diff --git a/llvm/test/Object/ar-create.test b/llvm/test/Object/ar-create.test
--- a/llvm/test/Object/ar-create.test
+++ b/llvm/test/Object/ar-create.test
@@ -1,3 +1,4 @@
+XFAIL: system-aix
 Test which operations create an archive and which don't.
 
 RUN: touch %t
diff --git a/llvm/test/Object/archive-extract-dir.test b/llvm/test/Object/archive-extract-dir.test
--- a/llvm/test/Object/archive-extract-dir.test
+++ b/llvm/test/Object/archive-extract-dir.test
@@ -1,3 +1,5 @@
+XFAIL: system-aix
+
 RUN: mkdir -p %t
 RUN: cd %t
 RUN: rm -rf foo
diff --git a/llvm/test/Object/archive-malformed-object.test b/llvm/test/Object/archive-malformed-object.test
--- a/llvm/test/Object/archive-malformed-object.test
+++ b/llvm/test/Object/archive-malformed-object.test
@@ -1,3 +1,4 @@
+# XFAIL: system-aix
 ## Show that the archive library emits error messages when adding malformed
 ## objects.
 
diff --git a/llvm/test/Object/archive-replace-pos.test b/llvm/test/Object/archive-replace-pos.test
--- a/llvm/test/Object/archive-replace-pos.test
+++ b/llvm/test/Object/archive-replace-pos.test
@@ -1,3 +1,5 @@
+XFAIL: system-aix
+
 Test adding a member to a particular position
 
 RUN: touch %t.foo
diff --git a/llvm/test/Object/archive-unknown-filetype.test b/llvm/test/Object/archive-unknown-filetype.test
--- a/llvm/test/Object/archive-unknown-filetype.test
+++ b/llvm/test/Object/archive-unknown-filetype.test
@@ -1,3 +1,4 @@
+# XFAIL: system-aix
 ## Show that the archive library does not emit an error or add any symbols to
 ## the archive symbol table, when it encounters an unknown file type, but still
 ## adds the file to the archive.
diff --git a/llvm/test/Object/archive-update.test b/llvm/test/Object/archive-update.test
--- a/llvm/test/Object/archive-update.test
+++ b/llvm/test/Object/archive-update.test
@@ -1,3 +1,5 @@
+XFAIL: system-aix
+
 Test the 'u' option of llvm-ar
 
 RUN: rm -rf %t && mkdir -p %t && cd %t
diff --git a/llvm/test/Object/directory.ll b/llvm/test/Object/directory.ll
--- a/llvm/test/Object/directory.ll
+++ b/llvm/test/Object/directory.ll
@@ -1,3 +1,5 @@
+; XFAIL: system-aix
+
 ;RUN: rm -rf %t && mkdir -p %t
 ;RUN: not llvm-ar r %t/test.a . 2>&1 | FileCheck -DMSG=%errc_EISDIR %s
 ;CHECK: .: [[MSG]]
diff --git a/llvm/test/tools/llvm-ar/Inputs/xcoff.yaml b/llvm/test/tools/llvm-ar/Inputs/xcoff.yaml
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/llvm-ar/Inputs/xcoff.yaml
@@ -0,0 +1,3 @@
+--- !XCOFF
+FileHeader:
+  MagicNumber:          0x1DF
diff --git a/llvm/test/tools/llvm-ar/count.test b/llvm/test/tools/llvm-ar/count.test
--- a/llvm/test/tools/llvm-ar/count.test
+++ b/llvm/test/tools/llvm-ar/count.test
@@ -1,3 +1,4 @@
+# XFAIL: system-aix
 # Test the 'N' count parameter.
 
 # Get a temp clean cwd to extract into.
diff --git a/llvm/test/tools/llvm-ar/create.test b/llvm/test/tools/llvm-ar/create.test
--- a/llvm/test/tools/llvm-ar/create.test
+++ b/llvm/test/tools/llvm-ar/create.test
@@ -1,3 +1,4 @@
+XFAIL: system-aix
 ## Test the creation warning and supression of that warning.
 
 RUN: touch %t1.txt
diff --git a/llvm/test/tools/llvm-ar/dash-before-letter.test b/llvm/test/tools/llvm-ar/dash-before-letter.test
--- a/llvm/test/tools/llvm-ar/dash-before-letter.test
+++ b/llvm/test/tools/llvm-ar/dash-before-letter.test
@@ -1,3 +1,4 @@
+XFAIL: system-aix
 # Test the use of dash before key letters.
 
 RUN: touch %t1.txt
diff --git a/llvm/test/tools/llvm-ar/default-xcoff.test b/llvm/test/tools/llvm-ar/default-xcoff.test
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/llvm-ar/default-xcoff.test
@@ -0,0 +1,10 @@
+; REQUIRES: system-aix
+;; Test llvm-ar does not support Big AIX archive write operation.
+
+; RUN: yaml2obj %S/Inputs/xcoff.yaml -o %t.obj
+; RUN: rm -f %t.ar
+; RUN: not llvm-ar cr %t.ar %t.obj 2>&1 | FileCheck %s
+; RUN: echo "test big archive" > %t.txt
+; RUN: not llvm-ar cr %t.ar %t.txt 2>&1 | FileCheck %s
+
+; CHECK: unsupported big AIX write operation
diff --git a/llvm/test/tools/llvm-ar/delete.test b/llvm/test/tools/llvm-ar/delete.test
--- a/llvm/test/tools/llvm-ar/delete.test
+++ b/llvm/test/tools/llvm-ar/delete.test
@@ -1,3 +1,4 @@
+# XFAIL: system-aix
 ## Test the deletion of members and that symbols are removed from the symbol table.
 
 # RUN: yaml2obj %s -o %t-delete.o --docnum=1
diff --git a/llvm/test/tools/llvm-ar/display-empty.test b/llvm/test/tools/llvm-ar/display-empty.test
--- a/llvm/test/tools/llvm-ar/display-empty.test
+++ b/llvm/test/tools/llvm-ar/display-empty.test
@@ -1,3 +1,4 @@
+# XFAIL: system-aix
 ## Test Display of empty archives.
 
 # RUN: rm -rf %t && mkdir -p %t
diff --git a/llvm/test/tools/llvm-ar/double-hyphen.test b/llvm/test/tools/llvm-ar/double-hyphen.test
--- a/llvm/test/tools/llvm-ar/double-hyphen.test
+++ b/llvm/test/tools/llvm-ar/double-hyphen.test
@@ -1,3 +1,4 @@
+# XFAIL: system-aix
 ## Test the use of "--" on the commandline
 
 # RUN: echo contents > %t.txt
diff --git a/llvm/test/tools/llvm-ar/error-opening-permission.test b/llvm/test/tools/llvm-ar/error-opening-permission.test
--- a/llvm/test/tools/llvm-ar/error-opening-permission.test
+++ b/llvm/test/tools/llvm-ar/error-opening-permission.test
@@ -1,3 +1,4 @@
+# XFAIL: system-aix
 ## Unsupported on windows as marking files "unreadable"
 ## is non-trivial on windows.
 # UNSUPPORTED: system-windows
diff --git a/llvm/test/tools/llvm-ar/extract.test b/llvm/test/tools/llvm-ar/extract.test
--- a/llvm/test/tools/llvm-ar/extract.test
+++ b/llvm/test/tools/llvm-ar/extract.test
@@ -1,5 +1,5 @@
 ## Test extract operation.
-# XFAIL: system-darwin
+# XFAIL: system-darwin,system-aix
 
 # RUN: rm -rf %t && mkdir -p %t/extracted/
 
diff --git a/llvm/test/tools/llvm-ar/flatten-thin-archive.test b/llvm/test/tools/llvm-ar/flatten-thin-archive.test
--- a/llvm/test/tools/llvm-ar/flatten-thin-archive.test
+++ b/llvm/test/tools/llvm-ar/flatten-thin-archive.test
@@ -1,3 +1,4 @@
+# XFAIL: system-aix
 # This test creates a thin archive that contains a thin archive, a regular
 # archive, and a file.
 #
diff --git a/llvm/test/tools/llvm-ar/full-path-option.test b/llvm/test/tools/llvm-ar/full-path-option.test
--- a/llvm/test/tools/llvm-ar/full-path-option.test
+++ b/llvm/test/tools/llvm-ar/full-path-option.test
@@ -1,3 +1,4 @@
+XFAIL: system-aix
 # Note: many of these tests depend on relative paths, so we have to cd to a
 # test directory first.
 RUN: mkdir -p %t && cd %t
diff --git a/llvm/test/tools/llvm-ar/full-to-thin-archive.test b/llvm/test/tools/llvm-ar/full-to-thin-archive.test
--- a/llvm/test/tools/llvm-ar/full-to-thin-archive.test
+++ b/llvm/test/tools/llvm-ar/full-to-thin-archive.test
@@ -1,3 +1,4 @@
+# XFAIL: system-aix
 ## Test archives do not convert to thin archives.
 
 # RUN: rm -rf %t && mkdir -p %t
diff --git a/llvm/test/tools/llvm-ar/insert-after.test b/llvm/test/tools/llvm-ar/insert-after.test
--- a/llvm/test/tools/llvm-ar/insert-after.test
+++ b/llvm/test/tools/llvm-ar/insert-after.test
@@ -1,3 +1,4 @@
+XFAIL: system-aix
 ## Test inserting files after a file.
 
 RUN: touch %t1.txt
diff --git a/llvm/test/tools/llvm-ar/insert-before.test b/llvm/test/tools/llvm-ar/insert-before.test
--- a/llvm/test/tools/llvm-ar/insert-before.test
+++ b/llvm/test/tools/llvm-ar/insert-before.test
@@ -1,3 +1,4 @@
+XFAIL: system-aix
 ## Test inserting files before a file.
 
 RUN: touch %t1.txt
diff --git a/llvm/test/tools/llvm-ar/move-after.test b/llvm/test/tools/llvm-ar/move-after.test
--- a/llvm/test/tools/llvm-ar/move-after.test
+++ b/llvm/test/tools/llvm-ar/move-after.test
@@ -1,3 +1,4 @@
+XFAIL: system-aix
 ## Test moving files after a file.
 
 RUN: touch %t1.txt
diff --git a/llvm/test/tools/llvm-ar/move-before.test b/llvm/test/tools/llvm-ar/move-before.test
--- a/llvm/test/tools/llvm-ar/move-before.test
+++ b/llvm/test/tools/llvm-ar/move-before.test
@@ -1,3 +1,4 @@
+XFAIL: system-aix
 ## Test moving files after a file.
 
 RUN: touch %t1.txt
diff --git a/llvm/test/tools/llvm-ar/mri-delete.test b/llvm/test/tools/llvm-ar/mri-delete.test
--- a/llvm/test/tools/llvm-ar/mri-delete.test
+++ b/llvm/test/tools/llvm-ar/mri-delete.test
@@ -1,3 +1,4 @@
+XFAIL: system-aix
 RUN: yaml2obj %S/Inputs/elf.yaml -o %t.o
 RUN: rm -f %t.ar
 
diff --git a/llvm/test/tools/llvm-ar/mri-nonascii.test b/llvm/test/tools/llvm-ar/mri-nonascii.test
--- a/llvm/test/tools/llvm-ar/mri-nonascii.test
+++ b/llvm/test/tools/llvm-ar/mri-nonascii.test
@@ -1,3 +1,4 @@
+XFAIL: system-aix
 # Test non-ascii archive members
 
 RUN: rm -rf %t && mkdir -p %t/extracted
diff --git a/llvm/test/tools/llvm-ar/non-windows-name-case.test b/llvm/test/tools/llvm-ar/non-windows-name-case.test
--- a/llvm/test/tools/llvm-ar/non-windows-name-case.test
+++ b/llvm/test/tools/llvm-ar/non-windows-name-case.test
@@ -1,3 +1,4 @@
+# XFAIL: system-aix
 ## Test that on windows, members are case insensitive.
 # UNSUPPORTED: system-windows
 
diff --git a/llvm/test/tools/llvm-ar/plugin.test b/llvm/test/tools/llvm-ar/plugin.test
--- a/llvm/test/tools/llvm-ar/plugin.test
+++ b/llvm/test/tools/llvm-ar/plugin.test
@@ -1,3 +1,4 @@
+# XFAIL: system-aix
 ## Test that --plugin is ignored.
 
 # RUN: rm -f %t.a
diff --git a/llvm/test/tools/llvm-ar/print.test b/llvm/test/tools/llvm-ar/print.test
--- a/llvm/test/tools/llvm-ar/print.test
+++ b/llvm/test/tools/llvm-ar/print.test
@@ -1,5 +1,5 @@
 ## Test Print output
-# XFAIL: system-darwin
+# XFAIL: system-darwin,system-aix
 
 # RUN: rm -rf %t && mkdir -p %t
 # RUN: echo file1 > %t/1.txt
diff --git a/llvm/test/tools/llvm-ar/quick-append.test b/llvm/test/tools/llvm-ar/quick-append.test
--- a/llvm/test/tools/llvm-ar/quick-append.test
+++ b/llvm/test/tools/llvm-ar/quick-append.test
@@ -1,3 +1,4 @@
+# XFAIL: system-aix
 ## Test quick append 
 
 # RUN: rm -rf %t && mkdir -p %t
diff --git a/llvm/test/tools/llvm-ar/read-only-archive.test b/llvm/test/tools/llvm-ar/read-only-archive.test
--- a/llvm/test/tools/llvm-ar/read-only-archive.test
+++ b/llvm/test/tools/llvm-ar/read-only-archive.test
@@ -1,3 +1,4 @@
+# XFAIL: system-aix
 ## Test that read-only archives cannot be edited
 
 # RUN: rm -rf %t && mkdir -p %t
diff --git a/llvm/test/tools/llvm-ar/response-utf8.test b/llvm/test/tools/llvm-ar/response-utf8.test
--- a/llvm/test/tools/llvm-ar/response-utf8.test
+++ b/llvm/test/tools/llvm-ar/response-utf8.test
@@ -1,3 +1,4 @@
+# XFAIL: system-aix
 ## Check that response files can cope with non-ascii characters.
 
 # RUN: echo 'contents' > %t-£.txt
diff --git a/llvm/test/tools/llvm-ar/thin-to-full-archive.test b/llvm/test/tools/llvm-ar/thin-to-full-archive.test
--- a/llvm/test/tools/llvm-ar/thin-to-full-archive.test
+++ b/llvm/test/tools/llvm-ar/thin-to-full-archive.test
@@ -1,3 +1,4 @@
+# XFAIL: system-aix
 ## Test thin archives do not siletly convert to full archives on write.
 
 # RUN: rm -f %tthin.a %tfull.a 
diff --git a/llvm/test/tools/llvm-libtool-darwin/archive-flattening.test b/llvm/test/tools/llvm-libtool-darwin/archive-flattening.test
--- a/llvm/test/tools/llvm-libtool-darwin/archive-flattening.test
+++ b/llvm/test/tools/llvm-libtool-darwin/archive-flattening.test
@@ -1,3 +1,4 @@
+# XFAIL: system-aix
 ## This test checks that an archive is flattened correctly.
 
 # RUN: yaml2obj %S/Inputs/input1.yaml -o %t-input1.o
diff --git a/llvm/test/tools/llvm-link/archivell.ll b/llvm/test/tools/llvm-link/archivell.ll
--- a/llvm/test/tools/llvm-link/archivell.ll
+++ b/llvm/test/tools/llvm-link/archivell.ll
@@ -1,4 +1,4 @@
-# RUN: llvm-ar cr %t.fg.a %S/Inputs/f.ll %S/Inputs/g.ll
+# RUN: llvm-ar --format=gnu cr %t.fg.a %S/Inputs/f.ll %S/Inputs/g.ll
 # RUN: not llvm-link %S/Inputs/h.ll %t.fg.a -o %t.linked.bc 2>&1 | FileCheck %s
 
 # RUN: rm -f %t.fg.a
diff --git a/llvm/test/tools/llvm-nm/option-X.test b/llvm/test/tools/llvm-nm/option-X.test
--- a/llvm/test/tools/llvm-nm/option-X.test
+++ b/llvm/test/tools/llvm-nm/option-X.test
@@ -56,7 +56,7 @@
 # RUN: yaml2obj --docnum=2 -DFLAG=0x1DF %s -o %t_xcoff32.o
 # RUN: yaml2obj --docnum=2 -DFLAG=0x1F7 %s -o %t_xcoff64.o
 # RUN: rm -f %t.a
-# RUN: llvm-ar -q -c %t.a %t_xcoff32.o %t_xcoff64.o
+# RUN: llvm-ar --format=gnu -q -c %t.a %t_xcoff32.o %t_xcoff64.o
 
 # RUN: llvm-nm --format=just-symbols -X32 %t_xcoff32.o | \
 # RUN:   FileCheck --check-prefixes=XCOFF32 %s --implicit-check-not={{.}}
diff --git a/llvm/test/tools/llvm-readobj/basic.test b/llvm/test/tools/llvm-readobj/basic.test
--- a/llvm/test/tools/llvm-readobj/basic.test
+++ b/llvm/test/tools/llvm-readobj/basic.test
@@ -16,7 +16,7 @@
 
 # Test that unrecognised files in archives are ignored.
 RUN: rm -f %t.a
-RUN: llvm-ar rc %t.a %t.empty
+RUN: llvm-ar --format=gnu rc %t.a %t.empty
 RUN: llvm-readobj --all %t.a 2>&1 | FileCheck --check-prefix=NO-OUTPUT --allow-empty %s
 NO-OUTPUT-NOT: {{.}}
 
diff --git a/llvm/test/tools/llvm-size/archive.test b/llvm/test/tools/llvm-size/archive.test
--- a/llvm/test/tools/llvm-size/archive.test
+++ b/llvm/test/tools/llvm-size/archive.test
@@ -5,21 +5,21 @@
 
 ## Case 1: Empty archive. No output expected.
 # RUN: rm -f %t1.a
-# RUN: llvm-ar rc %t1.a
+# RUN: llvm-ar --format=gnu 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
+# RUN: llvm-ar --format=gnu rc %t2.a %t1
 
 # RUN: llvm-size -B %t2.a | FileCheck %s -DARCHIVE=%t2.a --check-prefix=BERKELEY-1
 # RUN: llvm-size -A %t2.a | FileCheck %s -DARCHIVE=%t2.a --check-prefix=SYSV-1
 
 ## Case 3: Multiple members.
 # RUN: rm -f %t3.a
-# RUN: llvm-ar rc %t3.a %t1 %t2
+# RUN: llvm-ar --format=gnu rc %t3.a %t1 %t2
 
 # RUN: llvm-size -B %t3.a | FileCheck %s -DARCHIVE=%t3.a --check-prefixes=BERKELEY-1,BERKELEY-2
 # RUN: llvm-size -A %t3.a | FileCheck %s -DARCHIVE=%t3.a --check-prefixes=SYSV-1,SYSV-2
diff --git a/llvm/tools/llvm-ar/llvm-ar.cpp b/llvm/tools/llvm-ar/llvm-ar.cpp
--- a/llvm/tools/llvm-ar/llvm-ar.cpp
+++ b/llvm/tools/llvm-ar/llvm-ar.cpp
@@ -22,6 +22,7 @@
 #include "llvm/Object/MachO.h"
 #include "llvm/Object/ObjectFile.h"
 #include "llvm/Object/SymbolicFile.h"
+#include "llvm/Object/XCOFFObjectFile.h"
 #include "llvm/Support/Chrono.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/ConvertUTF.h"
@@ -881,9 +882,11 @@
 }
 
 static object::Archive::Kind getDefaultForHost() {
-  return Triple(sys::getProcessTriple()).isOSDarwin()
+  Triple HostTriple(sys::getProcessTriple());
+  return HostTriple.isOSDarwin()
              ? object::Archive::K_DARWIN
-             : object::Archive::K_GNU;
+             : (HostTriple.isOSAIX() ? object::Archive::K_AIXBIG
+                                     : object::Archive::K_GNU);
 }
 
 static object::Archive::Kind getKindFromMember(const NewArchiveMember &Member) {
@@ -894,7 +897,9 @@
   if (OptionalObject)
     return isa<object::MachOObjectFile>(**OptionalObject)
                ? object::Archive::K_DARWIN
-               : object::Archive::K_GNU;
+               : (isa<object::XCOFFObjectFile>(**OptionalObject)
+                      ? object::Archive::K_AIXBIG
+                      : object::Archive::K_GNU);
 
   // squelch the error in case we had a non-object file
   consumeError(OptionalObject.takeError());
@@ -939,6 +944,8 @@
     else
       Kind = !NewMembers.empty() ? getKindFromMember(NewMembers.front())
                                  : getDefaultForHost();
+    if (Kind == object::Archive::K_AIXBIG)
+      fail("unsupported big AIX write operation");
     break;
   case GNU:
     Kind = object::Archive::K_GNU;