Categorize and explain target features support

Author: mattico

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2128,7 +2128,13 @@ extern "C" {
     pub fn LLVMRustHasFeature(T: &TargetMachine, s: *const c_char) -> bool;
 
     pub fn LLVMRustPrintTargetCPUs(T: &TargetMachine);
-    pub fn LLVMRustPrintTargetFeatures(T: &TargetMachine);
+    pub fn LLVMRustGetTargetFeaturesCount(T: &TargetMachine) -> size_t;
+    pub fn LLVMRustGetTargetFeature(
+        T: &TargetMachine,
+        Index: size_t,
+        Feature: &mut *const c_char,
+        Desc: &mut *const c_char,
+    );
 
     pub fn LLVMRustGetHostCPUName(len: *mut usize) -> *const c_char;
     pub fn LLVMRustCreateTargetMachine(

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -10,6 +10,7 @@ use rustc_span::symbol::Symbol;
 use rustc_target::spec::{MergeFunctions, PanicStrategy};
 use std::ffi::{CStr, CString};
 
+use std::ptr;
 use std::slice;
 use std::str;
 use std::sync::atomic::{AtomicBool, Ordering};
@@ -192,15 +193,77 @@ pub fn print_passes() {
     }
 }
 
+fn llvm_target_features(tm: &llvm::TargetMachine) -> Vec<(&str, &str)> {
+    let len = unsafe { llvm::LLVMRustGetTargetFeaturesCount(tm) };
+    let mut ret = Vec::with_capacity(len);
+    for i in 0..len {
+        unsafe {
+            let mut feature = ptr::null();
+            let mut desc = ptr::null();
+            llvm::LLVMRustGetTargetFeature(tm, i, &mut feature, &mut desc);
+            if feature.is_null() || desc.is_null() {
+                bug!("LLVM returned a `null` target feature string");
+            }
+            let feature = CStr::from_ptr(feature).to_str().unwrap_or_else(|e| {
+                bug!("LLVM returned a non-utf8 feature string: {}", e);
+            });
+            let desc = CStr::from_ptr(desc).to_str().unwrap_or_else(|e| {
+                bug!("LLVM returned a non-utf8 feature string: {}", e);
+            });
+            ret.push((feature, desc));
+        }
+    }
+    ret
+}
+
+fn print_target_features(sess: &Session, tm: &llvm::TargetMachine) {
+    let mut target_features = llvm_target_features(tm);
+    let mut rustc_target_features = supported_target_features(sess)
+        .iter()
+        .filter_map(|(feature, _gate)| {
+            let llvm_feature = to_llvm_feature(sess, *feature);
+            // LLVM asserts that these are sorted. LLVM and Rust both use byte comparison for these strings.
+            target_features.binary_search_by_key(&llvm_feature, |(f, _d)| *f).ok().map(|index| {
+                let (_f, desc) = target_features.remove(index);
+                (*feature, desc)
+            })
+        })
+        .collect::<Vec<_>>();
+    rustc_target_features.extend_from_slice(&[(
+        "crt-static",
+        "Enables C Run-time Libraries to be statically linked",
+    )]);
+    let max_feature_len = target_features
+        .iter()
+        .chain(rustc_target_features.iter())
+        .map(|(feature, _desc)| feature.len())
+        .max()
+        .unwrap_or(0);
+
+    println!("Features supported by rustc for this target:");
+    for (feature, desc) in &rustc_target_features {
+        println!("    {1:0$} - {2}.", max_feature_len, feature, desc);
+    }
+    println!("\nCode-generation features supported by LLVM for this target:");
+    for (feature, desc) in &target_features {
+        println!("    {1:0$} - {2}.", max_feature_len, feature, desc);
+    }
+    if target_features.len() == 0 {
+        println!("    Target features listing is not supported by this LLVM version.");
+    }
+    println!("\nUse +feature to enable a feature, or -feature to disable it.");
+    println!("For example, rustc -C target-cpu=mycpu -C target-feature=+feature1,-feature2\n");
+    println!("Code-generation features cannot be used in cfg or #[target_feature],");
+    println!("and may be renamed or removed in a future version of LLVM or rustc.\n");
+}
+
 pub(crate) fn print(req: PrintRequest, sess: &Session) {
     require_inited();
     let tm = create_informational_target_machine(sess);
-    unsafe {
-        match req {
-            PrintRequest::TargetCPUs => llvm::LLVMRustPrintTargetCPUs(tm),
-            PrintRequest::TargetFeatures => llvm::LLVMRustPrintTargetFeatures(tm),
-            _ => bug!("rustc_codegen_llvm can't handle print request: {:?}", req),
-        }
+    match req {
+        PrintRequest::TargetCPUs => unsafe { llvm::LLVMRustPrintTargetCPUs(tm) },
+        PrintRequest::TargetFeatures => print_target_features(sess, tm),
+        _ => bug!("rustc_codegen_llvm can't handle print request: {:?}", req),
     }
 }
 

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -404,26 +404,21 @@ extern "C" void LLVMRustPrintTargetCPUs(LLVMTargetMachineRef TM) {
   printf("\n");
 }
 
-extern "C" void LLVMRustPrintTargetFeatures(LLVMTargetMachineRef TM) {
+extern "C" size_t LLVMRustGetTargetFeaturesCount(LLVMTargetMachineRef TM) {
   const TargetMachine *Target = unwrap(TM);
   const MCSubtargetInfo *MCInfo = Target->getMCSubtargetInfo();
   const ArrayRef<SubtargetFeatureKV> FeatTable = MCInfo->getFeatureTable();
-  unsigned MaxFeatLen = getLongestEntryLength(FeatTable);
-
-  printf("Available features for this target:\n");
-  for (auto &Feature : FeatTable)
-    printf("    %-*s - %s.\n", MaxFeatLen, Feature.Key, Feature.Desc);
-  printf("\nRust-specific features:\n");
-  printf("    %-*s - %s.\n",
-    MaxFeatLen,
-    "crt-static",
-    "Enables libraries with C Run-time Libraries(CRT) to be statically linked"
-  );
-  printf("\n");
+  return FeatTable.size();
+}
 
-  printf("Use +feature to enable a feature, or -feature to disable it.\n"
-         "For example, rustc -C -target-cpu=mycpu -C "
-         "target-feature=+feature1,-feature2\n\n");
+extern "C" void LLVMRustGetTargetFeature(LLVMTargetMachineRef TM, size_t Index,
+                                         const char** Feature, const char** Desc) {
+  const TargetMachine *Target = unwrap(TM);
+  const MCSubtargetInfo *MCInfo = Target->getMCSubtargetInfo();
+  const ArrayRef<SubtargetFeatureKV> FeatTable = MCInfo->getFeatureTable();
+  const SubtargetFeatureKV Feat = FeatTable[Index];
+  *Feature = Feat.Key;
+  *Desc = Feat.Desc;
 }
 
 #else
@@ -432,9 +427,11 @@ extern "C" void LLVMRustPrintTargetCPUs(LLVMTargetMachineRef) {
   printf("Target CPU help is not supported by this LLVM version.\n\n");
 }
 
-extern "C" void LLVMRustPrintTargetFeatures(LLVMTargetMachineRef) {
-  printf("Target features help is not supported by this LLVM version.\n\n");
+extern "C" size_t LLVMRustGetTargetFeaturesCount(LLVMTargetMachineRef) {
+  return 0;
 }
+
+extern "C" void LLVMRustGetTargetFeature(LLVMTargetMachineRef, const char**, const char**) {}
 #endif
 
 extern "C" const char* LLVMRustGetHostCPUName(size_t *len) {