GoTrace

AST-based function tracer for Go. Captures every function call with exact timings by rewriting source code at the AST level.

Unlike sampling profilers like pprof, GoTrace is deterministic — it traces every call, not a statistical sample.

Install

go install github.com/codeflash-ai/gotrace/cmd/gotrace@latest

Or build from source:

git clone https://github.com/codeflash-ai/gotrace.git
cd gotrace
go build -o gotrace ./cmd/gotrace

Usage

Trace a program

gotrace run ./cmd/server
gotrace run ./cmd/api listen --port 8080

The first argument is the package to build. Everything after is passed as arguments to the instrumented binary.

Trace tests

gotrace test ./pkg/auth
gotrace test ./pkg/auth -run TestLogin -v

Flamegraph output

Generates collapsed stacks compatible with Speedscope and Brendan Gregg's FlameGraph:

gotrace flame ./cmd/server > trace.folded

# Open in speedscope
speedscope trace.folded

# Or generate SVG
flamegraph.pl trace.folded > trace.svg

JSON output

gotrace json ./cmd/server

Produces a nested JSON tree:

[
  {
    "name": "main.main",
    "duration_ms": 231.45,
    "children": [
      {
        "name": "server.Start",
        "duration_ms": 192.3,
        "children": [...]
      }
    ]
  }
]

Filtering

Limit which packages get instrumented:

# Only instrument packages matching a pattern
gotrace run ./cmd --include="myapp/internal/*"

# Exclude specific packages
gotrace run ./cmd --exclude="myapp/generated/..."

By default, GoTrace instruments all packages within your module. Standard library and third-party dependencies are never instrumented.

Example Output

TOTAL: 2.31s

main.main                                       2.31s 100.0%
├── server.Start                                1.92s  83.1%
│   ├── db.Connect                              412ms  17.8%
│   ├── cache.Load                              182ms   7.9%
│   └── router.Init                             1.12s  48.5%
└── metrics.Start                               301ms  13.0%

How It Works

GoTrace operates in 5 stages:

1. Load

Uses golang.org/x/tools/go/packages to parse your project. Resolves all packages, ASTs, type information, and import graphs.

2. Rewrite

Walks the AST of every in-module package and injects tracing instrumentation:

Original:

func QueryDB(ctx context.Context) (*Result, error) {
    rows, err := db.Query(ctx, "SELECT ...")
    return parse(rows), err
}

Instrumented:

func QueryDB(ctx context.Context) (*Result, error) {
    __gotrace_token := __gotrace_tracer.Enter(__gotrace_fid_QueryDB)
    defer __gotrace_tracer.Exit(__gotrace_token)
    rows, err := db.Query(ctx, "SELECT ...")
    return parse(rows), err
}

The rewriter also transforms go statements for goroutine tracking:

// Before
go worker(item)

// After
go __gotrace_tracer.Go(func() { worker(item) })

This propagates parent-child relationships between goroutines.

3. Build

Copies your source to a temporary workspace, writes the rewritten files, injects the tracer runtime as a local module, and runs go build (or go test -c).

Your original source is never modified.

4. Execute

Runs the instrumented binary. The tracer records events into a lock-free pre-allocated buffer using atomic operations. Each event is ~29 bytes:

• Event type (enter/exit/spawn)
• Function ID
• Goroutine ID
• Nanosecond timestamp
• Parent goroutine ID

5. Render

After execution completes, reads the binary trace file, reconstructs the call tree from the event stream, and renders the output in the requested format.

What Gets Traced

• Regular functions
• Methods (with receiver type in the name)
• Generic functions
• Anonymous functions (named as pkg.Outer.func1)
• Goroutine spawn relationships

What Gets Skipped

• Standard library
• Third-party dependencies (anything outside your module)
• Generated files (*.pb.go, *_generated.go, files with // Code generated header)
• The tracer runtime itself

Commands

Command	Description
gotrace run <pkg> [args...]	Trace and print call tree
gotrace test <pkg> [test flags...]	Trace test execution
gotrace flame <pkg> [args...]	Output collapsed stacks
gotrace json <pkg> [args...]	Output JSON call tree
gotrace exec <pkg> [args...]	Build and run with tree output

Flags

Flag	Description
--include	Package patterns to instrument
--exclude	Package patterns to skip
-v, --verbose	Show workspace path and debug info

Requirements

• Go 1.21+
• Project must use Go modules

License

MIT