C! as written in 5.3 called RSHIFT and LSHIFT (which call SWAP) and OR.
It is now one straight-line case per byte position: the byte is shifted
up with a `2* unext` loop, that byte of the cell cleared with a constant
mask, and the two added. The word address is `2/ 2/` of the byte
address. The upper half of a 64-bit cell is still preserved.
Headroom (data cells under args / return entries): 5/4 -> 7/7.
Same checks as before (every byte of two adjacent words, eight values,
neighbours and the upper half untouched) at 32- and 64-bit cells,
optimised and ASan+UBSan (`make test`, `make sanitize`). Four mutations
fail, one of them only at 64-bit cells, where it differs.
test_pictured.c carries the same C!. The pictured words did not gain
return-stack room from this: <# #S #> still leaves 2 entries and the
signed picture 1. With C! shallow, the deepest point is now inside `#`,
which holds the high quotient on the return stack across its second
UM/MOD. Recorded in 5.8; `#` is unchanged.
Test results on the amd64 host only. This is a development check, not
acceptance (JUSTIFICATION.md section 16).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
