rustc: Add -C lto=val option

This commit primarily adds the ability to control what kind of LTO happens when
rustc performs LTO, namely allowing values to be specified to the `-C lto`
option, such as `-C lto=thin` and `-C lto=fat`. (where "fat" is the previous
kind of LTO, throw everything in one giant module)

Along the way this also refactors a number of fields which store information
about whether LTO/ThinLTO are enabled to unify them all into one field through
which everything is dispatched, hopefully removing a number of special cases
throughout.

This is intended to help mitigate #47409 but will require a backport as well,
and this would unfortunately need to be an otherwise insta-stable option.

src/librustc/session/config.rs

@@ -72,6 +72,26 @@ pub enum OptLevel {
     SizeMin, // -Oz
 }
 
+#[derive(Clone, Copy, PartialEq, Hash)]
+pub enum Lto {
+    /// Don't do any LTO whatsoever
+    No,
+
+    /// Do a full crate graph LTO. The flavor is determined by the compiler
+    /// (currently the default is "fat").
+    Yes,
+
+    /// Do a full crate graph LTO with ThinLTO
+    Thin,
+
+    /// Do a local graph LTO with ThinLTO (only relevant for multiple codegen
+    /// units).
+    ThinLocal,
+
+    /// Do a full crate graph LTO with "fat" LTO
+    Fat,
+}
+
 #[derive(Clone, Copy, PartialEq, Hash)]
 pub enum DebugInfoLevel {
     NoDebugInfo,
@@ -389,7 +409,7 @@ top_level_options!(
         // commands like `--emit llvm-ir` which they're often incompatible with
         // if we otherwise use the defaults of rustc.
         cli_forced_codegen_units: Option<usize> [UNTRACKED],
-        cli_forced_thinlto: Option<bool> [UNTRACKED],
+        cli_forced_thinlto_off: bool [UNTRACKED],
     }
 );
 
@@ -590,7 +610,7 @@ pub fn basic_options() -> Options {
         debug_assertions: true,
         actually_rustdoc: false,
         cli_forced_codegen_units: None,
-        cli_forced_thinlto: None,
+        cli_forced_thinlto_off: false,
     }
 }
 
@@ -780,11 +800,13 @@ macro_rules! options {
             Some("crate=integer");
         pub const parse_unpretty: Option<&'static str> =
             Some("`string` or `string=string`");
+        pub const parse_lto: Option<&'static str> =
+            Some("one of `thin`, `fat`, or omitted");
     }
 
     #[allow(dead_code)]
     mod $mod_set {
-        use super::{$struct_name, Passes, SomePasses, AllPasses, Sanitizer};
+        use super::{$struct_name, Passes, SomePasses, AllPasses, Sanitizer, Lto};
         use rustc_back::{LinkerFlavor, PanicStrategy, RelroLevel};
         use std::path::PathBuf;
 
@@ -978,6 +1000,16 @@ macro_rules! options {
                 _ => false,
             }
         }
+
+        fn parse_lto(slot: &mut Lto, v: Option<&str>) -> bool {
+            *slot = match v {
+                None => Lto::Yes,
+                Some("thin") => Lto::Thin,
+                Some("fat") => Lto::Fat,
+                Some(_) => return false,
+            };
+            true
+        }
     }
 ) }
 
@@ -994,7 +1026,7 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
         "extra arguments to append to the linker invocation (space separated)"),
     link_dead_code: bool = (false, parse_bool, [UNTRACKED],
         "don't let linker strip dead code (turning it on can be used for code coverage)"),
-    lto: bool = (false, parse_bool, [TRACKED],
+    lto: Lto = (Lto::No, parse_lto, [TRACKED],
         "perform LLVM link-time optimizations"),
     target_cpu: Option<String> = (None, parse_opt_string, [TRACKED],
         "select target processor (rustc --print target-cpus for details)"),
@@ -1677,7 +1709,7 @@ pub fn build_session_options_and_crate_config(matches: &getopts::Matches)
 
     let mut cg = build_codegen_options(matches, error_format);
     let mut codegen_units = cg.codegen_units;
-    let mut thinlto = None;
+    let mut disable_thinlto = false;
 
     // Issue #30063: if user requests llvm-related output to one
     // particular path, disable codegen-units.
@@ -1699,12 +1731,12 @@ pub fn build_session_options_and_crate_config(matches: &getopts::Matches)
                     }
                     early_warn(error_format, "resetting to default -C codegen-units=1");
                     codegen_units = Some(1);
-                    thinlto = Some(false);
+                    disable_thinlto = true;
                 }
             }
             _ => {
                 codegen_units = Some(1);
-                thinlto = Some(false);
+                disable_thinlto = true;
             }
         }
     }
@@ -1734,7 +1766,7 @@ pub fn build_session_options_and_crate_config(matches: &getopts::Matches)
         (&None, &None) => None,
     }.map(|m| PathBuf::from(m));
 
-    if cg.lto && incremental.is_some() {
+    if cg.lto != Lto::No && incremental.is_some() {
         early_error(error_format, "can't perform LTO when compiling incrementally");
     }
 
@@ -1934,7 +1966,7 @@ pub fn build_session_options_and_crate_config(matches: &getopts::Matches)
         debug_assertions,
         actually_rustdoc: false,
         cli_forced_codegen_units: codegen_units,
-        cli_forced_thinlto: thinlto,
+        cli_forced_thinlto_off: disable_thinlto,
     },
     cfg)
 }
@@ -2052,7 +2084,7 @@ mod dep_tracking {
     use std::hash::Hash;
     use std::path::PathBuf;
     use std::collections::hash_map::DefaultHasher;
-    use super::{Passes, CrateType, OptLevel, DebugInfoLevel,
+    use super::{Passes, CrateType, OptLevel, DebugInfoLevel, Lto,
                 OutputTypes, Externs, ErrorOutputType, Sanitizer};
     use syntax::feature_gate::UnstableFeatures;
     use rustc_back::{PanicStrategy, RelroLevel};
@@ -2107,6 +2139,7 @@ mod dep_tracking {
     impl_dep_tracking_hash_via_hash!(RelroLevel);
     impl_dep_tracking_hash_via_hash!(Passes);
     impl_dep_tracking_hash_via_hash!(OptLevel);
+    impl_dep_tracking_hash_via_hash!(Lto);
     impl_dep_tracking_hash_via_hash!(DebugInfoLevel);
     impl_dep_tracking_hash_via_hash!(UnstableFeatures);
     impl_dep_tracking_hash_via_hash!(Externs);
@@ -2180,6 +2213,7 @@ mod tests {
     use lint;
     use middle::cstore;
     use session::config::{build_configuration, build_session_options_and_crate_config};
+    use session::config::Lto;
     use session::build_session;
     use std::collections::{BTreeMap, BTreeSet};
     use std::iter::FromIterator;
@@ -2656,7 +2690,7 @@ mod tests {
 
         // Make sure changing a [TRACKED] option changes the hash
         opts = reference.clone();
-        opts.cg.lto = true;
+        opts.cg.lto = Lto::Fat;
         assert!(reference.dep_tracking_hash() != opts.dep_tracking_hash());
 
         opts = reference.clone();

src/librustc/session/mod.rs

@@ -498,9 +498,65 @@ impl Session {
             self.use_mir()
     }
 
-    pub fn lto(&self) -> bool {
-        self.opts.cg.lto || self.target.target.options.requires_lto
+    /// Calculates the flavor of LTO to use for this compilation.
+    pub fn lto(&self) -> config::Lto {
+        // If our target has codegen requirements ignore the command line
+        if self.target.target.options.requires_lto {
+            return config::Lto::Fat
+        }
+
+        // If the user specified something, return that. If they only said `-C
+        // lto` and we've for whatever reason forced off ThinLTO via the CLI,
+        // then ensure we can't use a ThinLTO.
+        match self.opts.cg.lto {
+            config::Lto::No => {}
+            config::Lto::Yes if self.opts.cli_forced_thinlto_off => {
+                return config::Lto::Fat
+            }
+            other => return other,
+        }
+
+        // Ok at this point the target doesn't require anything and the user
+        // hasn't asked for anything. Our next decision is whether or not
+        // we enable "auto" ThinLTO where we use multiple codegen units and
+        // then do ThinLTO over those codegen units. The logic below will
+        // either return `No` or `ThinLocal`.
+
+        // If processing command line options determined that we're incompatible
+        // with ThinLTO (e.g. `-C lto --emit llvm-ir`) then return that option.
+        if self.opts.cli_forced_thinlto_off {
+            return config::Lto::No
+        }
+
+        // If `-Z thinlto` specified process that, but note that this is mostly
+        // a deprecated option now that `-C lto=thin` exists.
+        if let Some(enabled) = self.opts.debugging_opts.thinlto {
+            if enabled {
+                return config::Lto::ThinLocal
+            } else {
+                return config::Lto::No
+            }
+        }
+
+        // If there's only one codegen unit and LTO isn't enabled then there's
+        // no need for ThinLTO so just return false.
+        if self.codegen_units() == 1 {
+            return config::Lto::No
+        }
+
+        // Right now ThinLTO isn't compatible with incremental compilation.
+        if self.opts.incremental.is_some() {
+            return config::Lto::No
+        }
+
+        // Now we're in "defaults" territory. By default we enable ThinLTO for
+        // optimized compiles (anything greater than O0).
+        match self.opts.optimize {
+            config::OptLevel::No => config::Lto::No,
+            _ => config::Lto::ThinLocal,
+        }
     }
+
     /// Returns the panic strategy for this compile session. If the user explicitly selected one
     /// using '-C panic', use that, otherwise use the panic strategy defined by the target.
     pub fn panic_strategy(&self) -> PanicStrategy {
@@ -804,38 +860,6 @@ impl Session {
         // scientific.
         16
     }
-
-    /// Returns whether ThinLTO is enabled for this compilation
-    pub fn thinlto(&self) -> bool {
-        // If processing command line options determined that we're incompatible
-        // with ThinLTO (e.g. `-C lto --emit llvm-ir`) then return that option.
-        if let Some(enabled) = self.opts.cli_forced_thinlto {
-            return enabled
-        }
-
-        // If explicitly specified, use that with the next highest priority
-        if let Some(enabled) = self.opts.debugging_opts.thinlto {
-            return enabled
-        }
-
-        // If there's only one codegen unit and LTO isn't enabled then there's
-        // no need for ThinLTO so just return false.
-        if self.codegen_units() == 1 && !self.lto() {
-            return false
-        }
-
-        // Right now ThinLTO isn't compatible with incremental compilation.
-        if self.opts.incremental.is_some() {
-            return false
-        }
-
-        // Now we're in "defaults" territory. By default we enable ThinLTO for
-        // optimized compiles (anything greater than O0).
-        match self.opts.optimize {
-            config::OptLevel::No => false,
-            _ => true,
-        }
-    }
 }
 
 pub fn build_session(sopts: config::Options,

src/librustc_trans/back/link.rs

@@ -16,7 +16,7 @@ use super::rpath::RPathConfig;
 use super::rpath;
 use metadata::METADATA_FILENAME;
 use rustc::session::config::{self, NoDebugInfo, OutputFilenames, OutputType, PrintRequest};
-use rustc::session::config::RUST_CGU_EXT;
+use rustc::session::config::{RUST_CGU_EXT, Lto};
 use rustc::session::filesearch;
 use rustc::session::search_paths::PathKind;
 use rustc::session::Session;
@@ -503,7 +503,8 @@ fn link_staticlib(sess: &Session,
         });
         ab.add_rlib(path,
                     &name.as_str(),
-                    sess.lto() && !ignored_for_lto(sess, &trans.crate_info, cnum),
+                    is_full_lto_enabled(sess) &&
+                        !ignored_for_lto(sess, &trans.crate_info, cnum),
                     skip_object_files).unwrap();
 
         all_native_libs.extend(trans.crate_info.native_libraries[&cnum].iter().cloned());
@@ -1211,7 +1212,8 @@ fn add_upstream_rust_crates(cmd: &mut Linker,
             lib.kind == NativeLibraryKind::NativeStatic && !relevant_lib(sess, lib)
         });
 
-        if (!sess.lto() || ignored_for_lto(sess, &trans.crate_info, cnum)) &&
+        if (!is_full_lto_enabled(sess) ||
+            ignored_for_lto(sess, &trans.crate_info, cnum)) &&
            crate_type != config::CrateTypeDylib &&
            !skip_native {
             cmd.link_rlib(&fix_windows_verbatim_for_gcc(cratepath));
@@ -1264,7 +1266,7 @@ fn add_upstream_rust_crates(cmd: &mut Linker,
                 // file, then we don't need the object file as it's part of the
                 // LTO module. Note that `#![no_builtins]` is excluded from LTO,
                 // though, so we let that object file slide.
-                let skip_because_lto = sess.lto() &&
+                let skip_because_lto = is_full_lto_enabled(sess) &&
                     is_rust_object &&
                     (sess.target.target.options.no_builtins ||
                      !trans.crate_info.is_no_builtins.contains(&cnum));
@@ -1301,7 +1303,7 @@ fn add_upstream_rust_crates(cmd: &mut Linker,
     fn add_dynamic_crate(cmd: &mut Linker, sess: &Session, cratepath: &Path) {
         // If we're performing LTO, then it should have been previously required
         // that all upstream rust dependencies were available in an rlib format.
-        assert!(!sess.lto());
+        assert!(!is_full_lto_enabled(sess));
 
         // Just need to tell the linker about where the library lives and
         // what its name is
@@ -1409,3 +1411,13 @@ fn link_binaryen(sess: &Session,
                             e));
     }
 }
+
+fn is_full_lto_enabled(sess: &Session) -> bool {
+    match sess.lto() {
+        Lto::Yes |
+        Lto::Thin |
+        Lto::Fat => true,
+        Lto::No |
+        Lto::ThinLocal => false,
+    }
+}