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Llvm Ir And Passes by mohitmishra786

Name: Llvm Ir And Passes
Rating: 4.2 (5 reviews)
Author: mohitmishra786

Llvm Ir And Passes

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.

File contents

LLVM IR and Passes

Purpose

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.

When to Use

Understanding what -O2 changed in generated IR
Triaging miscompiles between Clang versions
Preparing to write a custom pass
Teaching SSA and dominance without writing C++ passes yet

Workflow

1. LLVM IR structure

; Function in SSA form
define i32 @add(i32 %a, i32 %b) {
entry:
  %sum = add i32 %a, %b
  ret i32 %sum
}

Key concepts: basic blocks, PHI nodes at merges, typed values (i32, ptr, vectors).

2. Emit IR from Clang

clang -S -emit-llvm -O0 -o foo.ll foo.c
clang -c -emit-llvm -O2 -o foo.bc foo.c
llvm-dis foo.bc -o foo.ll

3. Standard optimization pipeline (New PM)

Clang -O2 roughly runs:

SimplifyCFG → InstCombine → GVN → LICM → LoopVectorize → ...

Inspect with:

opt -passes='default<O2>' -S foo.ll -o foo-opt.ll
opt -passes='default<O2>' -print-passes foo.ll 2>&1 | head

4. Useful analysis passes

Pass	Shows
`-passes=print<domtree>`	Dominator tree
`-passes=print<loops>`	Loop nests
`-passes=print-alias-sets`	Alias sets

opt -passes='print<domtree>' -disable-output foo.ll

5. Reading PHI and UB

merge:
  %v = phi i32 [ %a, %then ], [ %b, %else ]

undef and poison in IR model LLVM poison semantics — distinct from C UB but related.

6. Compare before/after

opt -passes='instcombine,simplifycfg' -S foo.ll -o - | diff -u foo.ll -

7. Agent usage

/llvm-ir-and-passes Explain this PHI node and which pass likely created it

Common Problems

Symptom	Cause	Fix
Pass not found	LLVM version rename	`opt --print-passes` for your build
IR mismatch	Different LLVM major	Match clang/opt versions
"optnone" blocks opts	`-O0` attribute	Compile with `-O1+`
Huge IR	Inlined headers	`-fno-discard-value-names` off; filter function
Wrong pipeline	Legacy vs NPM	Use `-passes=` syntax

Related Skills

skills/compilers/llvm — toolchain overview
skills/compiler-internals/llvm-passes — writing PassPlugins
skills/compiler-internals/compiler-optimizations-deep — RA and ISel
skills/compiler-internals/compiler-frontend — IR generation
skills/rust/rustc-basics — rustc --emit=llvm-ir

---name: llvm-ir-and-passesdescription: 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.--- # LLVM IR and Passes ## Purpose 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. ## When to Use - Understanding what `-O2` changed in generated IR - Triaging miscompiles between Clang versions - Preparing to write a custom pass - Teaching SSA and dominance without writing C++ passes yet ## Workflow ### 1. LLVM IR structure ```llvm ; Function in SSA form define i32 @add(i32 %a, i32 %b) { entry: %sum = add i32 %a, %b ret i32 %sum } ``` Key concepts: basic blocks, PHI nodes at merges, typed values (`i32`, `ptr`, vectors). ### 2. Emit IR from Clang ```bash clang -S -emit-llvm -O0 -o foo.ll foo.c clang -c -emit-llvm -O2 -o foo.bc foo.c llvm-dis foo.bc -o foo.ll ``` ### 3. Standard optimization pipeline (New PM) Clang `-O2` roughly runs: ``` SimplifyCFG → InstCombine → GVN → LICM → LoopVectorize → ... ``` Inspect with: ```bash opt -passes='default<O2>' -S foo.ll -o foo-opt.ll opt -passes='default<O2>' -print-passes foo.ll 2>&1 | head ``` ### 4. Useful analysis passes | Pass | Shows | |------|-------| | `-passes=print<domtree>` | Dominator tree | | `-passes=print<loops>` | Loop nests | | `-passes=print-alias-sets` | Alias sets | ```bash opt -passes='print<domtree>' -disable-output foo.ll ``` ### 5. Reading PHI and UB ```llvm merge: %v = phi i32 [ %a, %then ], [ %b, %else ] ``` `undef` and `poison` in IR model LLVM poison semantics — distinct from C UB but related. ### 6. Compare before/after ```bash opt -passes='instcombine,simplifycfg' -S foo.ll -o - | diff -u foo.ll - ``` ### 7. Agent usage ``` /llvm-ir-and-passes Explain this PHI node and which pass likely created it ``` ## Common Problems | Symptom | Cause | Fix | |---------|-------|-----| | Pass not found | LLVM version rename | `opt --print-passes` for your build | | IR mismatch | Different LLVM major | Match clang/opt versions | | "optnone" blocks opts | `-O0` attribute | Compile with `-O1+` | | Huge IR | Inlined headers | `-fno-discard-value-names` off; filter function | | Wrong pipeline | Legacy vs NPM | Use `-passes=` syntax | ## Related Skills - `skills/compilers/llvm` — toolchain overview - `skills/compiler-internals/llvm-passes` — writing PassPlugins - `skills/compiler-internals/compiler-optimizations-deep` — RA and ISel - `skills/compiler-internals/compiler-frontend` — IR generation - `skills/rust/rustc-basics` — `rustc --emit=llvm-ir`

Frequently asked questions