Add support for collecting time trace in the LLVM part

[mshockwave]

LLVM time profiler can collect time traces and present them in a hierarchical view. Which breakdowns the time spent in each Pass or even IR unit (e.g. how many time did we spent on optimizing & compiling a function). The result is also exported into a format that can be easily visualized by the Chrome browser.

Currently this features is only used for collecting time trace in the LLVM part and controlled by the following environment variables:

• ZIG_ENABLE_LLVM_TIME_TRACE toggles this feature
• ZIG_LLVM_TIME_TRACE_FILE specifies the output time trace file. Otherwise the result will be put in ".o.time-trace" or ".s.time-trace".
• ZIG_LLVM_TIME_TRACE_GRANULARITY controls the time granularity in ms (default to 500).

Here is an example usage:

1. $ env ZIG_ENABLE_LLVM_TIME_TRACE=1 ZIG_LLVM_TIME_TRACE_FILE=foo.time-trace.json zig build-exe foo.zig
2. Open Chrome and type "chrome://tracing" in the URL bar.
3. Click the "Load" button at the top-left corner to load foo.time-trace.json.

Then you'll get a nice breakdown on the time spent on each Pass and each function LLVM compiled. Alternatively you can also visualize the time trace file using this website: https://www.speedscope.app

The only thing I would like to discuss with everyone here is that should we use command line flag to toggle this feature rather than environment variable?

[jedisct1]

This is very nice, thank you!

Since this is mainly useful for optimizing the compiler itself, enabling it via environment variables looks like a good choice to me, but others may have a different opinion :)

Are two environment variables required, though? Why not just enable this if ZIG_LLVM_TIME_TRACE_FILE is defined?

[jedisct1]

Are two environment variables required, though? Why not just enable this if ZIG_LLVM_TIME_TRACE_FILE is defined?

Ping :)

[mshockwave]

Are two environment variables required, though? Why not just enable this if ZIG_LLVM_TIME_TRACE_FILE is defined?

Ping :)

I’m so sorry, somehow I didn’t get notified of your reply.
I’m glad people find this useful. And I agree it will be better to consolidate the first two env vars into one. I’ll push my changes later

[mshockwave]

Alright I've merged ZIG_LLVM_ENABLE_TIME_TRACE into ZIG_LLVM_TIME_TRACE_FILE. Now one only needs to do something like this:

env ZIG_LLVM_TIME_TRACE_FILE=foo.time-trace.json zig build-exe foo.zig