llvm-passes

Name: llvm-passes
Author: mohitmishra786

141stars19forks

How to install llvm-passes?

mohitmishra786/low-level-dev-skills/llvm-passes

$npx -y skills add mohitmishra786/low-level-dev-skills --skill llvm-passes

Installs into the current project.

Summary

LLVM passes skill for writing compiler optimizations. Use when writing FunctionPass or ModulePass, registering PassPlugins, running with opt, using analysis utilities, or testing with llvm-lit. Activates on queries about LLVM pass, PassPlugin, opt -passes, DominatorTree, llvm-lit

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SKILL.md

1	# LLVM Passes
2
3	## Purpose
4
5	Guide agents through writing LLVM optimization passes with the New Pass Manager: `FunctionPass` and `ModulePass` structure, `PassPluginLibraryInfo` registration, running via `opt -load-pass-plugin`, common analysis utilities (`DominatorTree`, `LoopInfo`, `AliasAnalysis`), IR modification patterns, `llvm-lit` testing, and debugging with `opt -print-after-all`.
6
7	## When to Use
8
9	- Adding a custom optimization to an LLVM-based compiler
10	- Writing an IR transformation pass (inlining, DCE, custom lowering)
11	- Analyzing control flow with dominator trees or loop info
12	- Testing passes with FileCheck and llvm-lit
13	- Debugging pass ordering and IR corruption
14	- Integrating passes into Clang via plugin
15
16	## Workflow
17
18	### 1. New Pass Manager architecture
19
20	```
21	opt / clang
22	├── ModulePassManager
23	│ └── FunctionPassManager (per function)
24	│ └── FunctionPass instances
25	└── AnalysisManager (cached analyses)
26	```
27
28	Passes declare analysis usage; analyses are invalidated on IR mutation.
29
30	### 2. Minimal FunctionPass (C++ plugin)
31
32	```cpp
33	// MyPass.cpp
34	#include "llvm/IR/PassManager.h"
35	#include "llvm/Passes/PassBuilder.h"
36	#include "llvm/Passes/PassPlugin.h"
37	#include "llvm/Support/raw_ostream.h"
38
39	using namespace llvm;
40
41	namespace {
42
43	struct MyPass : PassInfoMixin<MyPass> {
44	PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM) {
45	bool changed = false;
46	for (BasicBlock &BB : F) {
47	for (Instruction &I : BB) {
48	if (auto *Call = dyn_cast<CallInst>(&I)) {
49	if (Call->getCalledFunction() &&
50	Call->getCalledFunction()->getName() == "dead_func") {
51	Call->eraseFromParent();
52	changed = true;
53	}
54	}
55	}
56	}
57	return changed ? PreservedAnalyses::none() : PreservedAnalyses::all();
58	}
59	};
60
61	} // namespace
62
63	extern "C" LLVM_ATTRIBUTE_WEAK PassPluginLibraryInfo llvmGetPassPluginInfo() {
64	return {
65	LLVM_PLUGIN_API_VERSION, "MyPass", "v0.1",
66	[](PassBuilder &PB) {
67	PB.registerPipelineParsingCallback(
68	[](StringRef Name, FunctionPassManager &FPM,
69	ArrayRef<PassBuilder::PipelineElement>) {
70	if (Name == "my-pass") {
71	FPM.addPass(MyPass());
72	return true;
73	}
74	return false;
75	});
76	}
77	};
78	}
79	```
80
81	```cmake
82	# CMakeLists.txt
83	find_package(LLVM REQUIRED CONFIG)
84	add_library(MyPass MODULE MyPass.cpp)
85	target_include_directories(MyPass SYSTEM PRIVATE ${LLVM_INCLUDE_DIRS})
86	target_compile_definitions(MyPass PRIVATE ${LLVM_DEFINITIONS})
87	llvm_map_components_to_libnames(llvm_libs core passes support)
88	target_link_libraries(MyPass PRIVATE ${llvm_libs})
89	```
90
91	```bash
92	cmake -B build -DLLVM_DIR=$(llvm-config --cmakedir)
93	cmake --build build
94	```
95
96	### 3. Running with opt
97
98	```bash
99	# Run pass on IR file
100	opt -load-pass-plugin ./build/MyPass.so -passes=my-pass -S input.ll -o output.ll
101
102	# Print IR after each pass
103	opt -load-pass-plugin ./build/MyPass.so -passes=my-pass -print-after-all input.ll -o /dev/null
104
105	# Pass pipeline string
106	opt -passes="function(instcombine),my-pass,function(dce)" input.ll -S -o out.ll
107	```
108
109	### 4. ModulePass example
110
111	```cpp
112	struct MyModulePass : PassInfoMixin<MyModulePass> {
113	PreservedAnalyses run(Module &M, ModuleAnalysisManager &AM) {
114	for (Function &F : M) {
115	if (F.isDeclaration()) continue;
116	// module-level transformation
117	}
118	return PreservedAnalyses::none();
119	}
120	};
121	```
122
123	Register on `ModulePassManager` in plugin callback.
124
125	### 5. Analysis utilities
126
127	```cpp
128	#include "llvm/Analysis/DominatorTree.h"
129	#include "llvm/Analysis/LoopInfo.h"
130	#include "llvm/Analysis/AliasAnalysis.h"
131
132	PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM) {
133	auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
134	auto &LI = AM.getResult<LoopAnalysis>(F);
135	auto &AA = AM.getResult<AAManager>(F);
136
137	for (Loop *L : LI) {
138	BasicBlock *Header = L->getHeader();
139	// Loop invariant code motion, etc.
140	}
141
142	DominatorTreeNode *IDom = DT.getNode(&F.getEntryBlock());
143	(void)IDom;
144	return PreservedAnalyses::all();
145	}
146	```
147
148	Declare analyses used:
149
150	```cpp
151	AnalysisUsage MyLegacyPass::getAnalysisUsage(AnalysisUsage &AU) const {
152	AU.addRequired<DominatorTreeWrapperPass>();
153	AU.addRequired<LoopInfoWrapperPass>();
154	return AU;
155	}
156	```
157
158	(New PM: analyses requested via `AM.getResult<>` — dependency auto-tracked.)
159
160	### 6. IR modification patterns
161
162	```cpp
163	// Insert instruction before iter

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llvm-passes

141stars19forks

View on GitHub

How to install llvm-passes?

mohitmishra786/low-level-dev-skills/llvm-passes

$npx -y skills add mohitmishra786/low-level-dev-skills --skill llvm-passes

Installs into the current project.

Summary

Files · 1

View on GitHub

SKILL.md

1	# LLVM Passes
2
3	## Purpose
4
5	Guide agents through writing LLVM optimization passes with the New Pass Manager: `FunctionPass` and `ModulePass` structure, `PassPluginLibraryInfo` registration, running via `opt -load-pass-plugin`, common analysis utilities (`DominatorTree`, `LoopInfo`, `AliasAnalysis`), IR modification patterns, `llvm-lit` testing, and debugging with `opt -print-after-all`.
6
7	## When to Use
8
9	- Adding a custom optimization to an LLVM-based compiler
10	- Writing an IR transformation pass (inlining, DCE, custom lowering)
11	- Analyzing control flow with dominator trees or loop info
12	- Testing passes with FileCheck and llvm-lit
13	- Debugging pass ordering and IR corruption
14	- Integrating passes into Clang via plugin
15
16	## Workflow
17
18	### 1. New Pass Manager architecture
19
20	```
21	opt / clang
22	├── ModulePassManager
23	│ └── FunctionPassManager (per function)
24	│ └── FunctionPass instances
25	└── AnalysisManager (cached analyses)
26	```
27
28	Passes declare analysis usage; analyses are invalidated on IR mutation.
29
30	### 2. Minimal FunctionPass (C++ plugin)
31
32	```cpp
33	// MyPass.cpp
34	#include "llvm/IR/PassManager.h"
35	#include "llvm/Passes/PassBuilder.h"
36	#include "llvm/Passes/PassPlugin.h"
37	#include "llvm/Support/raw_ostream.h"
38
39	using namespace llvm;
40
41	namespace {
42
43	struct MyPass : PassInfoMixin<MyPass> {
44	PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM) {
45	bool changed = false;
46	for (BasicBlock &BB : F) {
47	for (Instruction &I : BB) {
48	if (auto *Call = dyn_cast<CallInst>(&I)) {
49	if (Call->getCalledFunction() &&
50	Call->getCalledFunction()->getName() == "dead_func") {
51	Call->eraseFromParent();
52	changed = true;
53	}
54	}
55	}
56	}
57	return changed ? PreservedAnalyses::none() : PreservedAnalyses::all();
58	}
59	};
60
61	} // namespace
62
63	extern "C" LLVM_ATTRIBUTE_WEAK PassPluginLibraryInfo llvmGetPassPluginInfo() {
64	return {
65	LLVM_PLUGIN_API_VERSION, "MyPass", "v0.1",
66	[](PassBuilder &PB) {
67	PB.registerPipelineParsingCallback(
68	[](StringRef Name, FunctionPassManager &FPM,
69	ArrayRef<PassBuilder::PipelineElement>) {
70	if (Name == "my-pass") {
71	FPM.addPass(MyPass());
72	return true;
73	}
74	return false;
75	});
76	}
77	};
78	}
79	```
80
81	```cmake
82	# CMakeLists.txt
83	find_package(LLVM REQUIRED CONFIG)
84	add_library(MyPass MODULE MyPass.cpp)
85	target_include_directories(MyPass SYSTEM PRIVATE ${LLVM_INCLUDE_DIRS})
86	target_compile_definitions(MyPass PRIVATE ${LLVM_DEFINITIONS})
87	llvm_map_components_to_libnames(llvm_libs core passes support)
88	target_link_libraries(MyPass PRIVATE ${llvm_libs})
89	```
90
91	```bash
92	cmake -B build -DLLVM_DIR=$(llvm-config --cmakedir)
93	cmake --build build
94	```
95
96	### 3. Running with opt
97
98	```bash
99	# Run pass on IR file
100	opt -load-pass-plugin ./build/MyPass.so -passes=my-pass -S input.ll -o output.ll
101
102	# Print IR after each pass
103	opt -load-pass-plugin ./build/MyPass.so -passes=my-pass -print-after-all input.ll -o /dev/null
104
105	# Pass pipeline string
106	opt -passes="function(instcombine),my-pass,function(dce)" input.ll -S -o out.ll
107	```
108
109	### 4. ModulePass example
110
111	```cpp
112	struct MyModulePass : PassInfoMixin<MyModulePass> {
113	PreservedAnalyses run(Module &M, ModuleAnalysisManager &AM) {
114	for (Function &F : M) {
115	if (F.isDeclaration()) continue;
116	// module-level transformation
117	}
118	return PreservedAnalyses::none();
119	}
120	};
121	```
122
123	Register on `ModulePassManager` in plugin callback.
124
125	### 5. Analysis utilities
126
127	```cpp
128	#include "llvm/Analysis/DominatorTree.h"
129	#include "llvm/Analysis/LoopInfo.h"
130	#include "llvm/Analysis/AliasAnalysis.h"
131
132	PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM) {
133	auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
134	auto &LI = AM.getResult<LoopAnalysis>(F);
135	auto &AA = AM.getResult<AAManager>(F);
136
137	for (Loop *L : LI) {
138	BasicBlock *Header = L->getHeader();
139	// Loop invariant code motion, etc.
140	}
141
142	DominatorTreeNode *IDom = DT.getNode(&F.getEntryBlock());
143	(void)IDom;
144	return PreservedAnalyses::all();
145	}
146	```
147
148	Declare analyses used:
149
150	```cpp
151	AnalysisUsage MyLegacyPass::getAnalysisUsage(AnalysisUsage &AU) const {
152	AU.addRequired<DominatorTreeWrapperPass>();
153	AU.addRequired<LoopInfoWrapperPass>();
154	return AU;
155	}
156	```
157
158	(New PM: analyses requested via `AM.getResult<>` — dependency auto-tracked.)
159
160	### 6. IR modification patterns
161
162	```cpp
163	// Insert instruction before iter

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