Fix the bitfield record layout in codegen for big endian targets.

This was an egregious bug due to the several iterations of refactorings
that took place. Size no longer meant what it original did by the time
I finished, but this line of code never got updated. Unfortunately we
had essentially zero tests for this in the regression test suite. =[

I've added a PPC64 run over the bitfield test case I've been primarily
using. I'm still looking at adding more tests and making sure this is
the *correct* bitfield access code on PPC64 linux, but it looks pretty
close to me, and it is *worlds* better than before this patch as it no
longer asserts! =] More commits to follow with at least additional tests
and maybe more fixes.

Sorry for the long breakage due to this....

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@169691 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 7 insertions, 3 deletions

diff --git a/lib/CodeGen/CGRecordLayoutBuilder.cpp b/lib/CodeGen/CGRecordLayoutBuilder.cpp
index 123848b8fc..92c5672c73 100644
--- a/lib/CodeGen/CGRecordLayoutBuilder.cpp
+++ b/lib/CodeGen/CGRecordLayoutBuilder.cpp
@@ -256,9 +256,13 @@ CGBitFieldInfo CGBitFieldInfo::MakeInfo(CodeGenTypes &Types,
     Size = TypeSizeInBits;
   }
 
-  // Reverse the bit offsets for big endian machines.
-  if (Types.getDataLayout().isBigEndian())
-    Offset = Size - Offset - 1;
+  // Reverse the bit offsets for big endian machines. Because we represent
+  // a bitfield as a single large integer load, we can imagine the bits
+  // counting from the most-significant-bit instead of the
+  // least-significant-bit.
+  if (Types.getDataLayout().isBigEndian()) {
+    Offset = StorageSize - (Offset + Size);
+  }
 
   return CGBitFieldInfo(Offset, Size, IsSigned, StorageSize, StorageAlignment);
 }