ashrMInt caveat

[Sandeep Dasgupta]

This is regarding the implementation of ashrMInt

What is the Problem

ashrMInt(mi(4, -8), 1) gives -4 //i.e ashrMInt(1000, 1) gives 1100, which is correct
ashrMInt(mi(4, 8), 1) gives 4 //i.e ashrMInt(1000, 1) gives 0100, which is not Correct

Why this is a problem

• As both Mints mi(4, 8) and mi(4, -8) represent the same bit vector (1000), arithmatic right shifts on them should not be affected by how the Mints are created.

Root Cause

MInts ( or BitVector) are stored as a value: BigInteger and a bitwidth: Int. Now right shift is performed on the BigInteger which expects the BigInteger to be a signed value and hence the problem.

Fix suggestion

• Either Implement the hook ashrMInt as

syntax  MInt ::= ashrMInt(MInt, Int) [function]
rule ashrMInt(MI, I) => ashrMIntHelper(mi(bitwidthMInt(MI), svalueMInt(MI)), I)
syntax  MInt ::= ashrMIntHelper(MInt, Int) [function] [function, hook(MINT.ashr)] 
 

I can send the patch, if you need.

• Or adding a comment in domains.k about the usage model of ashrMInt