Code Heatmaps

BOLT has gained the ability to print code heatmaps based on sampling-based LBR profiles generated by perf. The output is produced in colored ASCII to be displayed in a color-capable terminal. It looks something like this:

Heatmaps can be generated for BOLTed and non-BOLTed binaries. You can use them to compare the code layout before and after optimizations.

To generate a heatmap, start with running your app under perf:

$ perf record -e cycles:u -j any,u -- <executable with args>

or if you want to monitor the existing process(es):

$ perf record -e cycles:u -j any,u [-p PID|-a] -- sleep <interval>

Running with LBR (-j any,u or -b) is recommended. Heatmaps can be generated from basic events by using the llvm-bolt-heatmap option -nl (no LBR) but such heatmaps do not have the coverage provided by LBR and may only be useful for finding event hotspots at larger code block granularities.

Once the run is complete, and is generated, run llvm-bolt-heatmap:

$ llvm-bolt-heatmap -p <executable>

By default the heatmap will be dumped to stdout. You can change it with -o <heatmapfile> option. Each character/block in the heatmap shows the execution data accumulated for corresponding 64 bytes of code. You can change this granularity with a -block-size option. E.g. set it to 4096 to see code usage grouped by 4K pages. Other useful options are:

-line-size=<uint>   - number of entries per line (default 256)
-max-address=<uint> - maximum address considered valid for heatmap (default 4GB)

If you prefer to look at the data in a browser (or would like to share it that way), then you can use an HTML conversion tool. E.g.:

$ aha -b -f <heatmapfile> > <heatmapfile>.html