Clearing and setting a flag bit
Straight from SFA's crfueltank_update: a timer helper decides which branch runs, and each branch flips the same flag bit in opposite directions. Because the function calls something, you get a real prologue/epilogue with a saved register.
typedef struct { f32 timer; s16 flags; u8 fadeTimer; u8 pad; } CrFuelTankObject;
extern int timerCountDown(f32* t);
extern void ObjHits_EnableObject(CrFuelTankObject* o);
Two idioms, same field. A single-bit clear and a single-bit set on the same s16 flags field compile differently:
flags & ~MASK → rlwinm (rotate-and-mask; clears exactly one bit)flags | MASK → ori (OR immediate; sets exactly one bit)
Writing flags &= 0xbfff instead of flags &= ~0x4000 would emit andi — a different instruction that won't match.
The (s16) cast around each result keeps the field halfword-wide: reads use lha and writes use sth.
To see both idioms in a simpler context, here is a particle-timer variant that operates on bit 0x2000 instead:
# nonzero branch:
lha r3, 4(r31)
li r0, 128
rlwinm r3, r3, 0, 19, 17 # flags &= ~0x2000
sth r3, 4(r31)
stb r0, 6(r31) # alpha = 0x80
b .end
# zero branch:
lha r0, 4(r31)
ori r0, r0, 8192 # flags |= 0x2000
sth r0, 4(r31)
Note rlwinm r3, r3, 0, 19, 17: rotate by 0, keep bits from MB=19 through ME=17 (wrapping), which zeros every bit except those in that range — the excluded bit is at MSB-position 18, i.e. 0x2000. The ori 8192 is 0x2000 in decimal.
In the target assembly below, read the rlwinm MB/ME fields to work out which bit is cleared, and read the ori immediate to confirm the same bit is set. Read the li immediate to find the byte-field value.
stwu r1, -16(r1)
mflr r0
...
bl timerCountDown
cmpwi r3, 0
beq- .else
mr r3, r31
bl ObjHits_EnableObject
lha r3, 4(r31)
li r0, 255
rlwinm r3, r3, 0, 18, 16
sth r3, 4(r31)
stb r0, 6(r31)
b .end
.else:
lha r0, 4(r31)
ori r0, r0, 16384
sth r0, 4(r31)
.end:
...
blr
Your task
With the struct above, write crfueltank_tick to reproduce the assembly above.