Add lifetimes command to analyze stack variable lifetimes

[dylandreimerink]

This works adds a new command which analyzes the lifetimes of stack variables. It outputs an SVG graph which show each stack offset on the vertical axis and the instructions on the horizontal axis. Colored bars represent a lifetime, where a lifetime is the time between the first write and the last read of a stack variable. The writes are marked by black dots and the reads are marked by white dots.

The idea of this is that we should be able to see which lifetimes overlap and thus why more space was allocated. In theory showing us the places in the program where the least stack re-use was possible. Focusing on reducing stack usage at that point the the program should yield best results when reducing stack usage.

The code itself works by doing static analysis of the assembly code to get basic blocks. Then visiting all basic blocks and walking the instructions, keeping track of register and stack state. Sort of like a very limited shitty verifier. We record all reads and writes to the stack, including those done by helper functions.

Once we have all reads and writes, we iterate over all the reads and traverse the graph of basic blocks backwards, finding writes to the same stack offset. When we find them, we record a lifetime spanning the stack of basic blocks we traversed backwards. Overlapping lifetimes get merged since these are likely modifications of the same variable or variable reuse. Each lifetime gets its own color to allow users to see when a stack offset gets reused.

AI disclosure: AIL 2, core is handwritten, AI made modifications after review, everything manually reviewed.

Fixes: #2

[Copilot]

Pull request overview

This PR introduces a new lifetimes CLI command intended to statically analyze eBPF assembly, derive stack-slot read/write lifetimes across basic blocks, and render the result as an SVG timeline to help explain stack allocation/reuse decisions.

Changes:

• Adds an internal/analyze package to build basic blocks / CFG metadata from asm.Instructions, with tests and benchmarks.
• Adds stackwhere lifetimes to generate an SVG visualization of stack-slot lifetimes, plus supporting helper modeling.
• Updates testdata build targets and introduces a new dump-dwarf command wiring in cmd/stackwhere/main.go, along with new Go test dependencies.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
testdata/spill.c	Refactors test C source includes/macros for BPF test program.
testdata/Makefile	Adds lifetimes to the testdata build target list.
internal/analyze/util.go	Adds helper types/utilities for resolving jump targets and connecting call relationships.
internal/analyze/blocks.go	Implements basic-block construction, iterators, and backtracking over a CFG.
internal/analyze/blocks_test.go	Adds unit tests and benchmarks for block construction and iteration.
go.mod	Adds testify and new indirect dependencies for tests.
go.sum	Updates checksums for added dependencies.
cmd/stackwhere/main.go	Registers the new lifetimes and dump-dwarf subcommands.
cmd/stackwhere/lifetimes.go	Implements the lifetimes command, analysis visitor, lifetime discovery, and SVG output.
cmd/stackwhere/dwarf.go	Adds a dump-dwarf command to inspect DWARF/subprogram mapping.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 9 changed files in this pull request and generated 10 comments.