[lld][MachO] Silence warnings about --read-workers parsing (#156608)

The parsing of the --read-workers argument v is implemented like this:

  unsigned threads = 0
  if (!llvm::to_integer(v, threads, 0) || threads < 0) {
  ...

As reported by a compiler warning, the value of the "threads < 0"
expession is never going to be true. It could only evaluate to true if v
represents a negative number, but in this case llvm::to_integer returns
false since threads is unsigned and hence the second operand of the ||
operator will not be evaluated.

This patch removes the useless || operand to silence compiler warnings.
Since I had to first find out if --read-workers=0 is valid or not (it is),
I also added a test to document the valid values for the option and I adjusted
the error message on invalid values to clearly state that 0 is valid.

1 files changed, 6 insertions, 5 deletions

diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
index bcba759b2bbe..3db638e1ead9 100644
--- a/lld/MachO/Driver.cpp
+++ b/lld/MachO/Driver.cpp
@@ -1833,11 +1833,12 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
 
   if (auto *arg = args.getLastArg(OPT_read_workers)) {
     StringRef v(arg->getValue());
-    unsigned threads = 0;
-    if (!llvm::to_integer(v, threads, 0) || threads < 0)
-      error(arg->getSpelling() + ": expected a positive integer, but got '" +
-            arg->getValue() + "'");
-    config->readWorkers = threads;
+    unsigned workers = 0;
+    if (!llvm::to_integer(v, workers, 0))
+      error(arg->getSpelling() +
+            ": expected a non-negative integer, but got '" + arg->getValue() +
+            "'");
+    config->readWorkers = workers;
   }
   if (auto *arg = args.getLastArg(OPT_threads_eq)) {
     StringRef v(arg->getValue());