Inconsistent output on calling function with same args

[shuklaayush]

Aim

Getting this weird error where the same function results in a different output based on whether it's called directly or inside a complex call stack

Expected Behavior

Output should be the same

Bug

The shl1 functoin in noir-bigint shifts a BigInt left by 1 bit.

Running it on a constant input gives the right result

nargo test shl6 --show-output
[biguint] Running 1 test functions
[biguint] Testing test_shl6...
BigUint56 { limbs: [0x80000000000000, 0x00, 0x00, 0x00, 0x00] }
BigUint56 { limbs: [0x00, 0x01, 0x00, 0x00, 0x00] }
ok

While when it is called from within div, it outputs 0 which is wrong

nargo test div3 --show-output
[biguint] Running 1 test functions
[biguint] Testing test_div3...
...
--------------------------------------------------------------------------------
BigUint56 { limbs: [0x80000000000000, 0x00, 0x00, 0x00, 0x00] }
BigUint56 { limbs: [0x00, 0x00, 0x00, 0x00, 0x00] }

To Reproduce

1. Clone repository git clone https://github.com/shuklaayush/noir-bigint.git
2. Switch to branch git checkout bug/inconsistent-shl1
3. Run nargo test shl6 --show-output
4. Run nargo test div3 --show-output and see last log

Installation Method

Binary

Nargo Version

nargo version = 0.19.4 noirc version = 0.19.4+1ef854686a416eb547cc0919d669627f43d01450 (git version hash: 1ef8546, is dirty: false)

Additional Context

No response

Would you like to submit a PR for this Issue?

Maybe

Support Needs

No response

[guipublic]

After a lot of efforts and sweating, I managed to strip this issue down to a minimal example (30 opcodes):

use dep::biguint::BigUint56;

fn main( ) { 
    let a = BigUint56 { limbs: [0x02, 0x00, 0x00, 0x00, 0x00] };
    let b = BigUint56 { limbs: [0x10000000000000, 0x00, 0x00, 0x00, 0x00] };
    let c = BigUint56 { limbs: [0x00, 0x08, 0x00, 0x00, 0x00] };
    let (q, r) = indiv(a, b,c); 
    assert(q.eq(BigUint56 { limbs: [0, 1, 0, 0, 0] }));
}

fn indiv(mut rem: BigUint56, mut quo: BigUint56, mut c: BigUint56) -> (BigUint56, BigUint56) {
    for _i in 0..4 {
        if rem.limbs[0]==c.limbs[0] {
            assert(false);
            quo = quo.shl1();
        } else {
            quo = quo.shl1();
        }
        c = c.shr1();                 
    }
    (quo, rem)
}

Executing this program fails, but if you comment the first quo = quo.shl1(); (just after the assert(false)), then it works.
One can see that it is the same as the original issue by adding this at the end of shl_limb (in biguint):

if( (self.limbs[0]!= 0)  & (res.limbs[0] ==0)) {
    assert(self.limbs[0] == 0x80000000000000);
    println(self.limbs[0]>> 55);
}

When it fails, it prints 0x0 (which is not correct), and when it succeed (by commenting the first quo = quo.shl1();), then it prints 0x1 (which is correct).