Use the target cpu from the target spec when -Ctarget-cpu isn't used

Rather than nehalem on x86_64 and the cranelift default on other
targets.

Fixes #1148

Author: bjorn3

src/lib.rs

@@ -312,9 +312,9 @@ fn build_isa(sess: &Session, backend_config: &BackendConfig) -> Arc<dyn TargetIs
 
     let flags = settings::Flags::new(flags_builder);
 
-    let isa_builder = match sess.opts.cg.target_cpu.as_deref() {
-        Some("native") => cranelift_native::builder_with_options(true).unwrap(),
-        Some(value) => {
+    let isa_builder = match sess.opts.cg.target_cpu.as_deref().unwrap_or(sess.target.cpu.as_ref()) {
+        "native" => cranelift_native::builder_with_options(true).unwrap(),
+        value => {
             let mut builder =
                 cranelift_codegen::isa::lookup(target_triple.clone()).unwrap_or_else(|err| {
                     sess.dcx().fatal(format!("can't compile for {}: {}", target_triple, err));
@@ -325,18 +325,6 @@ fn build_isa(sess: &Session, backend_config: &BackendConfig) -> Arc<dyn TargetIs
             }
             builder
         }
-        None => {
-            let mut builder =
-                cranelift_codegen::isa::lookup(target_triple.clone()).unwrap_or_else(|err| {
-                    sess.dcx().fatal(format!("can't compile for {}: {}", target_triple, err));
-                });
-            if target_triple.architecture == target_lexicon::Architecture::X86_64 {
-                // Don't use "haswell" as the default, as it implies `has_lzcnt`.
-                // macOS CI is still at Ivy Bridge EP, so `lzcnt` is interpreted as `bsr`.
-                builder.enable("nehalem").unwrap();
-            }
-            builder
-        }
     };
 
     match isa_builder.finish(flags) {