Add a setting to choose a register allocator algorithm to use with MachBackend;

cranelift/codegen/meta/src/shared/settings.rs

@@ -3,6 +3,36 @@ use crate::cdsl::settings::{SettingGroup, SettingGroupBuilder};
 pub(crate) fn define() -> SettingGroup {
     let mut settings = SettingGroupBuilder::new("shared");
 
+    settings.add_enum(
+        "regalloc",
+        r#"Register allocator to use with the MachInst backend.
+
+        This selects the register allocator as an option among those offered by the `regalloc.rs`
+        crate. Please report register allocation bugs to the maintainers of this crate whenever
+        possible.
+
+        Note: this only applies to target that use the MachInst backend. As of 2020-04-17, this
+        means the x86_64 backend doesn't use this yet.
+
+        Possible values:
+
+        - `backtracking` is a greedy, backtracking register allocator as implemented in
+        Spidermonkey's optimizing tier IonMonkey. It may take more time to allocate registers, but
+        it should generate better code in general, resulting in better throughput of generated
+        code.
+        - `backtracking_checked` is the backtracking allocator with additional self checks that may
+        take some time to run, and thus these checks are disabled by default.
+        - `experimental_linear_scan` is an experimental linear scan allocator. It may take less
+        time to allocate registers, but generated code's quality may be inferior. As of
+        2020-04-17, it is still experimental and it should not be used in production settings.
+    "#,
+        vec![
+            "backtracking",
+            "backtracking_checked",
+            "experimental_linear_scan",
+        ],
+    );
+
     settings.add_enum(
         "opt_level",
         r#"