Index: lib/MC/MCParser/AsmParser.cpp
===================================================================
--- lib/MC/MCParser/AsmParser.cpp
+++ lib/MC/MCParser/AsmParser.cpp
@@ -3199,7 +3199,7 @@
   int64_t MaxBytesToFill = 0;
 
   auto parseAlign = [&]() -> bool {
-    if (checkForValidSection() || parseAbsoluteExpression(Alignment))
+    if (parseAbsoluteExpression(Alignment))
       return true;
     if (parseOptionalToken(AsmToken::Comma)) {
       // The fill expression can be omitted while specifying a maximum number of
@@ -3218,6 +3218,13 @@
     return parseToken(AsmToken::EndOfStatement);
   };
 
+  if (checkForValidSection())
+    return addErrorSuffix(" in directive");
+  // GAS-Compatibility: Ignore empty '.p2align', but add a warning
+  if (IsPow2 && (ValueSize == 1) && getTok().is(AsmToken::EndOfStatement)) {
+    Warning(AlignmentLoc, "p2align directive with no operand(s) is ignored");
+    return parseToken(AsmToken::EndOfStatement);
+  }
   if (parseAlign())
     return addErrorSuffix(" in directive");
 
Index: test/MC/AsmParser/directive_align.s
===================================================================
--- test/MC/AsmParser/directive_align.s
+++ test/MC/AsmParser/directive_align.s
@@ -1,4 +1,5 @@
-# RUN: not llvm-mc -triple i386-apple-darwin9 %s | FileCheck %s
+# RUN: not llvm-mc -triple i386-apple-darwin9 %s 2> %t.err | FileCheck %s
+# RUN: FileCheck < %t.err %s --check-prefix=CHECK-WARN
 
 # CHECK: TEST0:
 # CHECK: .p2align 1
@@ -14,3 +15,7 @@
 # CHECK: .balign 3, 10
 TEST2:  
         .balign 3,10
+
+# CHECK-WARN: p2align directive with no operand(s) is ignored
+TEST3:  
+        .p2align