Signed error with lshift

Support
1

Hi All,

I want to shift a signal left if the msbit is false in a clocked process… (Verilog has no issue with this)

shift.next = mult << ~mult[WIDTH2M-1]

i.e. of mult[WIDTH2M-1] is false, shift left otherwise do not.

This gives an error
File “D:\Lumetrica_Projects\myHDL_IP\math\floating_point\fp_multiply.py”, line 41, in comb2_logic
shift.next = mult << int(~mult[WIDTH2M-1])
File “d:\lumetrica_projects\adc_interface\src\myhdl\myhdl_Signal.py”, line 434, in lshift
return self._val << other
File “d:\lumetrica_projects\adc_interface\src\myhdl\myhdl_intbv.py”, line 257, in lshift
return intbv(int(self._val) << other)
ValueError: negative shift count

My Signals are modbv() and not intbv - if that matters

If there is a way to cast this to avoid the error I haven’t found it yet.

Turning it into an ‘if … else’ is really quite messy.- but I guess I can do this.

Do I register this as a bug or is it correct behaviour?

Regards,

Steve.

2

Hi Steve,

This is in simulation, I assume.
~a[x] resolves to either -1 or -2 which is a negative shift and raises a ValueError in Python.
Correct behavior.
c.next = b << (not b[3]) will simulate correctly and convert to:
assign c = (b << (!b[3]));
and
c <= shift_left(b, to_integer((not bool(b(3)))));
The VHDL is correct. I assume that Verilog will have no issue either.

Regards,
Josy

3

Hi Josy,

This was in verilog generation and not simulation.

You gave me the clue though the shift distance has to be cast as an int so …
mult.next = mult << int(~mult[WIDTH2M-1]) gave the correct result

assign shift = (mult << (~mult[(16 - 1)]));

Thank you (again)

4

Hi Josy, …

It was in simulation also (now that I checked again) and so now the mult << int(not mult[… ) is the correct answer.

Steve.