Added default target cpu to --print target-cpus output and updated docs

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2249,7 +2249,7 @@ extern "C" {
 
     pub fn LLVMRustHasFeature(T: &TargetMachine, s: *const c_char) -> bool;
 
-    pub fn LLVMRustPrintTargetCPUs(T: &TargetMachine);
+    pub fn LLVMRustPrintTargetCPUs(T: &TargetMachine, cpu: *const c_char);
     pub fn LLVMRustGetTargetFeaturesCount(T: &TargetMachine) -> size_t;
     pub fn LLVMRustGetTargetFeature(
         T: &TargetMachine,

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -329,7 +329,14 @@ pub(crate) fn print(req: PrintRequest, sess: &Session) {
     require_inited();
     let tm = create_informational_target_machine(sess);
     match req {
-        PrintRequest::TargetCPUs => unsafe { llvm::LLVMRustPrintTargetCPUs(tm) },
+        PrintRequest::TargetCPUs => {
+            // SAFETY generate a C compatible string from a byte slice to pass
+            // the target CPU name into LLVM, the lifetime of the reference is
+            // at least as long as the C function
+            let cpu_cstring = CString::new(handle_native(sess.target.cpu.as_ref()))
+                .unwrap_or_else(|e| bug!("failed to convert to cstring: {}", e));
+            unsafe { llvm::LLVMRustPrintTargetCPUs(tm, cpu_cstring.as_ptr()) };
+        }
         PrintRequest::TargetFeatures => print_target_features(sess, tm),
         _ => bug!("rustc_codegen_llvm can't handle print request: {:?}", req),
     }

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -307,7 +307,7 @@ static size_t getLongestEntryLength(ArrayRef<KV> Table) {
   return MaxLen;
 }
 
-extern "C" void LLVMRustPrintTargetCPUs(LLVMTargetMachineRef TM) {
+extern "C" void LLVMRustPrintTargetCPUs(LLVMTargetMachineRef TM, const char* TargetCPU) {
   const TargetMachine *Target = unwrap(TM);
   const MCSubtargetInfo *MCInfo = Target->getMCSubtargetInfo();
   const Triple::ArchType HostArch = Triple(sys::getDefaultTargetTriple()).getArch();
@@ -323,9 +323,18 @@ extern "C" void LLVMRustPrintTargetCPUs(LLVMTargetMachineRef TM) {
     printf("    %-*s - Select the CPU of the current host (currently %.*s).\n",
       MaxCPULen, "native", (int)HostCPU.size(), HostCPU.data());
   }
-  for (auto &CPU : CPUTable)
-    printf("    %-*s\n", MaxCPULen, CPU.Key);
-  printf("\n");
+  for (auto &CPU : CPUTable) {
+    // Compare cpu against current target to label the default
+    if (strcmp(CPU.Key, TargetCPU) == 0) {
+      printf("    %-*s - This is the default target CPU"
+      " for the current build target (currently %s).",
+        MaxCPULen, CPU.Key, Target->getTargetTriple().str().c_str());
+    }
+    else {
+      printf("    %-*s", MaxCPULen, CPU.Key);
+    }
+    printf("\n");
+  }
 }
 
 extern "C" size_t LLVMRustGetTargetFeaturesCount(LLVMTargetMachineRef TM) {

src/doc/rustc/src/codegen-options/index.md

@@ -574,7 +574,8 @@ change in the future.
 This instructs `rustc` to generate code specifically for a particular processor.
 
 You can run `rustc --print target-cpus` to see the valid options to pass
-here. Each target has a default base CPU. Special values include:
+and the default target CPU for the current buid target.
+Each target has a default base CPU. Special values include:
 
 * `native` can be passed to use the processor of the host machine.
 * `generic` refers to an LLVM target with minimal features but modern tuning.