[MC/ELF] Accept zero for .align directive

.align directive refuses alignment 0 -- a comment in the code hints this is
done for GNU as compatibility, but it seems GNU as accepts .align 0
(and silently rounds up alignment to 1).

Differential Revision:	 http://reviews.llvm.org/D12682

llvm-svn: 247048

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/lib/MC/MCParser/AsmParser.cpp b/llvm/lib/MC/MCParser/AsmParser.cpp
index 5e174de8a93..7126a909b9e 100644
--- a/llvm/lib/MC/MCParser/AsmParser.cpp
+++ b/llvm/lib/MC/MCParser/AsmParser.cpp
@@ -2706,7 +2706,11 @@ bool AsmParser::parseDirectiveAlign(bool IsPow2, unsigned ValueSize) {
 
     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");
   }