feat: simplify jmpifs by reversing branches if condition is negated

[TomAFrench]

Description

Problem*

Resolves

Summary*

Currently if an if-else block has a negated condition we generate an extra instruction to calculate the condition rather than just swapping the "if" and "else" branches. I've added a check for this in the simplify_cfg pass.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] 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.

[github-actions]

Changes to Brillig bytecode sizes

Generated at commit: 23deeb387938a697768c8ee112f0029e424a0f50, compared to commit: 2303615815a2a60de8ac3dd53349f85201660917

🧾 Summary (10% most significant diffs)

Program	Brillig opcodes (+/-)	%
acir_inside_brillig_recursion	-1 ✅	-0.94%
brillig_recursion	-1 ✅	-1.09%
fold_fibonacci	-1 ✅	-1.09%
brillig_slices	-8 ✅	-1.74%
references	-13 ✅	-4.87%

---

Full diff report 👇

Program	Brillig opcodes (+/-)	%
regression_4709	133,780 (-2)	-0.00%
regression_5252	5,145 (-1)	-0.02%
debug_logs	4,991 (-1)	-0.02%
poseidonsponge_x5_254	4,318 (-1)	-0.02%
u128	3,106 (-1)	-0.03%
nested_array_dynamic	2,460 (-1)	-0.04%
bigint	2,039 (-1)	-0.05%
nested_array_in_slice	1,237 (-1)	-0.08%
uhashmap	23,493 (-22)	-0.09%
brillig_cow_regression	1,965 (-2)	-0.10%
sha2_byte	3,145 (-4)	-0.13%
slice_dynamic_index	2,571 (-4)	-0.16%
sha256_regression	6,881 (-12)	-0.17%
eddsa	11,157 (-22)	-0.20%
array_dynamic_nested_blackbox_input	959 (-2)	-0.21%
ecdsa_secp256k1	953 (-2)	-0.21%
sha256_var_size_regression	1,898 (-4)	-0.21%
aes128_encrypt	458 (-1)	-0.22%
6	1,746 (-4)	-0.23%
conditional_1	1,248 (-3)	-0.24%
regression_4449	799 (-2)	-0.25%
array_dynamic_blackbox_input	1,145 (-3)	-0.26%
hashmap	26,303 (-69)	-0.26%
brillig_sha256	740 (-2)	-0.27%
array_sort	339 (-1)	-0.29%
conditional_regression_short_circuit	1,317 (-4)	-0.30%
sha256_var_witness_const_regression	1,270 (-4)	-0.31%
sha256	1,855 (-6)	-0.32%
slice_regex	2,422 (-17)	-0.70%
slices	2,007 (-19)	-0.94%
acir_inside_brillig_recursion	105 (-1)	-0.94%
brillig_recursion	91 (-1)	-1.09%
fold_fibonacci	91 (-1)	-1.09%
brillig_slices	453 (-8)	-1.74%
references	254 (-13)	-4.87%

[TomAFrench]

I'm really unsure on why exactly this is causing test failures. The ordering of the branches in a jmpif shouldn't cause any changes to behaviour afaik.

[TomAFrench]

This branch changes the order of the jmpif as expected in the simplify_cfg pass however the issue comes up in flattening as we end up merging the values incorrectly.

Master

acir(inline) fn main f0 {
  b0(v0: Field):
    v1 = allocate
    v2 = allocate
    store Field 0 at v2
    v5 = eq v0, Field 10
    v6 = not v5
    enable_side_effects v6
    v7 = load v2
    store Field 2 at v2
    store v7 at v2
    enable_side_effects u1 1
    v10 = cast v6 as Field
    v11 = cast v5 as Field
    v12 = mul v10, Field 2 // NOT(v5) * 2
    v13 = mul v11, v7 // v5 * 0
    v14 = add v12, v13
    store v14 at v2
    v15 = load v2
    v16 = eq v15, Field 2
    constrain v15 == Field 2
    return 
}

This branch

acir(inline) fn main f0 {
  b0(v0: Field):
    v1 = allocate
    v2 = allocate
    store Field 0 at v2
    v5 = eq v0, Field 10
    v6 = not v5
    enable_side_effects v5
    v7 = load v2
    store Field 2 at v2
    v9 = not v5 
    store v7 at v2
    enable_side_effects u1 1
    v11 = cast v5 as Field
    v12 = cast v9 as Field
    v13 = mul v11, Field 2 // v5 * 2
    v14 = mul v12, v7 // NOT(v5) * 0
    v15 = add v13, v14
    store v15 at v2
    v16 = load v2
    v17 = eq v16, Field 2
    constrain v16 == Field 2
    return 
}

[vezenovm]

This branch changes the order of the jmpif as expected in the simplify_cfg pass however the issue comes up in flattening as we end up merging the values incorrectly.

If we do not see an simple or obvious way to have this optimization work w/ flattening we could make this optimization just work on Brillig for now.

[jfecher]

I'd like if we investigated why flattening isn't adapting to the new changes. Could be related to the bug I've been looking at.

[TomAFrench]

Agreed, we should at least understand the root cause of this first.