Start converting MIR to SSA TIR.

This change:

 - Starts adding support for statements to TIR.
   Only simple assignments are translated. Everything else is made into
   an "unimplemented" statement.

 - Does the first part of the SSA conversion, namely:
   * Computes the dominance frontier.
   * Inserts PHI nodes in the right place and for the correct TIR variables.

So what we get is a CFG with PHIs, but which is not yet in SSA. The
remaining step is to walk the dominator tree and assign each definition
of a variable with a new SSA variable name.

Manual inspection of CFG graphs with assignments and PHIs shown
look good, but tests will follow later.

Author: vext01

.buildbot.sh

@@ -5,6 +5,9 @@
 ./x.py clean  # We don't clone afresh to save time and bandwidth.
 git clean -dffx # If upstream removes a submodule, remove the files from disk.
 
+# Ensure the build fails if it uses excessive amounts of memory.
+ulimit -d $((1024 * 1024 * 8)) # 8 GiB
+
 # Note that the gdb must be Python enabled.
-PATH=/opt/gdb-8.2/bin:${PATH} RUST_BACKTRACE=1 YK_DEBUG_SECTIONS=1 \
-    ./x.py test --config .buildbot.toml
+/usr/bin/time -v env PATH=/opt/gdb-8.2/bin:${PATH} \
+    RUST_BACKTRACE=1 ./x.py test --config .buildbot.toml

Cargo.lock

@@ -3062,8 +3062,10 @@ version = "0.0.0"
 name = "rustc_yk_sections"
 version = "0.0.0"
 dependencies = [
+ "log 0.4.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc 0.0.0",
  "rustc_codegen_utils 0.0.0",
+ "rustc_data_structures 0.0.0",
  "rustc_yk_link 0.0.0",
  "ykpack 0.1.0 (git+https://github.com/softdevteam/ykpack)",
 ]
@@ -4044,7 +4046,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 [[package]]
 name = "ykpack"
 version = "0.1.0"
-source = "git+https://github.com/softdevteam/ykpack#e0fd6162b9522fd04b6bbb77a232986c4ea5c257"
+source = "git+https://github.com/softdevteam/ykpack#358a7403a5b8c03809f5c5b07933a6850060afa4"
 dependencies = [
  "fallible-iterator 0.1.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "rmp-serde 0.14.0 (git+https://github.com/3Hren/msgpack-rust?rev=40b3d480b20961e6eeceb416b32bcd0a3383846a)",

src/librustc_yk_sections/Cargo.toml

@@ -14,3 +14,5 @@ rustc = { path = "../librustc" }
 rustc_yk_link = { path = "../librustc_yk_link" }
 ykpack = { git = "https://github.com/softdevteam/ykpack" }
 rustc_codegen_utils = { path = "../librustc_codegen_utils" }
+rustc_data_structures = { path = "../librustc_data_structures" }
+log = "0.4.5"

src/librustc_yk_sections/lib.rs

@@ -13,5 +13,7 @@ extern crate rustc;
 extern crate rustc_yk_link;
 extern crate rustc_codegen_utils;
 extern crate ykpack;
+extern crate rustc_data_structures;
+#[macro_use] extern crate log;
 
 pub mod mir_cfg;