MC: Support aligned COMMON symbols for COFF

link.exe:
Fuzz testing has shown that COMMON symbols with size > 32 will always
have an alignment of at least 32 and all symbols with size < 32 will
have an alignment of at least the largest power of 2 less than the size
of the symbol.

binutils:
The BFD linker essentially work like the link.exe behavior but with
alignment 4 instead of 32.  The BFD linker also supports an extension to
COFF which adds an -aligncomm argument to the .drectve section which
permits specifying a precise alignment for a variable but MC currently
doesn't support editing .drectve in this way.

With all of this in mind, we decide to play a little trick: we can
ensure that the alignment will be respected by bumping the size of the
global to it's alignment.

llvm-svn: 218201

1 files changed, 1 insertions, 3 deletions

diff --git a/llvm/lib/MC/MCObjectFileInfo.cpp b/llvm/lib/MC/MCObjectFileInfo.cpp
index da707d860a6..547630ea37d 100644
--- a/llvm/lib/MC/MCObjectFileInfo.cpp
+++ b/llvm/lib/MC/MCObjectFileInfo.cpp
@@ -595,9 +595,7 @@ void MCObjectFileInfo::InitELFMCObjectFileInfo(Triple T) {
 void MCObjectFileInfo::InitCOFFMCObjectFileInfo(Triple T) {
   bool IsWoA = T.getArch() == Triple::arm || T.getArch() == Triple::thumb;
 
-  // The object file format cannot represent common symbols with explicit
-  // alignments.
-  CommDirectiveSupportsAlignment = false;
+  CommDirectiveSupportsAlignment = true;
 
   // COFF
   BSSSection =