Drew T
2.4 KiB
§432 ★★★ — DEFEAT cse's MERGE OF TWO IDENTICAL MASKS BY SPELLING ONE AS A SHIFT PAIR (P31 S72/S73; main/func_8002DC68, MATCH 198/198)
The shape. The target masks the same value TWICE — a compare into $v1 and a second andi into
$v0 that reorg then steals for a beqz delay slot:
andi $v1, $s2, 0x7F # the compare
andi $v0, $s2, 0x7F # a SECOND, identical mask -> reorg fills the branch delay slot with it
Written the obvious way — param_2 & 0x7F on both sides — cse merges the two (and:SI) RTXs,
there is only one mask left, and the delay slot comes out EMPTY. No amount of statement reordering
recovers it, because the two expressions really are identical and cse is right.
The lever. Spell ONE of them as a shift pair:
(param_2 << 25) >> 25 /* instead of param_2 & 0x7F */
cse sees (lshiftrt (ashift ...)), not (and ...), so it does not merge — and then combine's
simplify_shift_const folds the pair straight back to andi 0x7F. You get two masks in the RTL
and one instruction each in the output. Byte-verified with the shift on EITHER side, so use whichever
reads more naturally.
Why it generalises. This is the inverse of the usual advice. Normally you fight gcc by making two
expressions identical so cse merges them; here you must make them textually different in RTL but
identical after combine. Any x & ((1<<n)-1) has a shift-pair twin that survives cse and folds back:
the mask and the shift pair are the same value to combine and different values to cse, and the
passes run in that order. Reach for it whenever a duplicated narrowing operation is missing from your
output and the second use is a delay-slot filler.
Verification standard this one met (§405-A + §409): the agent checked the 18-entry
jtbl_80072F3C order and all 51 reloc symbols by hand before reporting, because match_one
sees neither.
Companion finding — a §378c declaration blocker, and it is the norm on main now. The draft is
s32 func_8002DC68(u32, u32) while src/800_b.c carries five extern void func_8002DC68(s32, s32); declarations. That is a §376 gate blocker, not a body problem: align the
declarations to the definition, prove the alignment byte-neutral with NO draft substituted, COMMIT
it, and only then gate (the gate reverts src/*.c as its first action — §431).