fix(ssa): Tracking slices stores correctly for parent blocks and handling in ACIR

[vezenovm]

Description

Problem*

Resolves #4202

Summary*

There are two fixes done in this PR, one in the SSA and one in the ACIR:

SSA Update

When storing slice stores from the outer block we were only checking JmpIf terminators. The issue has provided an example SSA where we jump to a block which performs a jump if without the block with the jump if storing any values.

Here is the SSA for the basic failing program in the linked issue:

After Mem2Reg:
acir fn main f0 {
  b0():
    v54 = allocate
    store Field 4 at v54
    inc_rc [u32 1, u32 2, u32 3, u32 4]
    v56 = allocate
    store [u32 1, u32 2, u32 3, u32 4] at v56
    jmp b2()
  b2():
    jmpif u1 0 then: b3, else: b4
  b3():
    store Field 4 at v54
    store [u32 1, u32 2, u32 3, u32 6] at v56
    jmp b4()
  b4():
    return 
}

We now just check blocks with jump terminators for stores as well.

ACIR Update

After the flattening pass, the following mem2reg pass may resolve a constant array into the array_get that gets inserted inside of merge_slice_values. This constant array can potentially be smaller than the constant index that gets specified during flattening if we are merging slices of two different sizes.

Inside ACIR when handling constant indices we now allow returning dummy data when we have a constant index that is greater than the array size. This should be safe to do as any slice accesses specified by the user will always codegen a check against the dynamic slice length. The only time we will have a raw array get for a slice like this is due to instructions inserted by the compiler such as with flattening.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [Exceptional Case] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[vezenovm]

In this PR for merging nested slices #3979 the slice capacity tracker that was written for the fill internal slices pass was used by the flattening pass to track slice capacities. This let us remove the back tracking done by the get_slice_length method and the outer_block_stores field in flattening. I was hoping the simple fix I just pushed would be a quick bandaid but it looks to be causing other issues with more complex examples.

So, I am converting this PR back to a draft. I wanted to bring back the capacity tracker anyway to remove outer_block_stores, but now I will try it as a part of this PR.

EDIT: Going to do the capacity_tracker in a followup to this PR. It turns out this bug required two fixes. One for accurately tracking slices in parent blocks which could be done with the simple bandaid in this PR, and then ACIR updates outlined in the main PR description.

[vezenovm]

Have one more idea I want to try out to avoid the ACIR changes so converting back to a draft temporarily.

[vezenovm]

Have one more idea I want to try out to avoid the ACIR changes so converting back to a draft temporarily.

I was able to resolve this by fixing up #4240 and making a truly accurate slice capacity tracker. Closing this PR.