driver: Pick a better default C cross compiler on non-Apple Posix (#4983)

Try to pick a better default instead of `cc` when
cross-compiling. When targeting Linux on a non-Linux build machine,
for example, try to pick `cc` from:
- `<triple>-gcc`
- `<triple>-clang`
- `clang --target=<triple>`

This should be a better UX when cross-compiling with simpler build
systems like `dub` or even without one at all, offering a
out-of-the-box functional setup, on most systems.

The advantage to performing this check in the compiler code as opposed
to hardcoding a default value in the config file is that:
1. Multiple programs can be searched, instead of just one
2. The `$CC` value continues to be respected, which keeps more
advanced cross-compiling setups working (for example Meson)

Signed-off-by: Andrei Horodniceanu <[email protected]>

Author: the-horo

.github/actions/5a-android-x86/action.yml

@@ -30,5 +30,4 @@ runs:
           "${flags[@]}" \
           ANDROID_ABI="$abi" # override the one in CROSS_CMAKE_FLAGS
 
-        # TODO: append default -gcc switch (x86_64-linux-android30-clang, i686-linux-android29-clang)
         cat install/etc/ldc2.conf/55-target-android-$arch.conf

CHANGELOG.md

@@ -6,6 +6,12 @@
 - ldc2.conf can now be a directory. All the files inside it, ordered naturally, will be concatenated and treated like a big config. (#4954)
   - Running `ldc-build-runtime --installWithSuffix` now includes installing a target-specific .conf file to that directory. (#4978)
 - **Breaking change for ldc2.conf cmake generation**: The `cmake` build process now generates the `ldc2.conf` and `ldc2_install.conf` as directories. `ldc2*.conf.in` and `ADDITIONAL_DEFAULT_LDC_SWITCHES` have been removed, if you need to add switches check out `makeConfSection` in `LdcConfig.cmake`. (#4954)
+- When cross-compiling, the fallback value for the (cross) C compiler will be picked based on some heuristics.
+  The old behavior was to default to `cc`.
+  As an example, when cross-compiling for `aarch64-linux-gnu` the compilers that are checked are:
+  - `aarch64-linux-gnu-gcc`
+  - `aarch64-linux-gnu-clang`
+  - `clang --target=aarch64-linux-gnu`
 - The prebuilt arm64/universal macOS packages additionally bundle the arm64 iOS-*simulator* libraries, for out-of-the-box cross-compilation support via e.g. `-mtriple=arm64-apple-ios12.0-simulator`. (#4974)
 
 #### Platform support

driver/cpreprocessor.cpp

@@ -30,33 +30,6 @@ const char *getPathToImportc_h(Loc loc) {
   return cached;
 }
 
-const std::string &getCC(bool isMSVC,
-                         std::vector<std::string> &additional_args) {
-  static std::string cached_cc;
-  static std::vector<std::string> cached_args;
-  if (cached_cc.empty()) {
-    std::string fallback = "cc";
-    if (isMSVC) {
-#ifdef _WIN32
-      // by default, prefer clang-cl.exe (if in PATH) over cl.exe
-      // (e.g., no echoing of source filename being preprocessed to stderr)
-      auto found = llvm::sys::findProgramByName("clang-cl.exe");
-      if (found) {
-        fallback = found.get();
-      } else {
-        fallback = "cl.exe";
-      }
-#else
-      fallback = "clang-cl";
-#endif
-    }
-    cached_cc = getGcc(cached_args, fallback.c_str());
-  }
-
-  additional_args.insert(additional_args.end(), cached_args.cbegin(), cached_args.cend());
-  return cached_cc;
-}
-
 FileName getOutputPath(Loc loc, const char *csrcfile) {
   llvm::SmallString<64> buffer;
 
@@ -96,7 +69,7 @@ FileName runCPreprocessor(FileName csrcfile, Loc loc, OutBuffer &defines) {
   FileName ipath = getOutputPath(loc, csrcfile.toChars());
 
   std::vector<std::string> args;
-  const std::string &cc = getCC(isMSVC, args);
+  const std::string &cc = getCC(args);
 
   args.push_back(isMSVC ? "/std:c11" : "-std=c11");
 

driver/linker-gcc.cpp

@@ -844,7 +844,7 @@ int linkObjToBinaryGcc(llvm::StringRef outputPath,
     tool = getProgram("wasm-ld", &opts::linker);
   } else {
     argsBuilder = std::make_unique<ArgsBuilder>();
-    tool = getGcc(argsBuilder->args);
+    tool = getCC(argsBuilder->args);
   }
 
   // build arguments

driver/toobj.cpp

@@ -156,7 +156,7 @@ void codegenModule(llvm::TargetMachine &Target, llvm::Module &m,
 static void assemble(const std::string &asmpath, const std::string &objpath) {
   std::vector<std::string> args;
   std::string gcc;
-  gcc = getGcc(args);
+  gcc = getCC(args);
 
   args.push_back("-O3");
   args.push_back("-c");

driver/tool.cpp

@@ -21,6 +21,12 @@
 #include "llvm/Support/Path.h"
 #include "llvm/Target/TargetMachine.h"
 
+#if LDC_LLVM_VER >= 1600
+#include "llvm/TargetParser/Host.h"
+#else
+#include "llvm/Support/Host.h"
+#endif
+
 #ifdef _WIN32
 #include <Windows.h>
 #endif
@@ -52,6 +58,69 @@ static std::string findProgramByName(llvm::StringRef name) {
 
 //////////////////////////////////////////////////////////////////////////////
 
+namespace {
+llvm::SmallVector<std::string, 2> findCCFallback() {
+  const auto &triple = *global.params.targetTriple;
+  llvm::Triple nativeTriple{ llvm::sys::getDefaultTargetTriple() };
+  const auto isNativeBuild =
+    opts::mTargetTriple.empty() || triple.isCompatibleWith(nativeTriple);
+
+  llvm::SmallVector<llvm::SmallVector<std::string, 2>, 3> choices;
+  if (triple.isWindowsMSVCEnvironment()) {
+#ifdef _WIN32
+    // by default, prefer clang-cl.exe (if in PATH) over cl.exe
+    // (e.g., no echoing of source filename being preprocessed to stderr)
+    choices = {
+      { "clang-cl.exe" },
+      { "cl.exe" },
+    };
+#else
+    choices = { { "clang-cl" } };
+#endif
+  } else if (isNativeBuild) {
+    choices = { { "cc" } };
+  } else if (triple.isOSDarwin()) {
+    // Cross-compiling for Apple is most probably done on an Apple machine
+    choices = { { "cc" } };
+  } else {
+    const auto tripleString = triple.getTriple();
+    choices = {
+      { tripleString + "-gcc" },
+      { tripleString + "-clang" },
+      { "clang", "--target=" + tripleString },
+    };
+  }
+
+
+  const auto verbose = global.params.v.verbose;
+  const auto logCross = verbose && !isNativeBuild;
+  if (logCross) message("Trying to find a C cross-compiler");
+
+  for (auto &choice : choices) {
+    auto fullPath = findProgramByName(choice[0]);
+    if (fullPath.empty()) {
+      if (logCross) message("Did not find C cross-compiler: `%s`", choice[0].c_str());
+      continue;
+    }
+
+    if (logCross) message("Found C cross-compiler: `%s`", fullPath.c_str());
+    choice[0] = fullPath;
+    return choice;
+  }
+
+  if (isNativeBuild) {
+    error(Loc(), "could not find C compiler `%s`", choices[0][0].c_str());
+  } else {
+    error(Loc(), "could not find a C cross-compiler for this cross-compilation");
+    tip("make sure you have a compiler like `%s` installed and set $CC or pass -gcc accordingly", choices[0][0].c_str());
+  }
+
+  fatal();
+}
+} // anonymous namespace
+
+////////////////////////////////////////////////////////////////////////////////
+
 std::string getProgram(const char *fallbackName,
                        const llvm::cl::opt<std::string> *opt,
                        const char *envVar) {
@@ -76,19 +145,24 @@ std::string getProgram(const char *fallbackName,
 
 ////////////////////////////////////////////////////////////////////////////////
 
-std::string getGcc(std::vector<std::string> &additional_args,
-                   const char *fallback) {
+namespace {
+std::string getCCImpl(std::vector<std::string> &additional_args) {
+  if (!gcc.empty())
+    return getProgram(nullptr, &gcc);
+
+  std::string cc = env::get("CC");
+  if (cc.empty()) {
+    auto fallback = findCCFallback();
+    additional_args.insert(additional_args.end(), fallback.begin() + 1, fallback.end());
+    return fallback[0];
+  }
+
 #ifdef _WIN32
   // spaces in $CC are to be expected on Windows
   // (e.g., `C:\Program Files\LLVM\bin\clang-cl.exe`)
-  return getProgram(fallback, &gcc, "CC");
+  return getProgram(cc.c_str(), &gcc);
 #else
   // Posix: in case $CC contains spaces split it into a command and arguments
-  std::string cc = env::get("CC");
-  if (cc.empty())
-    return getProgram(fallback, &gcc);
-
-  // $CC is set so fallback doesn't matter anymore.
   if (cc.find(' ') == cc.npos)
     return getProgram(cc.c_str(), &gcc);
 
@@ -103,6 +177,18 @@ std::string getGcc(std::vector<std::string> &additional_args,
   return getProgram(args[0].str().c_str(), &gcc);
 #endif
 }
+} // anonymous namespace
+
+std::string getCC(std::vector<std::string> &additional_args) {
+  static std::string cachedResult;
+  static std::vector<std::string> cachedArgs;
+
+  if (cachedResult.empty())
+    cachedResult = getCCImpl(cachedArgs);
+
+  additional_args.insert(additional_args.end(), cachedArgs.begin(), cachedArgs.end());
+  return cachedResult;
+}
 
 ////////////////////////////////////////////////////////////////////////////////
 

driver/tool.h

@@ -25,8 +25,7 @@ namespace opts {
 extern llvm::cl::opt<std::string> linker;
 }
 
-std::string getGcc(std::vector<std::string> &additional_args,
-		   const char *fallback = "cc");
+std::string getCC(std::vector<std::string> &additional_args);
 void appendTargetArgsForGcc(std::vector<std::string> &args);
 
 std::string getProgram(const char *fallbackName,

tests/driver/cross_cc_fallback.d

@@ -0,0 +1,8 @@
+// RUN: env PATH= CC= not %ldc -mtriple aarch64-unknown-linux-gnut64 -v -defaultlib= %s 2>&1 | FileCheck %s
+
+// CHECK-DAG: aarch64-unknown-linux-gnut64-gcc
+// CHECK-DAG: aarch64-unknown-linux-gnut64-clang
+
+// UNSUPPORTED: Windows
+
+extern(C) void main () {}