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cgen: str tuple-positional element read -> 3-word -- Phase 2 C4.6 S3 (both stages)

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Reading a str-typed tuple element by position (t.N) loaded 2 words (ptr,len), dropping cap -- the C4.6 coda with NO adjacent slice-element arm to mirror. Author the 3-word triple directly to the canonical {ptr,len,cap} ABI (AX,BX,CX off the BP frame slot; add cap->CX at +16). Kind-gated (TY_STR / isstrtype, never size==24). cstage==wwstage byte-identical -- cross-stage identity is the correctness oracle here, since there is no local slice sibling.

test/wcc/935: table-driven runtime .cap-survives over let s = t.1; poison rides the return ABI (R8) into the slot's cap word; a CX-clobbering call ensures a 2-word read cannot coincidentally pass. fail-before/pass-after verified INDEPENDENTLY on both stages.

main.combined.ww regenerated via the canonical make path (md5-stable).
This commit is contained in:
Hojun-Cho
2026-05-24 14:22:25 +09:00
parent 634cbefc22
commit 877a1af6d3
6 changed files with 209 additions and 13 deletions
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11
selfhost/cmd/w6c/main.combined.ww
View File

@@ -15336,9 +15336,11 @@ fn cgdot(c: *cgen, n: *node) void = {
// Walk the tuple element type list summing slotsize
// (matches the (scalar, str) init layout: scalar in an
// 8B slot, str in 24B — str IS []u8, #1/Phase 3). For a
// str element, load (ptr, len) into (AX, BX); the cap
// stays in the slot (the 2-word str-field read, like
// every other chained/dot str leaf read — task #14).
// str element, load (ptr, len, cap) into (AX, BX, CX),
// the canonical slice-header ABI. No slice-element
// sibling here, so the triple is hand-authored; base is
// BP (frame, not a target reg) so ptr/len/cap order has
// no clobber risk.
if (lkind == nkind.N_TTUPLE) {
let idx: i32 = fldnumidx(fld);
if (idx >= 0) {
@@ -15362,6 +15364,9 @@ fn cgdot(c: *cgen, n: *node) void = {
emitline("\tMOVQ\t");
emitoff((lc.off + foff + 8): i64);
emitline("(BP), BX\n");
emitline("\tMOVQ\t");
emitoff((lc.off + foff + 16): i64);
emitline("(BP), CX\n");
return;
};
let sz: i32 = slotsize(c, tpt);

11
selfhost/cmd/wcc/cgenexpr.ww
View File

@@ -1508,9 +1508,11 @@ fn cgdot(c: *cgen, n: *node) void = {
// Walk the tuple element type list summing slotsize
// (matches the (scalar, str) init layout: scalar in an
// 8B slot, str in 24B — str IS []u8, #1/Phase 3). For a
// str element, load (ptr, len) into (AX, BX); the cap
// stays in the slot (the 2-word str-field read, like
// every other chained/dot str leaf read — task #14).
// str element, load (ptr, len, cap) into (AX, BX, CX),
// the canonical slice-header ABI. No slice-element
// sibling here, so the triple is hand-authored; base is
// BP (frame, not a target reg) so ptr/len/cap order has
// no clobber risk.
if (lkind == nkind.N_TTUPLE) {
let idx: i32 = fldnumidx(fld);
if (idx >= 0) {
@@ -1534,6 +1536,9 @@ fn cgdot(c: *cgen, n: *node) void = {
emitline("\tMOVQ\t");
emitoff((lc.off + foff + 8): i64);
emitline("(BP), BX\n");
emitline("\tMOVQ\t");
emitoff((lc.off + foff + 16): i64);
emitline("(BP), CX\n");
return;
};
let sz: i32 = slotsize(c, tpt);

11
selfhost/cmd/wwdump/main.combined.ww
View File

@@ -15336,9 +15336,11 @@ fn cgdot(c: *cgen, n: *node) void = {
// Walk the tuple element type list summing slotsize
// (matches the (scalar, str) init layout: scalar in an
// 8B slot, str in 24B — str IS []u8, #1/Phase 3). For a
// str element, load (ptr, len) into (AX, BX); the cap
// stays in the slot (the 2-word str-field read, like
// every other chained/dot str leaf read — task #14).
// str element, load (ptr, len, cap) into (AX, BX, CX),
// the canonical slice-header ABI. No slice-element
// sibling here, so the triple is hand-authored; base is
// BP (frame, not a target reg) so ptr/len/cap order has
// no clobber risk.
if (lkind == nkind.N_TTUPLE) {
let idx: i32 = fldnumidx(fld);
if (idx >= 0) {
@@ -15362,6 +15364,9 @@ fn cgdot(c: *cgen, n: *node) void = {
emitline("\tMOVQ\t");
emitoff((lc.off + foff + 8): i64);
emitline("(BP), BX\n");
emitline("\tMOVQ\t");
emitoff((lc.off + foff + 16): i64);
emitline("(BP), CX\n");
return;
};
let sz: i32 = slotsize(c, tpt);