Remove option to select regalloc algorithm

The main branch of Cranelift has switched to a new register allocator.
This register allocator doesn't have any alternative regalloc algorithms
unlike the old register allocator.

Author: bjorn3

src/config.rs

@@ -48,12 +48,6 @@ pub struct BackendConfig {
     /// Can be set using `-Cllvm-args=display_cg_time=...`.
     pub display_cg_time: bool,
 
-    /// The register allocator to use.
-    ///
-    /// Defaults to the value of `CG_CLIF_REGALLOC` or `backtracking` otherwise. Can be set using
-    /// `-Cllvm-args=regalloc=...`.
-    pub regalloc: String,
-
     /// Enable the Cranelift ir verifier for all compilation passes. If not set it will only run
     /// once before passing the clif ir to Cranelift for compilation.
     ///
@@ -80,8 +74,6 @@ impl Default for BackendConfig {
                 args.split(' ').map(|arg| arg.to_string()).collect()
             },
             display_cg_time: bool_env_var("CG_CLIF_DISPLAY_CG_TIME"),
-            regalloc: std::env::var("CG_CLIF_REGALLOC")
-                .unwrap_or_else(|_| "backtracking".to_string()),
             enable_verifier: cfg!(debug_assertions) || bool_env_var("CG_CLIF_ENABLE_VERIFIER"),
             disable_incr_cache: bool_env_var("CG_CLIF_DISABLE_INCR_CACHE"),
         }
@@ -101,7 +93,6 @@ impl BackendConfig {
                 match name {
                     "mode" => config.codegen_mode = value.parse()?,
                     "display_cg_time" => config.display_cg_time = parse_bool(name, value)?,
-                    "regalloc" => config.regalloc = value.to_string(),
                     "enable_verifier" => config.enable_verifier = parse_bool(name, value)?,
                     "disable_incr_cache" => config.disable_incr_cache = parse_bool(name, value)?,
                     _ => return Err(format!("Unknown option `{}`", name)),

src/lib.rs

@@ -256,8 +256,6 @@ fn build_isa(sess: &Session, backend_config: &BackendConfig) -> Box<dyn isa::Tar
 
     flags_builder.set("enable_llvm_abi_extensions", "true").unwrap();
 
-    flags_builder.set("regalloc", &backend_config.regalloc).unwrap();
-
     use rustc_session::config::OptLevel;
     match sess.opts.optimize {
         OptLevel::No => {