llvm-ir-and-passes

Name: llvm-ir-and-passes
Author: mohitmishra786

141stars19forks

How to install llvm-ir-and-passes?

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

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

Installs into the current project.

Summary

LLVM IR and passes skill for reading compiler IR. Use when analyzing LLVM IR, understanding SSA, pass pipeline order, or running opt on bitcode. Activates on queries about LLVM IR, SSA form, opt passes, llvm-dis, pass pipeline, or reading .ll files.

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SKILL.md

1	# LLVM IR and Passes
2
3	## Purpose
4
5	Teach agents to read and reason about LLVM IR (SSA, types, terminators), navigate the standard pass pipeline, and use `opt`/`llvm-dis` for inspection. Complements `skills/compilers/llvm` (toolchain) and `skills/compiler-internals/llvm-passes` (writing plugins) — not merged.
6
7	## When to Use
8
9	- Understanding what `-O2` changed in generated IR
10	- Triaging miscompiles between Clang versions
11	- Preparing to write a custom pass
12	- Teaching SSA and dominance without writing C++ passes yet
13
14	## Workflow
15
16	### 1. LLVM IR structure
17
18	```llvm
19	; Function in SSA form
20	define i32 @add(i32 %a, i32 %b) {
21	entry:
22	%sum = add i32 %a, %b
23	ret i32 %sum
24	}
25	```
26
27	Key concepts: basic blocks, PHI nodes at merges, typed values (`i32`, `ptr`, vectors).
28
29	### 2. Emit IR from Clang
30
31	```bash
32	clang -S -emit-llvm -O0 -o foo.ll foo.c
33	clang -c -emit-llvm -O2 -o foo.bc foo.c
34	llvm-dis foo.bc -o foo.ll
35	```
36
37	### 3. Standard optimization pipeline (New PM)
38
39	Clang `-O2` roughly runs:
40
41	```
42	SimplifyCFG → InstCombine → GVN → LICM → LoopVectorize → ...
43	```
44
45	Inspect with:
46
47	```bash
48	opt -passes='default<O2>' -S foo.ll -o foo-opt.ll
49	opt -passes='default<O2>' -print-passes foo.ll 2>&1 \| head
50	```
51
52	### 4. Useful analysis passes
53
54	\| Pass \| Shows \|
55	\|------\|-------\|
56	\| `-passes=print<domtree>` \| Dominator tree \|
57	\| `-passes=print<loops>` \| Loop nests \|
58	\| `-passes=print-alias-sets` \| Alias sets \|
59
60	```bash
61	opt -passes='print<domtree>' -disable-output foo.ll
62	```
63
64	### 5. Reading PHI and UB
65
66	```llvm
67	merge:
68	%v = phi i32 [ %a, %then ], [ %b, %else ]
69	```
70
71	`undef` and `poison` in IR model LLVM poison semantics — distinct from C UB but related.
72
73	### 6. Compare before/after
74
75	```bash
76	opt -passes='instcombine,simplifycfg' -S foo.ll -o - \| diff -u foo.ll -
77	```
78
79	### 7. Agent usage
80
81	```
82	/llvm-ir-and-passes Explain this PHI node and which pass likely created it
83	```
84
85	## Common Problems
86
87	\| Symptom \| Cause \| Fix \|
88	\|---------\|-------\|-----\|
89	\| Pass not found \| LLVM version rename \| `opt --print-passes` for your build \|
90	\| IR mismatch \| Different LLVM major \| Match clang/opt versions \|
91	\| "optnone" blocks opts \| `-O0` attribute \| Compile with `-O1+` \|
92	\| Huge IR \| Inlined headers \| `-fno-discard-value-names` off; filter function \|
93	\| Wrong pipeline \| Legacy vs NPM \| Use `-passes=` syntax \|
94
95	## Related Skills
96
97	- `skills/compilers/llvm` — toolchain overview
98	- `skills/compiler-internals/llvm-passes` — writing PassPlugins
99	- `skills/compiler-internals/compiler-optimizations-deep` — RA and ISel
100	- `skills/compiler-internals/compiler-frontend` — IR generation
101	- `skills/rust/rustc-basics` — `rustc --emit=llvm-ir`

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llvm-ir-and-passes

141stars19forks

View on GitHub

How to install llvm-ir-and-passes?

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

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

Installs into the current project.

Summary

Files · 1

View on GitHub

SKILL.md

1	# LLVM IR and Passes
2
3	## Purpose
4
5	Teach agents to read and reason about LLVM IR (SSA, types, terminators), navigate the standard pass pipeline, and use `opt`/`llvm-dis` for inspection. Complements `skills/compilers/llvm` (toolchain) and `skills/compiler-internals/llvm-passes` (writing plugins) — not merged.
6
7	## When to Use
8
9	- Understanding what `-O2` changed in generated IR
10	- Triaging miscompiles between Clang versions
11	- Preparing to write a custom pass
12	- Teaching SSA and dominance without writing C++ passes yet
13
14	## Workflow
15
16	### 1. LLVM IR structure
17
18	```llvm
19	; Function in SSA form
20	define i32 @add(i32 %a, i32 %b) {
21	entry:
22	%sum = add i32 %a, %b
23	ret i32 %sum
24	}
25	```
26
27	Key concepts: basic blocks, PHI nodes at merges, typed values (`i32`, `ptr`, vectors).
28
29	### 2. Emit IR from Clang
30
31	```bash
32	clang -S -emit-llvm -O0 -o foo.ll foo.c
33	clang -c -emit-llvm -O2 -o foo.bc foo.c
34	llvm-dis foo.bc -o foo.ll
35	```
36
37	### 3. Standard optimization pipeline (New PM)
38
39	Clang `-O2` roughly runs:
40
41	```
42	SimplifyCFG → InstCombine → GVN → LICM → LoopVectorize → ...
43	```
44
45	Inspect with:
46
47	```bash
48	opt -passes='default<O2>' -S foo.ll -o foo-opt.ll
49	opt -passes='default<O2>' -print-passes foo.ll 2>&1 \| head
50	```
51
52	### 4. Useful analysis passes
53
54	\| Pass \| Shows \|
55	\|------\|-------\|
56	\| `-passes=print<domtree>` \| Dominator tree \|
57	\| `-passes=print<loops>` \| Loop nests \|
58	\| `-passes=print-alias-sets` \| Alias sets \|
59
60	```bash
61	opt -passes='print<domtree>' -disable-output foo.ll
62	```
63
64	### 5. Reading PHI and UB
65
66	```llvm
67	merge:
68	%v = phi i32 [ %a, %then ], [ %b, %else ]
69	```
70
71	`undef` and `poison` in IR model LLVM poison semantics — distinct from C UB but related.
72
73	### 6. Compare before/after
74
75	```bash
76	opt -passes='instcombine,simplifycfg' -S foo.ll -o - \| diff -u foo.ll -
77	```
78
79	### 7. Agent usage
80
81	```
82	/llvm-ir-and-passes Explain this PHI node and which pass likely created it
83	```
84
85	## Common Problems
86
87	\| Symptom \| Cause \| Fix \|
88	\|---------\|-------\|-----\|
89	\| Pass not found \| LLVM version rename \| `opt --print-passes` for your build \|
90	\| IR mismatch \| Different LLVM major \| Match clang/opt versions \|
91	\| "optnone" blocks opts \| `-O0` attribute \| Compile with `-O1+` \|
92	\| Huge IR \| Inlined headers \| `-fno-discard-value-names` off; filter function \|
93	\| Wrong pipeline \| Legacy vs NPM \| Use `-passes=` syntax \|
94
95	## Related Skills
96
97	- `skills/compilers/llvm` — toolchain overview
98	- `skills/compiler-internals/llvm-passes` — writing PassPlugins
99	- `skills/compiler-internals/compiler-optimizations-deep` — RA and ISel
100	- `skills/compiler-internals/compiler-frontend` — IR generation
101	- `skills/rust/rustc-basics` — `rustc --emit=llvm-ir`

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6 picks

Type