CostBasedFusion aggregate_operations crash

[aromanro]

Informations

• Qiskit Aer version: Latest dev main branch sources
• Python version: Irrelevant, as I'm using it from a c++ project
• Operating system: Windows 11

What is the current behavior?

During some tests I do, CostBasedFusion::aggregate_operations crashes.

Steps to reproduce the problem

I'm executing some randomly generated circuits, I'm not yet sure how to reproduce it, but I think I could point out to a reason and a solution.

This is the problem code:

        double estimated_cost =
            estimate_cost(ops, (uint_t)j,
                          i) // fusion gate from j-th to i-th, and
            + (j == 0 ? 0.0 : costs[j - 1 - fusion_start]); // cost of (j-1)-th

With debug, I end up with j being 2, fusion_start also 2 and costs size being 2. This results in a costs[-1], resulting in a crash.

What is the expected behavior?

Not crashing, obviously :)

Suggested solutions

Replace the j == 0 check with j == fusion_start.

[aromanro]

More info:

I think this commit introduced the issue: a5e3aa2

[hhorii]

We believe that #2075 fixes this issue.