BE-255: HashQL: Implement Copy Propagation (CP) pass for MIR

[indietyp]

🌟 What is the purpose of this PR?

This PR adds a new Copy Propagation optimization pass to the MIR compiler. The pass identifies when local variables hold constant values and replaces uses of those locals with the constants directly, this is used after InstSimplify, to enable efficient fix point iteration.

🔍 What does this change?

• Adds a new CopyPropagation transform pass that propagates constant values through the MIR
• Implements helper methods as_place() and as_constant() on Operand for easier pattern matching
• Extracts the block parameter propagation logic into a reusable function that can be shared between passes
• Includes comprehensive test cases covering various scenarios like constant chains, block parameters, and loops

Pre-Merge Checklist 🚀

🚢 Has this modified a publishable library?

This PR:

• does not modify any publishable blocks or libraries, or modifications do not need publishing

📜 Does this require a change to the docs?

The changes in this PR:

• are internal and do not require a docs change

🕸️ Does this require a change to the Turbo Graph?

The changes in this PR:

• do not affect the execution graph

🛡 What tests cover this?

• Comprehensive test suite added in cp/tests.rs covering various scenarios:
  • Basic constant propagation
  • Constant chains through multiple loads
  • Block parameter propagation with unanimous and disagreeing predecessors
  • Handling of effectful predecessors
  • Projections and loop back-edges

[cursor]

PR Summary

Adds a new MIR optimization and centralizes block-parameter constant propagation logic.

• Introduces CopyPropagation pass that replaces simple local loads/uses with Constant when provably constant; integrates visitor to rewrite operands and track constants across assignments
• Extracts propagate_block_params into cp/mod.rs to compute per-parameter constant consensus from predecessors; reused by InstSimplify
• Enhances Operand with as_place() and as_constant() helpers for cleaner matching
• Updates InstSimplify to use shared propagation and evaluated.lookup(...); minor internal refactors
• Exposes pass via transform/mod.rs and adds snapshot tests covering constant chains, block params (agree/disagree), effectful preds, projections, and loop back-edges

^{Written by Cursor Bugbot for commit 049e36c. This will update automatically on new commits. Configure here.}

[indietyp]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• BE-227: HashQL: Implement call graph analysis for MIR #8214
• BE-258: HashQL: Add Changed enum to track MIR transformation pass modifications #8210
• BE-255: HashQL: Implement Copy Propagation (CP) pass for MIR #8207 👈 (View in Graphite)
• BE-231: Add InstSimplify pass for MIR instruction simplification and constant folding #8206
• BE-254: Add CodSpeed benchmarking for HashQL MIR transformation passes #8204
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[codspeed-hq]

CodSpeed Performance Report

Merging #8207 will not alter performance

_{Comparing bm/be-255-hashql-add-copy-propagation-pass (049e36c) with bm/be-231-hashql-peephole-optimization (08246f0)}

Summary

✅ 17 untouched

[codecov]

Codecov Report

❌ Patch coverage is 92.32614% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.14%. Comparing base (08246f0) to head (049e36c).

Files with missing lines	Patch %	Lines
...ibs/@local/hashql/mir/src/pass/transform/cp/mod.rs	82.60%	17 Missing and 3 partials ⚠️
libs/@local/hashql/mir/src/body/operand.rs	0.00%	10 Missing ⚠️
...hashql/mir/src/pass/transform/inst_simplify/mod.rs	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@                            Coverage Diff                             @@
##           bm/be-231-hashql-peephole-optimization    #8207      +/-   ##
==========================================================================
+ Coverage                                   59.03%   59.14%   +0.11%     
==========================================================================
  Files                                        1195     1197       +2     
  Lines                                      113298   113673     +375     
  Branches                                     5046     5056      +10     
==========================================================================
+ Hits                                        66880    67234     +354     
- Misses                                      45644    45663      +19     
- Partials                                      774      776       +2     

Flag	Coverage Δ
rust.hashql-compiletest	46.65% <ø> (ø)
rust.hashql-mir	88.29% <92.32%> (+0.20%)	⬆️

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