Drew T
2.3 KiB
§416 ★★ — FOUR LEVERS FROM THE S71 OVERNIGHT LANE, none of which the cookbook held (P31 S71)
All four are byte-proven, each on the function named.
1. RE-READ THE STORE, DO NOT PASS THE VALUE — CSE store-forwarding.
ov_SC06_000/func_80183398 (119 ins). The target does sh and then an in-place sll/sra on the
same location, which is gcc forwarding its own store. Writing f((s16)val) keeps the pseudo live in
$v1 and emits sll $a1,$v1,16 BEFORE the sh. Passing a RE-READ of the slot instead —
f(*(s16 *)(a0 + 0xA)) after the store — is what reproduces it. When the target reads back
something it just wrote, write that read in the source.
2. (&SYM)[3] vs p = &SYM; *p = 0; are different ADDRESSING, not style.
ov_SC01_084/func_80181310 (118 ins). addiu $s2,$a0,0xC proves D_801270D4 is written as
(&D_801270C8)[3]: loop.c hoists the constant address and -frerun-cse-after-loop re-relates it to
the base register. Spelling it r = &D_801270C8; *r = 0; puts the base in a register instead, and a
separate pointer local u = &D_801270D0; gives la + 0($v1) rather than two lo_sum forms. Three
spellings of one store, three different addressing modes.
3. ONE biv with +0/+2/+4, and a guard on the SYMBOL — not on the pointer.
Same function. combine_givs merges the +2/+4 accesses into $s0 at -0x2/0 only while there is a
SINGLE s16 * induction variable; introducing an explicit second pointer variable breaks both that
addressing and the preheader order (li $s4,1 / addiu $s2 / addiu $s0 — movables, then giv
inits). Guard on the separate symbol D_8018A9F0, never on *q.
4. A LOCAL'S WIDTH CHOOSES THE LOAD. Same function: an s32 local for a 16-bit global forces
lh; an s16 local emits lhu plus sll/sra. (This is §409 law 6 seen from the other side — there
the width cost two instructions per COMPARE, here it changes the LOAD itself.)
And one pack-quality note that recurs all night: the same-address "twin" hint was FALSE on
ov_SC06_000 (an unrelated 11-ins function), on ov_SC01_080 (two different functions), and on
ov_SC03_030. In every case the real fuel was a same-TU neighbour (§194-E). The address-twin
lead is worth checking and never worth trusting — law 1c exists for this.