Index: llvm/trunk/lib/MC/MCParser/AsmParser.cpp
===================================================================
--- llvm/trunk/lib/MC/MCParser/AsmParser.cpp
+++ llvm/trunk/lib/MC/MCParser/AsmParser.cpp
@@ -2706,7 +2706,11 @@
 
     Alignment = 1ULL << Alignment;
   } else {
-    // Reject alignments that aren't a power of two, for gas compatibility.
+    // Reject alignments that aren't either a power of two or zero,
+    // for gas compatibility. Alignment of zero is silently rounded
+    // up to one.
+    if (Alignment == 0)
+      Alignment = 1;
     if (!isPowerOf2_64(Alignment))
       Error(AlignmentLoc, "alignment must be a power of 2");
   }
Index: llvm/trunk/test/MC/ELF/align-zero.s
===================================================================
--- llvm/trunk/test/MC/ELF/align-zero.s
+++ llvm/trunk/test/MC/ELF/align-zero.s
@@ -0,0 +1,4 @@
+// Test that an alignment of zero is accepted.
+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o -
+
+  .align 0