[llvm-lib] Add /WX, warn by default on empty inputs, add opt-out

lib.exe by default exits successfully without writing an output
file when no inputs are passed. llvm-lib has the same behavior,
for compatibility.

This behavior interacts poorly with build systems: If a static
library target had no inputs, llvm-lib would not produce an output
file, causing ninja (or make, or a similar system) to successfully
run that step, but then re-run it on the next build.

After this patch, llvm-lib emits a warning in this case, that with
/WX can be turned into an error. That way, ninja (or make, or...)
will mark the initial build as failed.

People who don't like the warning can use /ignore:emptyoutput to
suppress it.

The warning also points out the existing flag /llvmlibempty which
forces creation of an empty .lib file (this is an extension to lib.exe).

Differential Revision: https://reviews.llvm.org/D123517

1 files changed, 21 insertions, 5 deletions

diff --git a/llvm/lib/ToolDrivers/llvm-lib/LibDriver.cpp b/llvm/lib/ToolDrivers/llvm-lib/LibDriver.cpp
index 8f69282..7ecde3c 100644
--- a/llvm/lib/ToolDrivers/llvm-lib/LibDriver.cpp
+++ b/llvm/lib/ToolDrivers/llvm-lib/LibDriver.cpp
@@ -229,10 +229,11 @@ static void appendFile(std::vector<NewArchiveMember> &Members,
         (Magic == file_magic::coff_object) ? getCOFFFileMachine(MB)
                                            : getBitcodeFileMachine(MB);
     if (!MaybeFileMachine) {
-      handleAllErrors(MaybeFileMachine.takeError(), [&](const ErrorInfoBase &EIB) {
-        llvm::errs() << MB.getBufferIdentifier() << ": " << EIB.message()
-                     << "\n";
-      });
+      handleAllErrors(MaybeFileMachine.takeError(),
+                      [&](const ErrorInfoBase &EIB) {
+                        llvm::errs() << MB.getBufferIdentifier() << ": "
+                                     << EIB.message() << "\n";
+                      });
       exit(1);
     }
     COFF::MachineTypes FileMachine = *MaybeFileMachine;
@@ -291,10 +292,25 @@ int llvm::libDriverMain(ArrayRef<const char *> ArgsArr) {
     return 0;
   }
 
+  // Parse /ignore:
+  llvm::StringSet<> IgnoredWarnings;
+  for (auto *Arg : Args.filtered(OPT_ignore))
+    IgnoredWarnings.insert(Arg->getValue());
+
   // If no input files and not told otherwise, silently do nothing to match
   // lib.exe
-  if (!Args.hasArgNoClaim(OPT_INPUT) && !Args.hasArg(OPT_llvmlibempty))
+  if (!Args.hasArgNoClaim(OPT_INPUT) && !Args.hasArg(OPT_llvmlibempty)) {
+    if (!IgnoredWarnings.contains("emptyoutput")) {
+      llvm::errs() << "warning: no input files, not writing output file\n";
+      llvm::errs() << "         pass /llvmlibempty to write empty .lib file,\n";
+      llvm::errs() << "         pass /ignore:emptyoutput to suppress warning\n";
+      if (Args.hasFlag(OPT_WX, OPT_WX_no, false)) {
+        llvm::errs() << "treating warning as error due to /WX\n";
+        return 1;
+      }
+    }
     return 0;
+  }
 
   if (Args.hasArg(OPT_lst)) {
     doList(Args);