(stderr,
+		    "match_global_field: %d/%d fixtures failed\n",
+		    fail, total);
+		return 1;
+	}
+	printf("match_global_field: %d/%d ok\n", total, total);
+	return 0;
+}