[AIX] fixed llvm-ar can not read empty big archive correctly.

[DiggerLin]

Summary

llvm-ar can not read empty big archive correctly. it output error as
error: unable to load 'empty.a': truncated or malformed archive (characters in size field in archive member header are not all decimal numbers: '<bigaf>

[jhenderson]

LGTM.

[jhenderson]

Are the dependent patches in the right order? I wouldn't expect this patch to depend on the writing patch necessarily.

[DiggerLin]

In D124017#3466962, @jhenderson wrote:

Are the dependent patches in the right order? I wouldn't expect this patch to depend on the writing patch necessarily.

the test cases(for example: llvm/test/tools/llvm-ar/quick-append.test) depend on the "writing patch " to generate a empty AIX big archive.

llvm/include/llvm/Object/Archive.h

iterator_range<symbol_iterator> symbols() const {
     return make_range(symbol_begin(), symbol_end());
   }
 
   static bool classof(Binary const *v) { return v->isArchive(); }
 
   // check if a symbol is in the archive
   Expected<Optional<Child>> findSym(StringRef name) const;
 
-  bool isEmpty() const;
+  virtual bool isEmpty() const;
   bool hasSymbolTable() const;
   StringRef getSymbolTable() const { return SymbolTable; }
   StringRef getStringTable() const { return StringTable; }
   uint32_t getNumberOfSymbols() const;
   virtual uint64_t getFirstChildOffset() const { return getArchiveMagicLen(); }
 
   std::vector<std::unique_ptr<MemoryBuffer>> takeThinBuffers() {
     return std::move(ThinBuffers);
public:
   const FixLenHdr *ArFixLenHdr;
   uint64_t FirstChildOffset = 0;
   uint64_t LastChildOffset = 0;
 
 public:
   BigArchive(MemoryBufferRef Source, Error &Err);
   uint64_t getFirstChildOffset() const override { return FirstChildOffset; }
   uint64_t getLastChildOffset() const { return LastChildOffset; }
+  bool isEmpty() const override {
+    return Data.getBufferSize() == sizeof(FixLenHdr);
+  };
 };
 
 } // end namespace object
 } // end namespace llvm
 
 #endif // LLVM_OBJECT_ARCHIVE_H

llvm/test/tools/llvm-ar/display-empty.test

 ## Test Display of empty archives.
 
 # RUN: rm -rf %t && mkdir -p %t
 
 ## Display empty archive:
-# RUN: llvm-ar cr --format=gnu %t/empty.a
+# RUN: llvm-ar cr %t/empty.a
 # RUN: llvm-ar tv %t/empty.a 2>&1 | count 0
 
 ## Display empty thin archive:
 # RUN: llvm-ar Trc %t/thin-archive-empty.a
 # RUN: llvm-ar tv %t/thin-archive-empty.a 2>&1 | count 0

llvm/test/tools/llvm-ar/full-path-option.test

 RUN: llvm-ar t display.a a/foo.txt | FileCheck %s --check-prefix=DISPLAY-FOUND --match-full-lines
 RUN: not llvm-ar tP display.a a/foo.txt 2>&1 | FileCheck %s --check-prefix=DISPLAY-NOT-FOUND
 
 DISPLAY-FOUND: foo.txt
 DISPLAY-NOT-FOUND: 'a/foo.txt' was not found
 
 # Deleting will fail with P because the members exist as foo.txt, not a/foo.txt.
 RUN: rm -f del1.a
-RUN: llvm-ar rcS --format=gnu del1.a foo.txt
+RUN: llvm-ar rcS del1.a foo.txt
 RUN: llvm-ar dP del1.a a/foo.txt
 RUN: llvm-ar t del1.a a/foo.txt | FileCheck %s --check-prefix=DISPLAY-FOUND --match-full-lines
 RUN: llvm-ar d del1.a a/foo.txt
 RUN: not llvm-ar t del1.a a/foo.txt 2>&1 | FileCheck %s --check-prefix=DISPLAY-NOT-FOUND
 
 # Run several checks that P is implied when using thin archives. None of these
 # checks explicitly use P.
 

llvm/test/tools/llvm-ar/quick-append.test

-# XFAIL: system-aix
 ## Test quick append 
 
 # RUN: rm -rf %t && mkdir -p %t
 # RUN: yaml2obj %s -o %t/1.o --docnum=1
 # RUN: yaml2obj %s -o %t/2.o --docnum=2
 
 ## Append single member:
 # RUN: llvm-ar qc %t/single.a %t/1.o

llvm/test/tools/llvm-lipo/create-archive-input.test

 # RUN: yaml2obj %p/Inputs/i386-slice.yaml -o %t-i386.o
 # RUN: yaml2obj %p/Inputs/x86_64-slice.yaml -o %t-x86_64.o
 # RUN: yaml2obj %p/Inputs/i386-x86_64-universal.yaml -o %t-universal.o
 # RUN: llvm-as %p/Inputs/armv7-ios.ll -o %t-ir-armv7.o
 # RUN: llvm-as %p/Inputs/x64-osx.ll -o %t-ir-x86_64.o
 
-# RUN: llvm-ar cr --format=gnu %t.empty.a
+# RUN: llvm-ar cr %t.empty.a
 # RUN: not llvm-lipo %t.empty.a -create -output /dev/null 2>&1 | FileCheck --check-prefix=EMPTY-ARCHIVE %s
 
 # RUN: llvm-ar cr %t.different_architectures.a %t-i386.o %t-x86_64.o
 # RUN: not llvm-lipo %t.different_architectures.a -create -output /dev/null 2>&1 | FileCheck --check-prefix=ARCHIVE-WITH-DIFFERENT-ARCHS %s
 
 # RUN: llvm-ar cr %t.contains_fat_binary.a %t-universal.o 
 # RUN: not llvm-lipo %t.contains_fat_binary.a -create -output /dev/null 2>&1 | FileCheck --check-prefix=ARCHIVE-WITH-FAT-BINARY %s
 

llvm/test/tools/llvm-size/archive.test

 ## Show how llvm-size behaves when passed in an archive.
 
 # RUN: yaml2obj %s --docnum=1 -o %t1
 # RUN: yaml2obj %s --docnum=2 -o %t2
 
 ## Case 1: Empty archive. No output expected.
 # RUN: rm -f %t1.a
-# RUN: llvm-ar rc --format=gnu %t1.a
+# RUN: llvm-ar rc %t1.a
 
 # RUN: llvm-size -B %t1.a | count 0
 # RUN: llvm-size -A %t1.a | count 0
 
 ## Case 2: Single member.
 # RUN: rm -f %t2.a
 # RUN: llvm-ar rc %t2.a %t1