Document the usage of cargo-llvm-lines and -Ztimings.

Author: Julian Wollersberger

src/profiling.md

@@ -12,5 +12,80 @@ Depending on what you're trying to measure, there are several different approach
     See [their docs](https://github.com/rust-lang/measureme/blob/master/summarize/Readme.md) for more information.
 
 - If you want function level performance data or even just more details than the above approaches:
-  - Consider using a native code profiler such as [perf](profiling/with_perf.html).
+  - Consider using a native code profiler such as [perf](profiling/with_perf.html) 
+  - or [tracy](https://github.com/nagisa/rust_tracy_client) for a nanosecond-precision, full-featured graphical interface.
 
+- If you want a nice visual representation of the compile times of your crate graph, 
+  you can use [cargo's `-Ztimings` flag](https://doc.rust-lang.org/cargo/reference/unstable.html#timings), eg. `cargo -Ztimings build`.
+  You can use this flag on the compiler itself with `CARGOFLAGS="-Ztimings" ./x.py build`
+  
+## Optimizing rustc's self-compile-times with cargo-llvm-lines
+
+Using [cargo-llvm-lines](https://github.com/dtolnay/cargo-llvm-lines) you can count the 
+number of lines of LLVM IR across all instantiations of a generic function.
+Since most of the time compiling rustc is spent in LLVM, the idea is that by
+reducing the amount of code passed to LLVM, compiling rustc gets faster.
+
+Example usage:
+```
+cargo install cargo-llvm-lines
+# On a normal crate you could now run `cargo llvm-lines`, but x.py isn't normal :P
+
+# Do a clean before every run, to not mix in the results from previous runs.
+./x.py clean
+RUSTFLAGS="--emit=llvm-ir" ./x.py build --stage 0 compiler/rustc
+
+# Single crate, eg. rustc_middle
+cargo llvm-lines --files ./build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/debug/deps/rustc_middle* > llvm-lines-middle.txt
+# Whole compiler at once
+cargo llvm-lines --files ./build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/debug/deps/*.ll > llvm-lines.txt
+```
+
+Example output:
+```
+  Lines            Copies        Function name
+  -----            ------        -------------
+  11802479 (100%)  52848 (100%)  (TOTAL)
+   1663902 (14.1%)   400 (0.8%)  rustc_query_system::query::plumbing::get_query_impl::{{closure}}
+    683526 (5.8%)  10579 (20.0%) core::ptr::drop_in_place
+    568523 (4.8%)    528 (1.0%)  rustc_query_system::query::plumbing::get_query_impl
+    472715 (4.0%)   1134 (2.1%)  hashbrown::raw::RawTable<T>::reserve_rehash
+    306782 (2.6%)   1320 (2.5%)  rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query::{{closure}}::{{closure}}::{{closure}}
+    212800 (1.8%)    514 (1.0%)  rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
+    194813 (1.7%)    124 (0.2%)  rustc_query_system::query::plumbing::force_query_impl
+    158488 (1.3%)      1 (0.0%)  rustc_middle::ty::query::<impl rustc_middle::ty::context::TyCtxt>::alloc_self_profile_query_strings
+    119768 (1.0%)    418 (0.8%)  core::ops::function::FnOnce::call_once
+    119644 (1.0%)      1 (0.0%)  rustc_target::spec::load_specific
+    104153 (0.9%)      7 (0.0%)  rustc_middle::ty::context::_DERIVE_rustc_serialize_Decodable_D_FOR_TypeckResults::<impl rustc_serialize::serialize::Decodable<__D> for rustc_middle::ty::context::TypeckResults>::decode::{{closure}}
+     81173 (0.7%)      1 (0.0%)  rustc_middle::ty::query::stats::query_stats
+     80306 (0.7%)   2029 (3.8%)  core::ops::function::FnOnce::call_once{{vtable.shim}}
+     78019 (0.7%)   1611 (3.0%)  stacker::grow::{{closure}}
+     69720 (0.6%)   3286 (6.2%)  <&T as core::fmt::Debug>::fmt
+     56327 (0.5%)    186 (0.4%)  rustc_query_system::query::plumbing::incremental_verify_ich
+     49714 (0.4%)     14 (0.0%)  rustc_mir::dataflow::framework::graphviz::BlockFormatter<A>::write_node_label
+```
+
+Since this doesn't seem to work with incremental compilation or `x.py check`, you will be compiling rustc _a lot_.
+I recommend changing a few settings in `config.toml` to make it bearable:
+```
+[rust]
+# A debug build takes _a fourth_ as long on my machine, 
+# but compiling more than stage0 rustc becomes unbearably slow.
+optimize = false
+
+# We can't use incremental anyway, so we disable it for a little speed boost.
+incremental = false
+# We won't be running it, so no point in compiling debug checks.
+debug = false
+
+# Caution: This changes the output of llvm-lines.
+# Using a single codegen unit gives more accurate output, but is slower to compile.
+# Changing it to the number of cores on my machine increased the output from 3.5GB to 4.1GB and decreased compile times from 5½ min to 4 min.
+codegen-units = 1
+#codegen-units = 0 # num_cpus
+```
+
+What I'm still not sure about is if inlining in MIR optimizations affect llvm-lines.
+The output with `-Zmir-opt-level=0` and `-Zmir-opt-level=1` is the same,
+but it feels like that some functions that show up at the top should be to small
+to have such a high impact. Inlining should only happens in LLVM though.