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wcc: struct ABI size maxalign-rounded via structabisize (#169)

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wwstage struct-return RECV and RETURN used unrounded / round-to-8 sizes where
cstage uses the maxalign-rounded lu->size / rt->size, so a struct with maxalign
8 and a sub-8 tail (e.g. struct{i64,i32}) — or a maxalign<8 struct on the
return path — unpacked with a different trailing-word width (MOVL vs MOVQ)
between stages. Value-correct either way, but a cs!=ww asm divergence.

Add a dedicated structabisize = round(natural, maxalign) used only at the two
register-ABI sites. structnaturalsize stays unrounded: cstage's >24B sret and
memory-move path (cgen.c:8150, Task #33) genuinely uses the unrounded natural
size, so the two are different sizes — rounding the shared metric breaks 995.
maxalign derives from each field's tinfo.align (mirrors cstage check.c:708),
not an fsz ladder (a ladder over-rounds composite [N]u8 fields).

Gate-blind (no bootstrap struct hits the maxalign-8+tail shape) — the
discriminator is the cs==ww .s byte-cmp; covered by probe 698.
This commit is contained in:
Hojun-Cho
2026-05-28 01:02:41 +09:00
parent a917533fbf
commit d4e500f61c
5 changed files with 223 additions and 48 deletions
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14
selfhost/cmd/wcc/cgenstmt.ww
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@@ -656,7 +656,10 @@ fn cgreturn(c: *cgen, n: *node) void = {
if (rname.len > 0) {
let rsi: *structinfo = structlookup(c, rname);
if (rsi != nil) {
let rsz: i32 = rsi.totsize;
// ≤24B register RETURN: cstage sizes by rt->size
// (maxalign-rounded), not the slot-padded totsize
// (round-to-8) — see structabisize (#169).
let rsz: i32 = structabisize(rsi);
if (rsz <= 24) {
let okrhs: bool = false;
if (rhs.kind == nkind.N_IDENT) {
@@ -1214,10 +1217,11 @@ fn cglet(c: *cgen, n: *node) void = {
if (sname.len > 0) {
let lsi: *structinfo = structlookup(c, sname);
if (lsi != nil) {
// si.totsize is slot-padded (rounded to 8) for
// stack-slot use; the receive ABI needs the
// TYPE's natural size — see structnaturalsize.
let lsz: i32 = structnaturalsize(lsi);
// ≤24B register RECV: the value arrives packed
// in AX/DX/CX, so size by the maxalign-rounded
// ABI size (cstage lu->size), not the natural
// extent — see structabisize (#169).
let lsz: i32 = structabisize(lsi);
let tlm: i32 = lsz - (lsz / 8) * 8;
if (lsz <= 24) {
if (tlm == 0 || tlm == 1