Revert "[AIX][BigArchive] Treat the archive is empty if the first child member offset is zero"

This reverts commit 7f0003c.

Reason: This broke the ASan buildbot, see the comments in
https://reviews.llvm.org/D138986 for more information.

Author: hctim

llvm/include/llvm/Object/Archive.h

@@ -410,7 +410,9 @@ class BigArchive : public Archive {
   BigArchive(MemoryBufferRef Source, Error &Err);
   uint64_t getFirstChildOffset() const override { return FirstChildOffset; }
   uint64_t getLastChildOffset() const { return LastChildOffset; }
-  bool isEmpty() const override { return getFirstChildOffset() == 0; }
+  bool isEmpty() const override {
+    return Data.getBufferSize() == sizeof(FixLenHdr);
+  };
 };
 
 } // end namespace object

llvm/lib/Object/Archive.cpp

@@ -949,15 +949,7 @@ Archive::child_iterator Archive::child_begin(Error &Err,
     return child_iterator::itr(
         Child(this, FirstRegularData, FirstRegularStartOfFile), Err);
 
-  uint64_t FirstChildOffset = getFirstChildOffset();
-  const char *Loc = Data.getBufferStart() + FirstChildOffset;
-  if (Loc >= Data.getBufferEnd()) {
-    Err = malformedError("First member offset " + Twine(FirstChildOffset) +
-                         " is beyond the data buffer which has size of " +
-                         Twine(Data.getBufferSize()));
-    return child_end();
-  }
-
+  const char *Loc = Data.getBufferStart() + getFirstChildOffset();
   Child C(this, Loc, &Err);
   if (Err)
     return child_end();

llvm/test/Object/archive-big-malformed-first-member.test

@@ -1,3 +1,3 @@
 # Test reading an empty archive with free list in it.
-# RUN: llvm-ar tv %p/Inputs/aix-empty-big-archive-with-freelist.a 2>&1 \
-# RUN:   | not grep 'truncated or malformed archive'
+# RUN: not llvm-ar tv %p/Inputs/aix-empty-big-archive-with-freelist.a 2>&1 \
+# RUN:   | grep 'truncated or malformed archive'