Timing analysis showed that even with the output register, timing
was still a bit tight in the output stage, where the carry has to
propagate all the way through the 64-bit negater, and we were then
testing the top 33 bits to determine if a 32-bit operation had
overflowed.
Instead of detecting overflow at the end, we watch for any 1
bits getting shifted into the top 32 bits of the quotient register
as we are doing the division. That is relatively easy to do and
simplifies the output stage.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
This puts the output of the divider through a register. With the
addition of the logic to detect overflow, the combinatorial output
logic of the divider was becoming a critical path. Adding the
output register adds a cycle to the latency of the divider but
helps make timing at 100MHz on the A7-100.
This also makes the valid, write_reg_enable and write_cr_enable
fields of the output be registered, which eliminates warnings
about register/latch pins with no clock.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
This adds logic to detect the cases where the quotient of the
division overflows the range of the output representation, and
return all zeroes in those cases, which is what POWER9 does.
To do this, we extend the dividend register by 1 bit and we do
an extra step in the division process to get a 2^64 bit of the
quotient, which ends up in the 'overflow' signal. This catches all
the cases where dividend >= 2^64 * divisor, including the case
where divisor = 0, and the divde/divdeu cases where |RA| >= |RB|.
Then, in the output stage, we also check that the result fits in
the representable range, which depends on whether the division is
a signed division or not, and whether it is a 32-bit or 64-bit
division. If dividend >= 2^64 or the result doesn't fit in the
representable range, write_data is set to 0 and write_cr_data to
0x20000000 (i.e. cr0.eq = 1).
POWER9 sets the top 32 bits of the result to zero for 32-bit signed
divisions, and sets CR0 when RC=1 according to the 64-bit value
(i.e. CR0.LT is always 0 for 32-bit signed divisions, even if the
32-bit result is negative). However, modsw with a negative result
sets the top 32 bits to all 1s. We follow suit.
This updates divider_tb to check the invalid cases as well as the
valid case.
This also fixes a small bug where the reset signal for the divider
was driven from rst when it should have been driven from core_rst.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
This moves the negation of negative operands for signed divide and
modulus operations out of the decode2 stage and into the divider.
If either of the operands for a signed divide or modulus operation
is negative, the divider now takes an extra cycle to negate the
operands that are negative.
The interface to the divider now has an 'is_signed' signal rather
than a 'neg_result' signal, and the dividend and divisor can be
negative, so divider_tb had to be updated for the new interface.
The reason for doing this is that one of the worst timing violations
on the Arty A7-100 at 100MHz involved the carry chain in the adders
that did the negation of the dividend and divisor in the decode stage.
Moving the negations to a separate cycle fixes that and also seems to
reduce the total number of slice LUTs used.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
These are intended to be combinatorial. The previous code was giving
warnings in vivado about registers/latches with no clock defined.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
This looks for cases where the next 8 bits of the quotient are obviously
going to be zero, because the top 72 bits of the 128-bit dividend
register are all zero. In those cases we shift 8 zero bits into the
quotient and increase count by 8. We only do this if count < 56.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
This adds a divider unit, connected to the core in much the same way
that the multiplier unit is connected. The division algorithm is
very simple-minded, taking 64 clock cycles for any division (even
32-bit division instructions).
The decoding is simplified by making use of regularities in the
instruction encoding for div* and mod* instructions. Instead of
having PPC_* encodings from the first-stage decoder for each of the
different div* and mod* instructions, we now just have PPC_DIV and
PPC_MOD, and the inputs to the divider that indicate what sort of
division operation to do are derived from instruction word bits.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Paul Mackerras