[lld-macho] Improve invalid fat binary warning

[keith]

Summary

This nearly mirrors ld64's error for this case:

ld: warning: ignoring file path/to/file, file is universal (armv7,arm64) but does not contain the x86_64 architecture: path/to/file

[int3]

Thanks!! FWIW we don't usually strive for 1:1 matching of ld64's error messages, if anything we try to be more similar to the other LLD ports, but this seems good

lld/MachO/InputFiles.cpp

std::optional<MemoryBufferRef> macho::readFile(StringRef path) {
   }
 
   llvm::BumpPtrAllocator &bAlloc = lld::bAlloc();
 
   // Object files and archive files may be fat files, which contain multiple
   // real files for different CPU ISAs. Here, we search for a file that matches
   // with the current link target and returns it as a MemoryBufferRef.
   const auto *arch = reinterpret_cast<const fat_arch *>(buf + sizeof(*hdr));
+  auto getArchName = [](uint32_t cpuType, uint32_t cpuSubtype) {
+    return getArchitectureName(getArchitectureFromCpuType(cpuType, cpuSubtype));
+  };
 
+  std::vector<StringRef> archs;
   for (uint32_t i = 0, n = read32be(&hdr->nfat_arch); i < n; ++i) {
     if (reinterpret_cast<const char *>(arch + i + 1) >
         buf + mbref.getBufferSize()) {
       error(path + ": fat_arch struct extends beyond end of file");
       return std::nullopt;
     }
 
     uint32_t cpuType = read32be(&arch[i].cputype);
     uint32_t cpuSubtype =
         read32be(&arch[i].cpusubtype) & ~MachO::CPU_SUBTYPE_MASK;
 
     // FIXME: LD64 has a more complex fallback logic here.
     // Consider implementing that as well?
     if (cpuType != static_cast<uint32_t>(target->cpuType) ||
-        cpuSubtype != target->cpuSubtype)
+        cpuSubtype != target->cpuSubtype) {
+      archs.emplace_back(getArchName(cpuType, cpuSubtype));
       continue;
+    }
 
     uint32_t offset = read32be(&arch[i].offset);
     uint32_t size = read32be(&arch[i].size);
     if (offset + size > mbref.getBufferSize())
       error(path + ": slice extends beyond end of file");
     if (tar)
       tar->append(relativeToRoot(path), mbref.getBuffer());
     return cachedReads[key] = MemoryBufferRef(StringRef(buf + offset, size),
                                               path.copy(bAlloc));
   }
 
-  warn("unable to find matching architecture in " + path);
+  auto targetArchName = getArchName(target->cpuType, target->cpuSubtype);
+  warn(path + ": ignoring file because it is universal (" + join(archs, ",") +
+       ") but does not contain the " + targetArchName + " architecture");
   return std::nullopt;
 }
 
 InputFile::InputFile(Kind kind, const InterfaceFile &interface)
     : id(idCount++), fileKind(kind), name(saver().save(interface.getPath())) {}
 
 // Some sections comprise of fixed-size records, so instead of splitting them at
 // symbol boundaries, we split them based on size. Records are distinct from

lld/test/MachO/fat-arch.s

 # REQUIRES: x86,aarch64
 ## FIXME: The tests doesn't run on windows right now because of llvm-mc (can't produce triple=arm64-apple-macos11.0)
 # UNSUPPORTED: system-windows
 
 # RUN: llvm-mc -filetype=obj -triple=i386-apple-darwin %s -o %t.i386.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t.x86_64.o
 # RUN: llvm-mc -filetype=obj -triple=arm64-apple-macos11.0 %s -o %t.arm64.o
 
 # RUN: llvm-lipo %t.i386.o %t.x86_64.o -create -o %t.fat.o
 # RUN: %lld -o /dev/null %t.fat.o
 # RUN: llvm-lipo %t.i386.o -create -o %t.noarch.o
 # RUN: not %no-fatal-warnings-lld -o /dev/null %t.noarch.o 2>&1 | \
 # RUN:    FileCheck %s -DFILE=%t.noarch.o
-# CHECK: warning: unable to find matching architecture in [[FILE]]
+# CHECK: warning: [[FILE]]: ignoring file because it is universal (i386) but does not contain the x86_64 architecture
+
+# RUN: not %lld -arch arm64 -o /dev/null %t.fat.o 2>&1 | \
+# RUN:    FileCheck --check-prefix=CHECK-FAT %s -DFILE=%t.fat.o
+# CHECK-FAT: error: [[FILE]]: ignoring file because it is universal (i386,x86_64) but does not contain the arm64 architecture
 
 ## Validates that we read the cpu-subtype correctly from a fat exec.
 # RUN: %lld -o %t.x86_64.out %t.x86_64.o
 # RUN: %lld -arch arm64 -o %t.arm64.out %t.arm64.o
 # RUN: llvm-lipo %t.x86_64.out %t.arm64.out -create -o %t.fat.exec.out
 # RUN: %lld -arch x86_64 %t.x86_64.o -bundle_loader %t.fat.exec.out -bundle -o %t.fat.bundle
 
 # RUN: llvm-otool -h %t.fat.bundle > %t.bundle_header.txt