Compiler Optimizations (Deep)

Purpose

Explain optimization phases beyond flags: mid-level IR opts, register allocation, instruction selection/scheduling, vectorization boundaries, PGO, and post-link BOLT — bridging skills/compilers/pgo and LLVM/GCC internals.

When to Use

-O3 did not vectorize a hot loop
Teaching why register pressure causes spills
Planning PGO or BOLT deployment
Understanding pass interaction (e.g., LICM before vectorize)

Workflow

1. Compiler pipeline map

Frontend → LLVM IR / GCC GIMPLE
├── Mid-level: DCE, GVN, LICM, inlining
├── Loop opts: unroll, vectorize
├── Codegen prep: legalize types
├── Instruction selection (DAG → machine ops)
├── Register allocation (greedy, linear scan)
└── Peephole / scheduling

2. Vectorization failure triage

clang -O3 -Rpass=loop-vectorize -Rpass-missed=loop-vectorize foo.c

Miss reason	Typical fix
Unknown trip count	peel loop; assert count
Dependence	reorder / separate accumulators
Function call in loop	inline or outline
Alignment unknown	`__builtin_assume_aligned`

3. Register allocation intuition

When live ranges exceed physical registers, the allocator spills to stack slots — costly loads/stores. Reducing live ranges (splitting variables, rematerialization) helps.

GCC/LLVM both use graph coloring variants (LLVM "greedy regalloc").

4. PGO workflow (Clang)

clang -fprofile-instr-generate -O2 -o app foo.c
./app   # training workload
llvm-profdata merge default.profraw -o default.profdata
clang -fprofile-instr-use=default.profdata -O2 -o app_pgo foo.c

Improves branch layout, inlining, and vectorization thresholds.

See skills/compilers/pgo for GCC and BOLT.

5. BOLT (post-link)

llvm-bolt -instrument app -o app.inst
./app.inst
llvm-bolt -data=perf.fdata -reorder-blocks=+ -o app.bolt app

Optimizes layout after linker — needs relocations (-Wl,--emit-relocs).

6. LICM example

Loop-invariant code motion hoists x * scale out of inner loop when legal — reduces work per iteration.

7. Agent usage

/compiler-optimizations-deep Why did LLVM fail to vectorize this reduction loop?

Common Problems

Symptom	Cause	Fix
PGO no gain	Unrepresentative training	Match production input
BOLT crash	Stripped binary	Keep symbols + relocs
Spills in asm	Register pressure	Simplify live ranges
`-O3` slower	Code bloat / cache	Try `-O2` or PGO
Different GCC/Clang	Pass ordering differs	Compare IR + asm

Related Skills

skills/compilers/pgo — PGO and BOLT detail
skills/compiler-internals/llvm-ir-and-passes — IR-level opts
skills/compiler-internals/code-generation-and-backends — ISel and backends
skills/computer-architecture/cpu-pipelines-and-hazards — scheduling context
skills/low-level-programming/simd-intrinsics — manual vectorization