This uses an algorithm for count leading/trailing zeroes that is
faster on FPGAs, which makes timing easier. cntlz* and cnttz*
still take two cycles, though.
For count trailing zeroes, we compute x & -x, which for non-zero x
has a single 1 bit in the position of the least-significant 1 bit
in x. This one-hot representation can then be converted to a bit
number with six 32-input OR gates. For count leading zeroes, we
simply do a bit-reversal on x and then use the same algorithm.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
This adds a register in the middle of the countzero computation,
so that we now have two cycles to count leading or trailing zeroes
instead of just one. Execute1 now outputs a one-cycle stall signal
when it encounters a cntlz* or cnttz* instruction. With this,
the countzero path no longer fails timing on the Artix-7 at 100MHz.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
By using 4:1 multiplexers rather than 2:1, this cuts the number of
levels of multiplexing from 4 to 2 and also reduces the total number
of slice LUTs required. Because we are now handling 4 bits at each
level, including the bottom level, the logic to do the priority
encoding can be factored out into a function that is used at each
level.
This rearranges the logic so that the encoding and selection of bits
is done whether or not the input operand is zero, and the if statement
testing whether the input is zero only affects what is assigned to
result. With this we don't get the inferred latches and we can go
back to using signals rather than variables.
Also add some comments about what is being done.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
The current code simulates correctly, but produces miscompares when synthesized
onto an FPGA. On closer inspection GHDL synthesis complains about inferred
latches and there does seem to be issues.
Convert it to variables that are always initialized to zero at the start of the
process.
Fixes: 24a4a796ce ("execute: Consolidate count-leading/trailing-zeroes implementations")
Signed-off-by: Anton Blanchard <anton@linux.ibm.com>
This adds combinatorial logic that does 32-bit and 64-bit count
leading and trailing zeroes in one unit, and consolidates the
four instructions under a single OP_CNTZ opcode.
This saves 84 slice LUTs on the Arty A7-100.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Paul Mackerras
Anton Blanchard