----------------------------------------------------------------------------- -- | -- Module : Documentation.SBV.Examples.Uninterpreted.Multiply -- Copyright : (c) Levent Erkok -- License : BSD3 -- Maintainer: erkokl@gmail.com -- Stability : experimental -- -- Demonstrates how to use uninterpreted function models to synthesize -- a simple two-bit multiplier. ----------------------------------------------------------------------------- {-# LANGUAGE ScopedTypeVariables #-} module Documentation.SBV.Examples.Uninterpreted.Multiply where import Data.SBV -- | The uninterpreted implementation of our 2x2 multiplier. We simply -- receive two 2-bit values, and return the high and the low bit of the -- resulting multiplication via two uninterpreted functions that we -- called @mul22_hi@ and @mul22_lo@. Note that there is absolutely -- no computation going on here, aside from simply passing the arguments -- to the uninterpreted functions and stitching it back together. -- -- NB. While definining @mul22_lo@ we used our domain knowledge that the -- low-bit of the multiplication only depends on the low bits of the inputs. -- However, this is merely a simplifying assumption; we could have passed -- all the arguments as well. mul22 :: (SBool, SBool) -> (SBool, SBool) -> (SBool, SBool) mul22 (a1, a0) (b1, b0) = (mul22_hi, mul22_lo) where mul22_hi = uninterpret "mul22_hi" a1 a0 b1 b0 mul22_lo = uninterpret "mul22_lo" a0 b0 -- | Synthesize a 2x2 multiplier. We use 8-bit inputs merely because that is -- the lowest bit-size SBV supports but that is more or less irrelevant. (Larger -- sizes would work too.) We simply assert this for all input values, extract -- the bottom two bits, and assert that our "uninterpreted" implementation in 'mul22' -- is precisely the same. We have: -- -- >>> sat synthMul22 -- Satisfiable. Model: -- mul22_hi :: Bool -> Bool -> Bool -> Bool -> Bool -- mul22_hi False True True False = True -- mul22_hi False True True True = True -- mul22_hi True False False True = True -- mul22_hi True False True True = True -- mul22_hi True True False True = True -- mul22_hi True True True False = True -- mul22_hi _ _ _ _ = False -- -- mul22_lo :: Bool -> Bool -> Bool -- mul22_lo True True = True -- mul22_lo _ _ = False -- -- It is easy to see that the low bit is simply the logical-and of the low bits. It takes a moment of -- staring, but you can see that the high bit is correct as well: The logical formula is @a1b xor a0b1@, -- and if you work out the truth-table presented, you'll see that it is exactly that. Of course, -- you can use SBV to prove this. First, define the model we were given to make it symbolic: -- -- >>> :{ -- mul22_hi :: SBool -> SBool -> SBool -> SBool -> SBool -- mul22_hi a1 a0 b1 b0 = ite ([a1, a0, b1, b0] .== [sFalse, sTrue , sTrue , sFalse]) sTrue -- \$ ite ([a1, a0, b1, b0] .== [sFalse, sTrue , sTrue , sTrue ]) sTrue -- \$ ite ([a1, a0, b1, b0] .== [sTrue , sFalse, sFalse, sTrue ]) sTrue -- \$ ite ([a1, a0, b1, b0] .== [sTrue , sFalse, sTrue , sTrue ]) sTrue -- \$ ite ([a1, a0, b1, b0] .== [sTrue , sTrue , sFalse, sTrue ]) sTrue -- \$ ite ([a1, a0, b1, b0] .== [sTrue , sTrue , sTrue , sFalse]) sTrue -- sFalse -- :} -- -- Now we can say: -- -- >>> prove \$ \a1 a0 b1 b0 -> mul22_hi a1 a0 b1 b0 .== (a1 .&& b0) .<+> (a0 .&& b1) -- Q.E.D. -- -- and rest assured that we have a correctly synthesized circuit! synthMul22 :: Goal synthMul22 = do a :: SWord8 <- forall "a" b :: SWord8 <- forall "b" let lsb2 x = let [x1, x0] = reverse \$ take 2 \$ blastLE x in (x1, x0) constrain \$ mul22 (lsb2 a) (lsb2 b) .== lsb2 (a * b)