Use more accurate estimate of generated LLVM IR with llvm-lines

The `--emit=llvm-ir` emits an optimized LLVM IR. For optimized builds it will be
highly inaccurate estimate of the amount IR generated initially.  While the
inaccuracy can be somewhat reduce after disabling the optimization, that in turn
has other unintended consequences, since opt-level controls the emission of
lifetime markers, sharing of generics between crates, instantiation of inline
functions, etc.

Use `-Csave-temps` and `no-opt` bitcode as a basis for more accurate estimate of
initial work handed of to the LLVM.

1 files changed, 41 insertions, 27 deletions

diff --git a/src/doc/rustc-dev-guide/src/profiling.md b/src/doc/rustc-dev-guide/src/profiling.md
index 429376167d5..155dda97dea 100644
--- a/src/doc/rustc-dev-guide/src/profiling.md
+++ b/src/doc/rustc-dev-guide/src/profiling.md
@@ -28,6 +28,12 @@ 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.
 
+To use `cargo-llvm-lines` together with somewhat custom rustc build process, you can use
+`-C save-temps` to obtain required LLVM IR. The option preserves temporary work products
+created during compilation. Among those is LLVM IR that represents an input to the
+optimization pipeline; ideal for our purposes. It is stored in files with `*.no-opt.bc`
+extension in LLVM bitcode format.
+
 Example usage:
 ```
 cargo install cargo-llvm-lines
@@ -35,36 +41,44 @@ cargo install cargo-llvm-lines
 
 # 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-a539a639bdab6513.ll > llvm-lines-middle.txt
-# Specify all crates of the compiler. (Relies on the glob support of your shell.)
-cargo llvm-lines --files ./build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/debug/deps/*.ll > llvm-lines.txt
+env RUSTFLAGS=-Csave-temps ./x.py build --stage 0 compiler/rustc
+
+# Single crate, e.g., rustc_middle. (Relies on the glob support of your shell.)
+# Convert unoptimized LLVM bitcode into a human readable LLVM assembly accepted by cargo-llvm-lines.
+for f in build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/deps/rustc_middle-*.no-opt.bc; do
+  ./build/x86_64-unknown-linux-gnu/llvm/bin/llvm-dis "$f"
+done
+cargo llvm-lines --files ./build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/deps/rustc_middle-*.ll > llvm-lines-middle.txt
+
+# Specify all crates of the compiler.
+for f in build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/deps/*.no-opt.bc; do
+  ./build/x86_64-unknown-linux-gnu/llvm/bin/llvm-dis "$f"
+done
+cargo llvm-lines --files ./build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/deps/*.ll > llvm-lines.txt
 ```
 
-Example output:
+Example output for the compiler:
 ```
-  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
+  Lines            Copies          Function name
+  -----            ------          -------------
+  45207720 (100%)  1583774 (100%)  (TOTAL)
+   2102350 (4.7%)   146650 (9.3%)  core::ptr::drop_in_place
+    615080 (1.4%)     8392 (0.5%)  std::thread::local::LocalKey<T>::try_with
+    594296 (1.3%)     1780 (0.1%)  hashbrown::raw::RawTable<T>::rehash_in_place
+    592071 (1.3%)     9691 (0.6%)  core::option::Option<T>::map
+    528172 (1.2%)     5741 (0.4%)  core::alloc::layout::Layout::array
+    466854 (1.0%)     8863 (0.6%)  core::ptr::swap_nonoverlapping_one
+    412736 (0.9%)     1780 (0.1%)  hashbrown::raw::RawTable<T>::resize
+    367776 (0.8%)     2554 (0.2%)  alloc::raw_vec::RawVec<T,A>::grow_amortized
+    367507 (0.8%)      643 (0.0%)  rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
+    355882 (0.8%)     6332 (0.4%)  alloc::alloc::box_free
+    354556 (0.8%)    14213 (0.9%)  core::ptr::write
+    354361 (0.8%)     3590 (0.2%)  core::iter::traits::iterator::Iterator::fold
+    347761 (0.8%)     3873 (0.2%)  rustc_middle::ty::context::tls::set_tlv
+    337534 (0.7%)     2377 (0.2%)  alloc::raw_vec::RawVec<T,A>::allocate_in
+    331690 (0.7%)     3192 (0.2%)  hashbrown::raw::RawTable<T>::find
+    328756 (0.7%)     3978 (0.3%)  rustc_middle::ty::context::tls::with_context_opt
+    326903 (0.7%)      642 (0.0%)  rustc_query_system::query::plumbing::try_execute_query
 ```
 
 Since this doesn't seem to work with incremental compilation or `x.py check`,