[MC/ELF] Accept zero for .align directive

[davide]

Summary

.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). This patch is an attempt to fix.

$ ./llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu blah.s -o blah
blah.s:4:8: error: alignment must be a power of 2
.align 0

^

$ as blah.s -o blah
$

$ cat blah.s
.global _start
.global bar

.align 0
.text
_start:
bar:

movl $bar, %edx

llvm/trunk/lib/MC/MCParser/AsmParser.cpp

if (IsPow2) {
     // FIXME: Diagnose overflow.
     if (Alignment >= 32) {
       Error(AlignmentLoc, "invalid alignment value");
       Alignment = 31;
     }
 
     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");
   }
 
   // Diagnose non-sensical max bytes to align.
   if (MaxBytesLoc.isValid()) {
     if (MaxBytesToFill < 1) {
       Error(MaxBytesLoc, "alignment directive can never be satisfied in this "

llvm/trunk/test/MC/ELF/align-zero.s

+// Test that an alignment of zero is accepted.
+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o -
+
+  .align 0