Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -25,6 +25,7 @@
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Path.h"
 #include "llvm/Support/TargetSelect.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cstdlib>
@@ -59,6 +60,10 @@
   return !HasError;
 }
 
+static std::string getOutputName() {
+  return sys::path::filename(Config->OutputFile);
+}
+
 // Parses a linker -m option.
 static std::tuple<ELFKind, uint16_t, uint8_t> parseEmulation(StringRef Emul) {
   uint8_t OSABI = 0;
@@ -88,9 +93,10 @@
 
   if (Ret.first == ELFNoneKind) {
     if (S == "i386pe" || S == "i386pep" || S == "thumb2pe")
-      error("Windows targets are not supported on the ELF frontend: " + Emul);
+      error(getOutputName() +
+            ": Windows targets are not supported on the ELF frontend: " + Emul);
     else
-      error("unknown emulation: " + Emul);
+      error(getOutputName() + ": unknown emulation: " + Emul);
   }
   return std::make_tuple(Ret.first, Ret.second, OSABI);
 }
@@ -152,7 +158,8 @@
     return;
   case file_magic::elf_shared_object:
     if (Config->Relocatable) {
-      error("attempted static link of dynamic object " + Path);
+      error(getOutputName() + ": attempted static link of dynamic object " +
+            Path);
       return;
     }
     Files.push_back(createSharedFile(MBRef));
@@ -171,7 +178,7 @@
 
   auto MBOrErr = MemoryBuffer::getFile(Path);
   if (auto EC = MBOrErr.getError()) {
-    error(EC, "cannot open " + Path);
+    error(EC, getOutputName() + ": cannot open " + Path);
     return None;
   }
   std::unique_ptr<MemoryBuffer> &MB = *MBOrErr;
@@ -188,7 +195,7 @@
 void LinkerDriver::addLibrary(StringRef Name) {
   std::string Path = searchLibrary(Name);
   if (Path.empty())
-    error("unable to find library -l" + Name);
+    error(getOutputName() + ": unable to find library -l" + Name);
   else
     addFile(Path);
 }
@@ -217,23 +224,25 @@
   // The MIPS ABI as of 2016 does not support the GNU-style symbol lookup
   // table which is a relatively new feature.
   if (Config->EMachine == EM_MIPS && Config->GnuHash)
-    error("the .gnu.hash section is not compatible with the MIPS target.");
+    error(getOutputName() +
+          ": the .gnu.hash section is not compatible with the MIPS target.");
 
   if (Config->EMachine == EM_AMDGPU && !Config->Entry.empty())
-    error("-e option is not valid for AMDGPU.");
+    error(getOutputName() + ": -e option is not valid for AMDGPU.");
 
   if (Config->Pie && Config->Shared)
-    error("-shared and -pie may not be used together");
+    error(getOutputName() + ": -shared and -pie may not be used together");
 
   if (Config->Relocatable) {
     if (Config->Shared)
-      error("-r and -shared may not be used together");
+      error(getOutputName() + ": -r and -shared may not be used together");
     if (Config->GcSections)
-      error("-r and --gc-sections may not be used together");
+      error(getOutputName() +
+            ": -r and --gc-sections may not be used together");
     if (Config->ICF)
-      error("-r and --icf may not be used together");
+      error(getOutputName() + ": -r and --icf may not be used together");
     if (Config->Pie)
-      error("-r and -pie may not be used together");
+      error(getOutputName() + ": -r and -pie may not be used together");
   }
 }
 
@@ -276,7 +285,7 @@
       Value = Value.substr(Pos + 1);
       uint64_t Result;
       if (Value.getAsInteger(0, Result))
-        error("invalid " + Key + ": " + Value);
+        error(getOutputName() + ": invalid " + Key + ": " + Value);
       return Result;
     }
   }
@@ -304,7 +313,8 @@
       Cpio->append("version.txt", getLLDVersion() + "\n");
     } else
       error(F.getError(),
-            Twine("--reproduce: failed to open ") + Path + ".cpio");
+            Twine(getOutputName() + ": --reproduce: failed to open ") + Path +
+                ".cpio");
   }
 
   readConfigs(Args);
@@ -347,7 +357,7 @@
       return UnresolvedPolicy::Ignore;
     if (S == "ignore-in-shared-libs" || S == "report-all")
       return UnresolvedPolicy::ReportError;
-    error("unknown --unresolved-symbols value: " + S);
+    error(getOutputName() + ": unknown --unresolved-symbols value: " + S);
   }
   return UnresolvedPolicy::ReportError;
 }
@@ -361,7 +371,7 @@
       return Target2Policy::Abs;
     if (S == "got-rel")
       return Target2Policy::GotRel;
-    error("unknown --target2 option: " + S);
+    error(getOutputName() + ": unknown --target2 option: " + S);
   }
   return Target2Policy::GotRel;
 }
@@ -371,7 +381,7 @@
     StringRef S = Arg->getValue();
     if (S == "binary")
       return true;
-    error("unknown --oformat value: " + S);
+    error(getOutputName() + ": unknown --oformat value: " + S);
   }
   return false;
 }
@@ -409,7 +419,7 @@
   if (S.startswith("0x"))
     S = S.drop_front(2);
   if (S.getAsInteger(16, VA))
-    error("invalid argument: " + stringize(Arg));
+    error(getOutputName() + ": invalid argument: " + stringize(Arg));
   return VA;
 }
 
@@ -438,7 +448,7 @@
   if (S == "name")
     return SortSectionPolicy::Name;
   if (!S.empty())
-    error("unknown --sort-section rule: " + S);
+    error(getOutputName() + ": unknown --sort-section rule: " + S);
   return SortSectionPolicy::Default;
 }
 
@@ -508,19 +518,25 @@
   Config->LtoAAPipeline = getString(Args, OPT_lto_aa_pipeline);
   Config->LtoNewPmPasses = getString(Args, OPT_lto_newpm_passes);
   Config->OutputFile = getString(Args, OPT_o);
+  // Default output filename is "a.out" by the Unix tradition.
+  if (Config->OutputFile.empty())
+    Config->OutputFile = "a.out";
+
   Config->SoName = getString(Args, OPT_soname);
   Config->Sysroot = getString(Args, OPT_sysroot);
 
   Config->Optimize = getInteger(Args, OPT_O, 1);
   Config->LtoO = getInteger(Args, OPT_lto_O, 2);
   if (Config->LtoO > 3)
-    error("invalid optimization level for LTO: " + getString(Args, OPT_lto_O));
+    error(getOutputName() + ": invalid optimization level for LTO: " +
+          getString(Args, OPT_lto_O));
   Config->LtoPartitions = getInteger(Args, OPT_lto_partitions, 1);
   if (Config->LtoPartitions == 0)
-    error("--lto-partitions: number of threads must be > 0");
+    error(getOutputName() +
+          ": --lto-partitions: number of threads must be > 0");
   Config->ThinLtoJobs = getInteger(Args, OPT_thinlto_jobs, -1u);
   if (Config->ThinLtoJobs == 0)
-    error("--thinlto-jobs: number of threads must be > 0");
+    error(getOutputName() + ": --thinlto-jobs: number of threads must be > 0");
 
   Config->ZCombreloc = !hasZOption(Args, "nocombreloc");
   Config->ZExecstack = hasZOption(Args, "execstack");
@@ -672,7 +688,7 @@
     Config->MipsN32Abi = Config->EMachine == EM_MIPS && isMipsN32Abi(F);
     return;
   }
-  error("target emulation unknown: -m or at least one .o file required");
+  error(getOutputName() + ": target emulation unknown: -m or at least one .o file required");
 }
 
 // Parses -image-base option.
@@ -712,10 +728,6 @@
       (Config->EMachine == EM_MIPS && Config->EKind == ELF64LEKind);
   Config->ImageBase = getImageBase(Args);
 
-  // Default output filename is "a.out" by the Unix tradition.
-  if (Config->OutputFile.empty())
-    Config->OutputFile = "a.out";
-
   // Handle --trace-symbol.
   for (auto *Arg : Args.filtered(OPT_trace_symbol))
     Symtab.trace(Arg->getValue());