[llvm-cov] Warn when -arch spec is missing/invalid for universal binary

llvm-cov reports a poor error message when the -arch specifier is
missing or invalid, and a binary has multiple slices. Make the error
message more specific.

rdar://40312677

include/llvm/ProfileData/Coverage/CoverageMapping.h

@@ -55,7 +55,8 @@ enum class coveragemap_error {
   unsupported_version,
   truncated,
   malformed,
-  decompression_failed
+  decompression_failed,
+  invalid_or_missing_arch_specifier
 };
 
 const std::error_category &coveragemap_category();

lib/ProfileData/Coverage/CoverageMapping.cpp

@@ -816,6 +816,8 @@ static std::string getCoverageMapErrString(coveragemap_error Err) {
     return "Malformed coverage data";
   case coveragemap_error::decompression_failed:
     return "Failed to decompress coverage data (zlib)";
+  case coveragemap_error::invalid_or_missing_arch_specifier:
+    return "`-arch` specifier is invalid or missing for universal binary";
   }
   llvm_unreachable("A value of coveragemap_error has no message.");
 }

lib/ProfileData/Coverage/CoverageMappingReader.cpp

@@ -950,6 +950,18 @@ loadBinaryFormat(std::unique_ptr<Binary> Bin, StringRef Arch) {
       BytesInAddress, Endian);
 }
 
+/// Determine whether \p Arch is invalid or empty, given \p Bin.
+static bool isArchSpecifierInvalidOrMissing(Binary *Bin, StringRef Arch) {
+  // If we have a universal binary and Arch doesn't identify any of its slices,
+  // it's user error.
+  if (auto *Universal = dyn_cast<MachOUniversalBinary>(Bin))
+    return none_of(Universal->objects(),
+                   [Arch](const MachOUniversalBinary::ObjectForArch &Obj) {
+                     return Obj.getArchFlagName() == Arch;
+                   });
+  return false;
+}
+
 Expected<std::vector<std::unique_ptr<BinaryCoverageReader>>>
 BinaryCoverageReader::create(
     MemoryBufferRef ObjectBuffer, StringRef Arch,
@@ -970,6 +982,10 @@ BinaryCoverageReader::create(
     return BinOrErr.takeError();
   std::unique_ptr<Binary> Bin = std::move(BinOrErr.get());
 
+  if (isArchSpecifierInvalidOrMissing(Bin.get(), Arch))
+    return make_error<CoverageMapError>(
+        coveragemap_error::invalid_or_missing_arch_specifier);
+
   // MachO universal binaries which contain archives need to be treated as
   // archives, not as regular binaries.
   if (auto *Universal = dyn_cast<MachOUniversalBinary>(Bin.get())) {

test/tools/llvm-cov/universal-binary.c

@@ -10,8 +10,9 @@ int main(int argc, const char *argv[]) {}
 // COMBINED: showTemplateInstantiations.cpp
 // COMBINED-NEXT: universal-binary.c
 
+// RUN: not llvm-cov show %S/Inputs/universal-binary -instr-profile %t.profdata -path-equivalence=/tmp,%S %s 2>&1 | FileCheck --check-prefix=WRONG-ARCH %s
 // RUN: not llvm-cov show %S/Inputs/universal-binary -instr-profile %t.profdata -path-equivalence=/tmp,%S %s -arch i386 2>&1 | FileCheck --check-prefix=WRONG-ARCH %s
-// WRONG-ARCH: Failed to load coverage
+// WRONG-ARCH: Failed to load coverage: `-arch` specifier is invalid or missing for universal binary
 
 // RUN: not llvm-cov show %S/Inputs/universal-binary -instr-profile %t.profdata -path-equivalence=/tmp,%S %s -arch definitly_a_made_up_architecture 2>&1 | FileCheck --check-prefix=MADE-UP-ARCH %s
 // MADE-UP-ARCH: Unknown architecture: definitly_a_made_up_architecture