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mul256x256_512_64bitlimbs.riscv64.s
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mul256x256_512_64bitlimbs.riscv64.s
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.globl mul256x256_512
.text
mul256x256_512: # computes mul256x256_512, where pointers: out in a0, a in a1, b in a2
# custom stuff: create mask lower 32-bits, useful for extracting lower 32-bits
#lui t6, 0xfffff # all 1's since sign extends
#srli t6, t6, 32 # now t6 is 0x00000000ffffffff
# bring a,b into registers a0-7
# first free a0, a1, and a2
mv t0, a0
mv t1, a1
mv t2, a2
ld a0, 0(t1)
ld a1, 8(t1)
ld a2, 16(t1)
ld a3, 24(t1)
ld a4, 0(t2)
ld a5, 8(t2)
ld a6, 16(t2)
ld a7, 24(t2)
# zero things
mv t1, zero
mv t2, zero
mv t3, zero
# done initializing, now let's compute something
# a0*b0 + carry
# a0*b0
mulhu t5, a0, a4
mul t4, a0, a4
mv t2, t5
mv t1, t4
# write result for this limb, prepare for next limb
sd t1, 0(t0) # write result for this limb
mv t1, t2
mv t2, zero
# a0*b1 + a1*b0 + carry
# a0*b1
mulhu t5, a0, a5
mul t4, a0, a5
add t1, t1, t4 # add low limb to accumulator t1
sltu t4, t1, t4 # check overflow
add t5, t5, t4 # add to overflow, even if add zero
add t2, t2, t5 # add high limb to accumulator t2
sltu t5, t2, t5 # check overflow
add t3, t3, t5 # add to overflow, even if add zero
# a1*b0
mulhu t5, a1, a4
mul t4, a1, a4
add t1, t1, t4 # add low limb to accumulator t1
sltu t4, t1, t4 # check overflow
add t5, t5, t4 # add to overflow, even if add zero
add t2, t2, t5 # add high limb to accumulator t2
sltu t5, t2, t5 # check overflow
add t3, t3, t5 # add to overflow, even if add zero
# write result for this limb, prepare for next limb
sd t1, 8(t0) # write result for this 32-bit limb
mv t1, t2
mv t2, t3
mv t3, zero
# a0*b2 + a1*b1 + a2*b0 + carry
# a0*b2
mulhu t5, a0, a6
mul t4, a0, a6
add t1, t1, t4 # add low limb to accumulator t1
sltu t4, t1, t4 # check overflow
add t5, t5, t4 # add to overflow, even if add zero
add t2, t2, t5 # add high limb to accumulator t2
sltu t5, t2, t5 # check overflow
add t3, t3, t5 # add to overflow, even if add zero
# a1*b1
mulhu t5, a1, a5
mul t4, a1, a5
add t1, t1, t4 # add low limb to accumulator t1
sltu t4, t1, t4 # check overflow
add t5, t5, t4 # add to overflow, even if add zero
add t2, t2, t5 # add high limb to accumulator t2
sltu t5, t2, t5 # check overflow
add t3, t3, t5 # add to overflow, even if add zero
# a2*b0
mulhu t5, a2, a4
mul t4, a2, a4
add t1, t1, t4 # add low limb to accumulator t1
sltu t4, t1, t4 # check overflow
add t5, t5, t4 # add to overflow, even if add zero
add t2, t2, t5 # add high limb to accumulator t2
sltu t5, t2, t5 # check overflow
add t3, t3, t5 # add to overflow, even if add zero
# write result for this limb, prepare for next limb
sd t1, 16(t0) # write result for this 32-bit limb
mv t1, t2
mv t2, t3
mv t3, zero
# a0*b3 + a1*b2 + a2*b1 + a3*b0 + carry
# a0*b3
mulhu t5, a0, a7
mul t4, a0, a7
add t1, t1, t4 # add low limb to accumulator t1
sltu t4, t1, t4 # check overflow
add t5, t5, t4 # add to overflow, even if add zero
add t2, t2, t5 # add high limb to accumulator t2
sltu t5, t2, t5 # check overflow
add t3, t3, t5 # add to overflow, even if add zero
# a1*b2
mulhu t5, a1, a6
mul t4, a1, a6
add t1, t1, t4 # add low limb to accumulator t1
sltu t4, t1, t4 # check overflow
add t5, t5, t4 # add to overflow, even if add zero
add t2, t2, t5 # add high limb to accumulator t2
sltu t5, t2, t5 # check overflow
add t3, t3, t5 # add to overflow, even if add zero
# a2*b1
mulhu t5, a2, a5
mul t4, a2, a5
add t1, t1, t4 # add low limb to accumulator t1
sltu t4, t1, t4 # check overflow
add t5, t5, t4 # add to overflow, even if add zero
add t2, t2, t5 # add high limb to accumulator t2
sltu t5, t2, t5 # check overflow
add t3, t3, t5 # add to overflow, even if add zero
# a3*b0
mulhu t5, a3, a4
mul t4, a3, a4
add t1, t1, t4 # add low limb to accumulator t1
sltu t4, t1, t4 # check overflow
add t5, t5, t4 # add to overflow, even if add zero
add t2, t2, t5 # add high limb to accumulator t2
sltu t5, t2, t5 # check overflow
add t3, t3, t5 # add to overflow, even if add zero
# write result for this limb, prepare for next limb
sd t1, 24(t0) # write result for this 32-bit limb
mv t1, t2
mv t2, t3
mv t3, zero
# a1*b3 + a2*b2 + a3*b1 + carry
# a1*b3
mulhu t5, a1, a7
mul t4, a1, a7
add t1, t1, t4 # add low limb to accumulator t1
sltu t4, t1, t4 # check overflow
add t5, t5, t4 # add to overflow, even if add zero
add t2, t2, t5 # add high limb to accumulator t2
sltu t5, t2, t5 # check overflow
add t3, t3, t5 # add to overflow, even if add zero
# a2*b2
mulhu t5, a2, a6
mul t4, a2, a6
add t1, t1, t4 # add low limb to accumulator t1
sltu t4, t1, t4 # check overflow
add t5, t5, t4 # add to overflow, even if add zero
add t2, t2, t5 # add high limb to accumulator t2
sltu t5, t2, t5 # check overflow
add t3, t3, t5 # add to overflow, even if add zero
# a3*b1
mulhu t5, a3, a5
mul t4, a3, a5
add t1, t1, t4 # add low limb to accumulator t1
sltu t4, t1, t4 # check overflow
add t5, t5, t4 # add to overflow, even if add zero
add t2, t2, t5 # add high limb to accumulator t2
sltu t5, t2, t5 # check overflow
add t3, t3, t5 # add to overflow, even if add zero
# write result for this limb, prepare for next limb
sd t1, 32(t0) # write result for this 32-bit limb
mv t1, t2
mv t2, t3
mv t3, zero
# a2*b3 + a3*b2 + carry
# a2*b3
mulhu t5, a2, a7
mul t4, a2, a7
add t1, t1, t4 # add low limb to accumulator t1
sltu t4, t1, t4 # check overflow
add t5, t5, t4 # add to overflow, even if add zero
add t2, t2, t5 # add high limb to accumulator t2
sltu t5, t2, t5 # check overflow
add t3, t3, t5 # add to overflow, even if add zero
# a3*b2
mulhu t5, a3, a6
mul t4, a3, a6
add t1, t1, t4 # add low limb to accumulator t1
sltu t4, t1, t4 # check overflow
add t5, t5, t4 # add to overflow, even if add zero
add t2, t2, t5 # add high limb to accumulator t2
sltu t5, t2, t5 # check overflow
add t3, t3, t5 # add to overflow, even if add zero
# write result for this limb, prepare for next limb
sd t1, 40(t0) # write result for this 32-bit limb
mv t1, t2
mv t2, t3
mv t3, zero
# a3*b3 carry
# a3*b3
mulhu t5, a3, a7
mul t4, a3, a7
add t1, t1, t4 # add low limb to accumulator t1
sltu t4, t1, t4 # check overflow
add t5, t5, t4 # add to overflow, even if add zero
add t2, t2, t5 # add high limb to accumulator t2
sltu t5, t2, t5 # check overflow
add t3, t3, t5 # add to overflow, even if add zero
# write result for this limb, prepare for next limb
sd t1, 48(t0) # write result for this 32-bit limb
sd t2, 56(t0) # write result for this 32-bit limb
#mv t1, t2
#mv t2, t3
#mv t3, zero
ret