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PPC630 (aka Power3) pipeline information:
Decoding is 4-way and issue is 8-way with some out-of-order capability.
Branches are handled separately, and are not part of the 4-way issue limit.
LS1 - ld/st unit 1
LS2 - ld/st unit 2
FXU1 - integer unit 1, handles any simple integer instruction
FXU2 - integer unit 2, handles any simple integer instruction
FXU3 - integer unit 3, handles integer multiply and divide
FPU1 - floating-point unit 1
FPU2 - floating-point unit 2
Memory: Any two memory operations can issue, but memory subsystem
can sustain just one store per cycle. No need for data
prefetch; the hardware has very sophisticated prefetch logic.
Simple integer: 2 operations (such as add, rl*)
Integer multiply: 1 operation every 9th cycle worst case; exact timing depends
on 2nd operand most significant bit position (10 bits per
cycle). Multiply unit is not pipelined, only one multiply
operation in progress is allowed.
Integer divide: ?
Floating-point: Any plain 2 arithmetic instructions (such as fmul, fadd, fmadd)
Latency = 4.
Floating-point square root:
Best possible times for the main loops:
shift: 1.5 cycles limited by integer unit contention.
With 63 special loops, one for each shift count, we could
reduce the needed integer instructions to 2, which would
reduce the best possible time to 1 cycle.
add/sub: 1.5 cycles, limited by ld/st unit contention.
mul: 18 cycles (average) unless floating-point operations are used,
but that would only help for multiplies of perhaps 10 and more
addmul/submul:Same situation as for mul.
*mul_1: Handling one limb using mulld/mulhdu and two limbs using
floating-point operations should give a performance of about 20 cycles
for 3 limbs, or 7 cycles/limb.
We should probably split the single-limb operand in 32-bit chunks, and
the multi-limb operand in 16-bit chunks, allowing us to accumulate
well in fp registers.
Problem is to get 32-bit or 16-bit words to the fp registers. Only
64-bit fp memops copies bits without fiddling with them. We might
therefore need to load to integer registers with zero extension, store
as 64 bits into temp space, and then load to fp regs. Alternatively,
load directly to fp space and add well-chosen constants to get
cancelation. (Other part after given by subsequent subtraction.)
Possible code mix for load-via-intregs variant: