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

Introduces a new MIR transform to replace uses of locals known to hold constants and to propagate constants through block parameters.

• New CopyPropagation pass (pass/transform/cp/) that records constants and rewrites Operand::Place to Operand::Constant when safe
• Adds Operand::as_place() and Operand::as_constant() for ergonomic matching
• Extracts propagate_block_params utility to compute consensus constants from predecessor targets; reused in inst_simplify
• Updates InstSimplify to call propagate_block_params and use LocalVec::lookup for evaluated locals
• Exposes the pass via pass/transform/mod.rs
• Adds targeted tests and snapshots in cp/tests.rs (constants, chains, block params, effectful predecessors, projections, loop back-edges)

^{Written by Cursor Bugbot for commit 82e5742. This will update automatically on new commits. Configure here.}

[indietyp]

• BE-273: HashQL: Profile and benchmark MIR interpreter #8251
• BE-272: HashQL: Add numeric addition and subtraction operators to MIR #8250
• BE-257: HashQL: Implement MIR interpreter with runtime value representation #8245
• 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

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[codspeed-hq]

Merging this PR will not alter performance

✅ 14 untouched benchmarks
🗄️ 3 archived benchmarks run¹

---

_{Comparing bm/be-255-hashql-add-copy-propagation-pass (82e5742) with main (372e9a1)}

Footnotes

1. 3 benchmarks were run, but are now archived. If they were deleted in another branch, consider rebasing to remove them from the report. Instead if they were added back, click here to restore them. ↩

[codecov]

Codecov Report

❌ Patch coverage is 86.19600% with 131 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.26%. Comparing base (1892655) to head (9b1c7bc).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...hashql/mir/src/pass/transform/inst_simplify/mod.rs	60.90%	80 Missing and 15 partials ⚠️
...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 ⚠️
libs/@local/hashql/core/src/id/vec.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8207      +/-   ##
==========================================
+ Coverage   59.01%   59.26%   +0.24%     
==========================================
  Files        1187     1191       +4     
  Lines      112497   113442     +945     
  Branches     4939     4982      +43     
==========================================
+ Hits        66394    67232     +838     
- Misses      45345    45434      +89     
- Partials      758      776      +18     

Flag	Coverage Δ
rust.hashql-compiletest	46.65% <ø> (ø)
rust.hashql-mir	88.29% <86.74%> (+0.10%)	⬆️

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Merge activity

• Jan 15, 2:06 PM UTC: Graphite rebased this pull request, because this pull request is set to merge when ready.
• Jan 16, 12:45 PM UTC: A user started a stack merge that includes this pull request via Graphite.