Compiler Explorer

Compiler Explorer

Optimize functions by analyzing compiler-generated assembly. This replicates the workflow of godbolt.org locally.

Workflow

1. Isolate the Function

Create a standalone file with the function to optimize. Export it to prevent inlining:

export fn targetFunction(x: i64) i64 {
    // function body
}

__attribute__((noinline)) int targetFunction(int x) {
    // function body
}

2. Generate Assembly

# Zig
zig build-obj src/target.zig -femit-asm -fno-emit-bin -O ReleaseFast

# C/C++ (clang)
clang -S -O3 -fno-inline -o target.s target.c

# Rust
rustc --emit=asm -C opt-level=3 target.rs

3. Locate and Read the Function

# Find the assembly file
find . -name "*.s" -newer src/target.zig

# Find function boundaries
grep -n "_targetFunction" output.s

# Count instructions
sed -n 'START,ENDp' output.s | grep -E '^\s+\w' | wc -l

4. Analyze and Iterate

Read through the assembly looking for:

• Unexpected instructions (type conversions, bounds checks, function calls)
• Repeated patterns that could be simplified
• Instructions that don't map to obvious source operations

Make a targeted change, regenerate assembly, compare instruction counts.

5. Benchmark

Always benchmark—fewer instructions ≠ always faster:

zig build-exe bench.zig -O ReleaseFast -o bench_v1
time ./bench_v1

Keep assembly outputs for comparison: function_v1.s, function_v2.s, etc.