Is there a way to construct a logarithm constraint? about pyvsc

[mballance]

Hi @kangshuo1,
clog2 as a constraint on a random variable/expression is typically implemented by the solver. We can implement a simple (likely slower) version using PyVSC's ability to generate constraints procedurally.
The thing to remember is that clog2 is answering the question "how many bits is required to represent N value combinations?".

I added a small test to implement the clog2 constraint here:

pyvsc/ve/unit/test_constraint_functions.py

Lines 6 to 34 in 6b8b9e8

	class TestConstraintFunctions(VscTestCase):
	def test_clog2(self):
	import vsc
	def clog2(lhs, rhs, nbits):
	with vsc.implies((rhs >= 0) & (rhs < 2)):
	lhs == 0
	for i in range(1,nbits):
	with vsc.implies(((rhs-1) >= (2**(i-1))) & ((rhs-1) < (2**i))):
	lhs == i
	@vsc.randobj
	class C(object):
	def __init__(self):
	self.a = vsc.rand_bit_t(8)
	self.b = vsc.rand_bit_t(8)
	@vsc.constraint
	def clog2_c(self):
	clog2(self.a, self.b, 8)
	c = C()
	for i in range(256):
	with c.randomize_with():
	c.b == i
	print("a=%d b=%d" % (c.a, c.b))

If you're finding that you need the clog2 constraint, please add an enhancement request for PyVSC to implement this.

-Matthew

[kangshuo1]

Thanks for your reply, it's very helpful to me