Hojun-Cho
a00d052833
The four wwstage whole-struct field-copy sites (cgenexpr.ww) copied
`ssi.totsize` — the slot-padded, round-8 structinfo size — instead of
the SOURCE struct's natural size. cstage copies `f->type->size` (the
field struct's aligned r.size; cmd/w6c/cgen.c:5302). wwstage over-copied
into the field's slot padding.
Fix: length = copysrcnatsize(c, n.rhs) = tichase(src.type_).size, read
from the SOURCE node's stamped tinfo (the checker's natural r.size,
check.ww N_TSTRUCT). This never reads structinfo / fi.foff / fi.fsz, so
it is correct at HEAD unconditionally and independent of the
registerstruct natural-offset change (#44/#55) that follows — a pure
wwstage convergence onto the length cstage already emits. Distinct from
the existing structnaturalsize (structinfo max(foff+fsz), a #44-coupled
source).
LENGTH ONLY. The ragged-tail completeness (both stages' field copies
inline a tail handling only {4,1}; a natural size %8 in {2,3,5,6,7}
falls through to an 8-byte MOVQ over-read) is a SEPARATE both-stage
class — cstage cgen.c:5302 has the identical incomplete tail — folded
into #73 (route both stages' field copies through the canonical greedy
aggcopy emitter). Touching only ww's tail here would create a gate-blind
cs!=ww on narrow-tail inputs, so it is deliberately left for the
both-stage fix.
No isolated runtime repro: the over-copy writes [natural, totsize),
which under HEAD's slot-padded field layout is the field's OWN padding
(the successor parks at the next slot). It only becomes a clobber once
#44 packs the successor at its natural offset (the 681 ragged_tail_12B
regression that forced this ordering). So this commit is byte-id-clean
and a no-op on the present corpus; its proof is the all-green run plus
the #44 commit that depends on it.