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 optimization and removes duplicated logic.

• New pass: CopyPropagation replaces uses of locals with known Constants and records constants through simple loads; ignores projections and effectful predecessors; no loop fixpoint
• Shared util: Extracts propagate_block_params(...) for constant agreement across predecessors; used by both CopyPropagation and InstSimplify
• API tweaks: Adds Operand::as_place() and Operand::as_constant() helpers; switches to LocalVec::lookup in simplification logic
• Wiring & tests: Exposes pass from transform/mod.rs and adds comprehensive snapshot tests under cp/

^{Written by Cursor Bugbot for commit 09dc1b6. 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-271: HashQL: Implement PostInline pass #8240
• BE-269: HashQL: Move PreInline out into Canonicalization pass #8239
• BE-268: HashQL: Rename PreInlining to PreInline for consistency #8238
• BE-270: HashQL: Optimize empty tuples to unit constants in MIR #8237
• BE-197: Implement MIR inline pass for HashQL #8236
• BE-267: Use bump scopes instead of full resets across transformation passes #8234
• BE-266: HashQL: Implement PreInlining pass for MIR optimization #8233
• BE-265: HashQL: Make CopyPropagation aware of local re-assignments #8231
• BE-264: HashQL: Rename SROA to ForwardSubstitution for better semantic clarity #8230
• BE-263: HashQL: add body! macro for declarative MIR construction in tests #8229
• BE-261: HashQL: Implement Administrative Reduction pass for MIR #8227
• BE-262: HashQL: Do not intern empty slices #8223
• BE-260: HashQL: Switch to bump allocator with scoping support #8218
• BE-259: HashQL: Add benchmarks for type system operations #8216
• 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: HashQL: 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 (09dc1b6) with bm/be-231-hashql-peephole-optimization (08246f0)¹}

Summary

✅ 17 untouched

Footnotes

1. No successful run was found on bm/be-231-hashql-peephole-optimization (7b8639a) during the generation of this report, so bab8f23 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[codecov]

Codecov Report

❌ Patch coverage is 92.32614% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.27%. Comparing base (7b8639a) to head (09dc1b6).

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                                   59.01%   59.27%   +0.25%     
==========================================================================
  Files                                        1187     1191       +4     
  Lines                                      112499   113436     +937     
  Branches                                     4939     4982      +43     
==========================================================================
+ Hits                                        66392    67234     +842     
- Misses                                      45349    45426      +77     
- Partials                                      758      776      +18     

Flag	Coverage Δ
rust.hashql-compiletest	46.65% <ø> (ø)
rust.hashql-mir	88.29% <92.32%> (+0.11%)	⬆️

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