Add support for --mcpu native

man/crystal.1

@@ -129,6 +129,7 @@ Dump LLVM assembly file to output directory.
 Pass additional flags to the linker. Though you can specify those flags on the source code, this is useful for passing environment specific information directly to the linker, like non-standard library paths or names. For more information on specifying linker flags on source, you can read the "C bindings" section of the documentation available on the official web site.
 .It Fl -mcpu Ar CPU
 Specify a specific CPU to generate code for. This will pass a -mcpu flag to LLVM, and is only intended to be used for cross-compilation. For a list of available CPUs, invoke "llvm-as < /dev/null | llc -march=xyz -mcpu=help".
+Passing --mcpu native will pass the host CPU name to tune performance for the host.
 .It Fl -mattr Ar CPU
 Override or control specific attributes of the target, such as whether SIMD operations are enabled or not. The default set of attributes is set by the current CPU. This will pass a -mattr flag to LLVM, and is only intended to be used for cross-compilation. For a list of available attributes, invoke "llvm-as < /dev/null | llc -march=xyz -mattr=help".
 .It Fl -mcmodel default|kernel|small|medium|large

src/compiler/crystal/command.cr

@@ -387,7 +387,11 @@ class Crystal::Command
           link_flags << some_link_flags
         end
         opts.on("--mcpu CPU", "Target specific cpu type") do |cpu|
-          compiler.mcpu = cpu
+          if cpu == "native"
+            compiler.mcpu = LLVM.host_cpu_name
+          else
+            compiler.mcpu = cpu
+          end
         end
         opts.on("--mattr CPU", "Target specific features") do |features|
           compiler.mattr = features

src/llvm.cr

@@ -83,6 +83,10 @@ module LLVM
     end
   end
 
+  def self.host_cpu_name : String
+    String.new LibLLVM.get_host_cpu_name
+  end
+
   def self.normalize_triple(triple : String)
     normalized = LibLLVMExt.normalize_target_triple(triple)
     normalized = LLVM.string_and_dispose(normalized)

src/llvm/lib_llvm.cr

@@ -183,6 +183,7 @@ lib LibLLVM
   fun get_named_function = LLVMGetNamedFunction(mod : ModuleRef, name : UInt8*) : ValueRef
   fun get_named_global = LLVMGetNamedGlobal(mod : ModuleRef, name : UInt8*) : ValueRef
   fun get_count_params = LLVMCountParams(fn : ValueRef) : UInt
+  fun get_host_cpu_name = LLVMGetHostCPUName : UInt8*
   fun get_param = LLVMGetParam(fn : ValueRef, index : Int32) : ValueRef
   fun get_param_types = LLVMGetParamTypes(function_type : TypeRef, dest : TypeRef*)
   fun get_params = LLVMGetParams(fn : ValueRef, params : ValueRef*)