auto merge of #15698 : Zoxc/rust/code-model, r=alexcrichton

The default code model is usually unsuitable for kernels,
so we add an option to specify which model we want.

Testing for this would be fragile and very architecture specific and is better left to LLVM.

src/librustc/back/link.rs

@@ -186,6 +186,22 @@ pub mod write {
                 }
             };
 
+            let code_model = match sess.opts.cg.code_model.as_slice() {
+                "default" => llvm::CodeModelDefault,
+                "small" => llvm::CodeModelSmall,
+                "kernel" => llvm::CodeModelKernel,
+                "medium" => llvm::CodeModelMedium,
+                "large" => llvm::CodeModelLarge,
+                _ => {
+                    sess.err(format!("{} is not a valid code model",
+                                     sess.opts
+                                         .cg
+                                         .code_model).as_slice());
+                    sess.abort_if_errors();
+                    return;
+                }
+            };
+
             let tm = sess.targ_cfg
                          .target_strs
                          .target_triple
@@ -195,7 +211,7 @@ pub mod write {
                     target_feature(sess).with_c_str(|features| {
                         llvm::LLVMRustCreateTargetMachine(
                             t, cpu, features,
-                            llvm::CodeModelDefault,
+                            code_model,
                             reloc_model,
                             opt_level,
                             true /* EnableSegstk */,

src/librustc/driver/config.rs

@@ -336,6 +336,8 @@ cgoptions!(
         "disable the use of the redzone"),
     relocation_model: String = ("pic".to_string(), parse_string,
          "choose the relocation model to use (llc -relocation-model for details)"),
+    code_model: String = ("default".to_string(), parse_string,
+         "choose the code model to use (llc -code-model for details)"),
     metadata: Vec<String> = (Vec::new(), parse_list,
          "metadata to mangle symbol names with"),
     extra_filename: String = ("".to_string(), parse_string,