LLVM Pass Collection

Custom LLVM optimization pass plugins built on LLVM's analysis infrastructure — DominatorTree, AliasAnalysis, ScalarEvolution, and LoopInfo.

Tech focus:
LLVM 18 · C++17 · CMake · New Pass Manager · Pass Plugin API

Pass focus:
Loop-Invariant Code Motion · [Pass 2] · [Pass 3] · [Pass 4]

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LICM — Loop-Invariant Code Motion

Moves loop-invariant computation out of loops — repeats while any transformation fires; hoisting can expose new invariants each pass.

Feature	Analyses
Preheader insertion — ensure loop preheader exists before hoisting	DT · LI
Core hoisting — hoist loop-invariant instructions with safety checks	DT · LI
Load hoisting — hoist loop-invariant loads using alias analysis	AA · LI
Call hoisting — hoist readonly/readnone calls with invariant arguments	AA · LI
Memory promotion — promote loop memory accesses to SSA registers via PHI-based load-store elimination	AA · DT · LI
Arithmetic + GEP reassociation — hoist hidden invariants via arithmetic and GEP reassociation	LI
Reciprocal multiplication — hoist reciprocal of loop-invariant divisor to preheader	LI
Instruction sinking — sink instructions used only in loop exit blocks	DT · LI
SE-unlocked hoist/sink — SE-unlocked hoisting and sinking of non-memory instructions unsafe to speculate *	SE · DT
LCSSA maintenance — maintain LCSSA form after all loop transformations	DT · LI · SE
Statistics + debug — pass statistics and debug logging	—

* LLVM covers do-while via dominator-based execution guarantees; the zero-trip path blocks that for for/while. SE proving TC > 0 guarantees the first iteration; every path from header to latch through BB guarantees no iteration skips it — covers the case LLVM conservatively skips.

See licm/README.md for full breakdown with before/after IR and CFG.

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[Pass Name] — [one-line description]

[Brief description of what the pass optimizes and how.]

Tech focus: [analyses / IR constructs used]

See [pass-dir/README.md] for full breakdown.

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[Pass Name] — [one-line description]

[Brief description of what the pass optimizes and how.]

Tech focus: [analyses / IR constructs used]

See [pass-dir/README.md] for full breakdown.

---

[Pass Name] — [one-line description]

[Brief description of what the pass optimizes and how.]

Tech focus: [analyses / IR constructs used]

See [pass-dir/README.md] for full breakdown.

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Build

Requirements: LLVM 18+, CMake 3.16+, C++17 compiler, Graphviz (for visualize.sh)

git clone https://github.com/andja45/llvm-passes.git
cd llvm-passes
cmake -B cmake-build-debug
cmake --build cmake-build-debug --target LICM

Run all examples — generates before/after IR and CFG PNGs for every feature:

./visualize.sh

Run a single example manually:

# compile to IR  (replace <example> with e.g. hoist_se, hoist_load, ...)
clang -S -emit-llvm -O0 -Xclang -disable-O0-optnone -fno-discard-value-names \
    examples/licm/<example>/input.c -o examples/licm/<example>/original.ll

# run pass
opt --load-pass-plugin=./cmake-build-debug/licm/LICM.so \
    --passes="mem2reg,loop(licm-pass)" \
    examples/licm/<example>/original.ll -S -o examples/licm/<example>/optimized.ll

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Authors

Pass	Author
LICM	Andjela Spasic
[Pass 2]	[Name]
[Pass 3]	[Name]
[Pass 4]	[Name]