Commands

bolt hot builds an optimized, sampled copy of the program (in target/hot/, apart from the normal builds), runs it, and reports the hottest functions (their own time and with what they call), the hottest .volt lines, and the hottest call paths. Code that got inlined counts where it’s written, and time spent in a shared library (libc’s malloc, say) is named too.

$ bolt hot bench/closures/main.volt -- 300
bolt hot: 200 samples of /tmp/bolt-hot-4242/main

  self   total  function
 45.5%   99.0%  pipeline<closure(i64) -> i64, closure(i64) -> bool> (bench/closures/main.volt:6), inlined
 40.5%   40.5%  a closure (bench/closures/main.volt:30:92), inlined
 13.0%   13.0%  a closure (bench/closures/main.volt:30:39), inlined
  1.0%  100.0%  main (bench/closures/main.volt:18), inlined

hottest lines
 53.5%  bench/closures/main.volt:30  total += pipeline(xs.items(), |factor| (x: i64) -> i64 { return x *...
 35.0%  bench/closures/main.volt:12  sum += y;
 10.5%  bench/closures/main.volt:9  for (x) in xs {

hottest paths
 45.5%  main -> pipeline<closure(i64) -> i64, closure(i64) -> bool>
 40.5%  main -> pipeline<closure(i64) -> i64, closure(i64) -> bool> -> closure@main.volt:30:92

inlined marks a function that every sample found inlined into its callers: its self time is spent at its lines there, not in calls to it. A C compiler’s copies of a function (work.constprop.0, made for a constant argument) are reported as that function.

bolt hot --html FILE writes the same profile as a web page too, one file that loads nothing from the network: the call tree as an icicle graph, a table of every function’s self and total time (click a column to sort by it, again to reverse), and the ten hottest lines. In the graph, the top box is all the samples; beneath it is main (and, when a stack was too deep to sample whole, the outermost function the sample reached), and beneath each function the ones it called, each box as wide as its share of the samples. Hover a box for its self and total time; click it to zoom in. Callees with under 0.1% of the samples are folded into one other box. bolt hot --folded FILE writes the call paths as folded stacks (main;work;mix 812, one line per path) for flamegraph.pl, speedscope or inferno.

The program samples itself (it asks for a sample every 100 µs of CPU time, and the kernel delivers them on its timer tick, usually 250 to 1000 times a second), so this needs no perf, debugger or root; it runs on Linux (x86-64 and aarch64), and names the samples with llvm-symbolizer or addr2line. Run the program long enough to get a few hundred samples.

bolt add also takes --dev, --optional, --features A,B and --no-default-features. add and remove edit bolt.toml in place, keeping its comments and layout. bolt new and bolt init name the package after its directory (lowercased, - as _) unless --name NAME says otherwise.

Options go before or after the command, and --option=value works too. b, c, r, t and rm are short for build, check, run, test and remove.