[lld] Infer the driver mode from the target.

[danalbert]

Summary

This is a largely untested work in progress as an alternative to
https://reviews.llvm.org/D76452. If we decide to go this route I'll
add the missing tests :)

[MaskRay]

lld/tools/lld/lld.cpp

134

This checks -m for lld-link, wasm-ld and ld64 which not be great.

We can place the check inside:

#if __APPLE__
  // Use Darwin driver for "ld" on Darwin.
  if (progname == "ld")
    return Darwin;
#endif

[MaskRay]

@ruiu @int3 If we can't really get rid of the #if __APPLE__ block, I would hope we can use

#if __APPLE__
  // Use Darwin driver for "ld" on Darwin.
  if (progname == "ld") {
    if (targetMatches(v, [](StringRef s) { ... }) return Gnu;
    return Darwin;
  }
#endif

This should address the Android folks' problem that ld on Darwin acts as a Mach-O linker (instead of an ELF linker). The approach seems cleaner than D76452.

-m is an ld64 obsoleted option.

-m          Don't treat multiple definitions as an error.  This is no
               longer supported. This option is obsolete.

[danalbert]

lld/tools/lld/lld.cpp

134

Okay, if we don't want this behavior to affect non-Android targets I think the Clang patch is the right answer.

Although removing the Darwin case also SGTM.

[MaskRay]

We went with D78837. This can be closed.

lld/tools/lld/lld.cpp

if (Triple(sys::getProcessTriple()).getOS() == Triple::Win32)
     return cl::TokenizeWindowsCommandLine;
   return cl::TokenizeGNUCommandLine;
 }
 
 static bool isPETargetName(StringRef s) {
   return s == "i386pe" || s == "i386pep" || s == "thumb2pe" || s == "arm64pe";
 }
 
-static bool isPETarget(std::vector<const char *> &v) {
+static bool targetMatches(std::vector<const char *> &v,
+                          std::function<bool(StringRef)> targetFilter) {
   for (auto it = v.begin(); it + 1 != v.end(); ++it) {
     if (StringRef(*it) != "-m")
       continue;
-    return isPETargetName(*(it + 1));
+    return targetFilter(*(it + 1));
   }
   // Expand response files (arguments in the form of @<filename>)
   // to allow detecting the -m argument from arguments in them.
   SmallVector<const char *, 256> expandedArgs(v.data(), v.data() + v.size());
   cl::ExpandResponseFiles(saver, getDefaultQuotingStyle(), expandedArgs);
   for (auto it = expandedArgs.begin(); it + 1 != expandedArgs.end(); ++it) {
     if (StringRef(*it) != "-m")
       continue;
-    return isPETargetName(*(it + 1));
+    return targetFilter(*(it + 1));
   }
   return false;
 }
 
 static Flavor parseProgname(StringRef progname) {
 #if __APPLE__
   // Use Darwin driver for "ld" on Darwin.
   if (progname == "ld")
if (v.size() <= 2)
       die("missing arg value for '-flavor'");
     Flavor f = getFlavor(v[2]);
     if (f == Invalid)
       die("Unknown flavor: " + StringRef(v[2]));
     v.erase(v.begin() + 1, v.begin() + 3);
     return f;
   }
 
+  // Deduce the flavor from the target.
+  if (targetMatches(v, [](StringRef s) {
+        return s.contains("linux") || s.contains("elf");
+      })) {
+    return Gnu;
+  }
+
   // Deduct the flavor from argv[0].
   StringRef arg0 = path::filename(v[0]);
   if (arg0.endswith_lower(".exe"))
     arg0 = arg0.drop_back(4);
   return parseProgname(arg0);
 }
 
 // If this function returns true, lld calls _exit() so that it quickly
 // exits without invoking destructors of globally allocated objects.
 //
 // We don't want to do that if we are running tests though, because
 // doing that breaks leak sanitizer. So, lit sets this environment variable,
 // and we use it to detect whether we are running tests or not.
 static bool canExitEarly() { return StringRef(getenv("LLD_IN_TEST")) != "1"; }
 
 /// Universal linker main(). This linker emulates the gnu, darwin, or
 /// windows linker based on the argv[0] or -flavor option.
 int main(int argc, const char **argv) {
   InitLLVM x(argc, argv);
 
   std::vector<const char *> args(argv, argv + argc);
   switch (parseFlavor(args)) {
   case Gnu:
-    if (isPETarget(args))
+    if (targetMatches(args, isPETargetName))
       return !mingw::link(args, canExitEarly(), llvm::outs(), llvm::errs());
     return !elf::link(args, canExitEarly(), llvm::outs(), llvm::errs());
   case WinLink:
     return !coff::link(args, canExitEarly(), llvm::outs(), llvm::errs());
   case Darwin:
     return !mach_o::link(args, canExitEarly(), llvm::outs(), llvm::errs());
   case DarwinNew:
     return !macho::link(args, canExitEarly(), llvm::outs(), llvm::errs());
   case Wasm:
     return !wasm::link(args, canExitEarly(), llvm::outs(), llvm::errs());
   default:
     die("lld is a generic driver.\n"
         "Invoke ld.lld (Unix), ld64.lld (macOS), lld-link (Windows), wasm-ld"
         " (WebAssembly) instead");
   }
 }