Fuse matching product scf.if regions

[Kuhai9801]

Summary

Fixes #300.

Adds conservative scf.if fusion to -llzk-fuse-product-loops for matching compute/constrain product regions.

The fusion handles narrow cases where matching compute and constrain scf.if regions have the same SSA condition and can be fused without changing member read/write ordering. It rejects constrain-side member reads and calls after mapped writes, so member reads remain tied to their original storage semantics until a more precise remapping analysis is added.

Testing

• git diff --check
• nix --extra-experimental-features "nix-command flakes" build -L .#debugClang
• Focused llzk-opt | FileCheck checks for:
  • test/Transforms/FuseProductLoops/fuse_if.llzk
  • test/Transforms/FuseProductLoops/no_fuse_if_condition_mismatch.llzk
  • test/Transforms/FuseProductLoops/no_fuse_if_internal_read.llzk

[iangneal]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: b4213c67d7

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Reject constrain calls that can read delayed writes

When the constrain branch contains a function.call (for example a nested @constrain(%self, ...)) and a member written between computeIf and constrainIf is read inside that callee, this check still allows fusion because it only rejects direct MemberReadOps in the region. The fused branch clones the call before the intervening struct.writem operations that are left after the new scf.if, so the callee can observe stale/unwritten member state compared with the original ordering.

Useful? React with 👍 / 👎.

[raghav198]

The constrain use is remapped to the branch-local compute value instead of hoisting the read.

I may be misunderstanding, but I think this is not safe when the struct.readm is a constrain op:

%0 = felt.add %a, %b {source = "compute"}
struct.writem %self[@foo] = %0 {source = "compute"}
%foo = struct.readm %self[@foo] {source = "constrain"}
constrain.eq %foo, %c {source = "constrain"}

emits a constraint guaranteeing that the @foo signal equals the result of %c, whereas in the remapped

%0 = felt.add %a, %b {source = "compute"}
struct.writem %self[@foo] = %0 {source = "compute"}
constrain.eq %0, %c {source = "constrain"}

the emitted constraint doesn't refer to a struct signal at all.

[Kuhai9801]

Thanks for catching this, Raghav. You're right. Remapping the struct.readm result to the branch local compute value can remove the member signal from the emitted constraint. I'll make this fusion case conservative for now and add a regression test so we keep the constraint tied to the member read.

[Copilot]

Pull request overview

This PR extends the -llzk-fuse-product-loops pass to also fuse matching compute/constrain scf.if regions in struct product programs (Fixes #300), and adds regression tests covering fusable and non-fusable scf.if patterns.

Changes:

• Add scf.if fusion logic that clones compute + constrain branches into a single fused scf.if when conditions and structural constraints match.
• Refactor pass entry/recursion to fuse both scf.if and scf.for control flow via a shared fuseMatchingRegionControlFlow routine.
• Add lit/FileCheck tests for scf.if fusion and “do not fuse” guardrails; add an unreleased changelog entry.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lib/Transforms/LLZKFuseProductLoopsPass.cpp	Implements scf.if fusion and refactors recursion over region control flow.
test/Transforms/FuseProductLoops/fuse_if_no_member_read.llzk	Positive test ensuring scf.if fusion occurs in a safe case.
test/Transforms/FuseProductLoops/fuse_loop_then_if.llzk	Ensures loop fusion still works and enables nested scf.if fusion after loop fusion.
test/Transforms/FuseProductLoops/no_fuse_if_call_after_write.llzk	Negative test: blocks fusion when a call occurs after a mapped write.
test/Transforms/FuseProductLoops/no_fuse_if_condition_mismatch.llzk	Negative test: blocks fusion when conditions differ.
test/Transforms/FuseProductLoops/no_fuse_if_internal_read.llzk	Negative test: blocks fusion when constrain side reads members inside the scf.if.
test/Transforms/FuseProductLoops/no_fuse_if_read_after_write.llzk	Negative test: blocks fusion when a member read appears between the ifs.
changelogs/unreleased/fuse-product-if-regions.yaml	Changelog entry for the new scf.if fusion capability.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.