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

2 files changed, 16 insertions, 5 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 =
diff --git a/llvm/lib/MC/WinCOFFStreamer.cpp b/llvm/lib/MC/WinCOFFStreamer.cpp
index 1d03e2d742f..2829a8ff0b5 100644
--- a/llvm/lib/MC/WinCOFFStreamer.cpp
+++ b/llvm/lib/MC/WinCOFFStreamer.cpp
@@ -183,8 +183,21 @@ void MCWinCOFFStreamer::EmitCommonSymbol(MCSymbol *Symbol, uint64_t Size,
           Symbol->getSection().getVariant() == MCSection::SV_COFF) &&
          "Got non-COFF section in the COFF backend!");
 
-  if (ByteAlignment > 32)
-    report_fatal_error("alignment is limited to 32-bytes");
+  const Triple &T = getContext().getObjectFileInfo()->getTargetTriple();
+  if (T.isKnownWindowsMSVCEnvironment()) {
+    if (ByteAlignment > 32)
+      report_fatal_error("alignment is limited to 32-bytes");
+  } else {
+    // The bfd linker from binutils only supports alignments less than 4 bytes
+    // without inserting -aligncomm arguments into the .drectve section.
+    // TODO: Support inserting -aligncomm into the .drectve section.
+    if (ByteAlignment > 4)
+      report_fatal_error("alignment is limited to 4-bytes");
+  }
+  // Round size up to alignment so that we will honor the alignment request.
+  // TODO: We don't need to do this if we are targeting the bfd linker once we
+  // add support for adding -aligncomm into the .drectve section.
+  Size = std::max(Size, static_cast<uint64_t>(ByteAlignment));
 
   AssignSection(Symbol, nullptr);