175: mask chains must be SEPARATE STATEMENTS -- one expression folds to a single and; three
statements emit the original's lw/and/and/and/sw. cc1 does not fold constants across statements.
176: a MECHANISM, not a tip. Three separate RMW statements give one lw, three ands and ONE sw
(cse forwards each load from the previous store within a block); an intervening early return makes
that store land in the beq delay slot and the tail's store stays alive because its load is
forwarded from a store in the PREVIOUS block. A local chain loses the delay-slot store (100 B);
volatile keeps both stores but flips the entry branch (100 B).
177: fold REASSOCIATES | -- one statement becomes v | (C1 | X), a 17-byte residual; two statements
give the original (v | C1) | X.
178: finding 170 has a CONSTANT-clobber variant -- the same entry-block copies arise because mask
constants are materialised into the parameters' own homes, with no call involved. The rule
generalises: a redundant entry copy means the value is live when its home is clobbered, whether by
a call's argument setup OR by constant materialisation.
Christopher Williams
bec90acd31