Add support for `-Ztimings` to `x.py`

[nnethercote]

Cargo's new experimental -Ztimings flag is really cool. See here for sample output. It would be great to add support for this to x.py. It would help a lot with #65031.

I tried and failed to get -Ztimings working with rustc, according to some instructions here. One complication is that rustc normally builds with a beta version of Cargo, but -Ztimings is nightly only.

@ehuss : what steps did you take to get the rustc measurements? Thanks!

[ehuss]

I didn't run it against rustc, that was Alex. You have to edit config.toml and set build.cargo to a nightly version of cargo (like from the .rustup directory, or build it yourself). Then edit the cargo function to add -Ztimings. It looks like someone has made a PR to make that read from an env.

[nnethercote]

You have to edit config.toml and set build.cargo to a nightly version of cargo

@alexcrichton: can you give the exact steps here? I tried to do this but failed. Thanks!

[ehuss]

What kind of error did you run into? Here's what my rust config.toml looks like (adjust for your setup):

[build]
cargo = "/Users/eric/.rustup/toolchains/nightly-x86_64-apple-darwin/bin/cargo"

And here's what the diff might look like:

diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
index 5d586f0c461..ef01c18eaa0 100644
--- a/src/bootstrap/builder.rs
+++ b/src/bootstrap/builder.rs
@@ -776,7 +776,8 @@ impl<'a> Builder<'a> {
         cargo
             .env("CARGO_TARGET_DIR", out_dir)
             .arg(cmd)
-            .arg("-Zconfig-profile");
+            .arg("-Zconfig-profile")
+            .arg("-Ztimings");

         let profile_var = |name: &str| {
             let profile = if self.config.rust_optimize {

I have nightly 2019-10-02, which seems to work. Sometimes bootstrapping won't work on the latest cargo because the bootstrap code needs updating, but it seems to be working at this point in time.

[alexcrichton]

@nnethercote oh I didn't actually add support to ./x.py when I generated my graphs, I literally just did:

$ cd src/rustc
$ cargo +nightly build -Z timings

and then fixed any missing env vars that the crates reported. I would expect that @ehuss's patch would work as expected, although it may also require a temporary check for if stage != 0 somewhere

[Zoxc]

You can now do this with CARGOFLAGS=-Ztimings.

[andjo403]

@nnethercote do you want more support then CARGOFLAGS_BOOTSTRAP=-Ztimings ./x.py build --stage 1 or can this be closed now?

[nnethercote]

I used CARGOFLAGS=-Ztimings ./x.py build for the first time yesterday and it worked great!