[cfe-dev] double/virtual offsets for MSVC broken in 3.1

[Don Williamson]

So, the problem is that the same approach needs to be applied to the vftable pointer, as well as the vbtable pointer.

I have a small diff of changes that pass all my tests... is this good enough to submit? (I'm working against 3.1 but the same is true of the trunk and I can sync to that to submit a patch)

Index: lib/AST/RecordLayoutBuilder.cpp
===================================================================
--- lib/AST/RecordLayoutBuilder.cpp     (revision 162839)
+++ lib/AST/RecordLayoutBuilder.cpp     (working copy)
@@ -1009,7 +1009,7 @@

   // Compute base subobject info.
   ComputeBaseSubobjectInfo(RD);
-
+
   // If we have a primary base class, lay it out.
   if (PrimaryBase) {
     if (PrimaryBaseIsVirtual) {
@@ -1044,8 +1044,13 @@
     CharUnits PtrAlign =
       Context.toCharUnitsFromBits(Context.getTargetInfo().getPointerAlign(0));
     EnsureVTablePointerAlignment(PtrAlign);
-    if (isMicrosoftCXXABI())
+    if (isMicrosoftCXXABI()) {
       VFPtrOffset = getSize();
+
+      // MSVC will extend the width of the vftable pointer to the max width
+         // of all fields in the record
+      PtrWidth = std::max(Alignment, PtrWidth);
+    }
     setSize(getSize() + PtrWidth);
     setDataSize(getSize());
   }
@@ -2222,6 +2227,16 @@
       Builder.Layout(RD);
     }

+       // The same is true in MSVC when there is a vftable pointer
+    CharUnits PtrWidth =
+      toCharUnitsFromBits(getTargetInfo().getPointerWidth(0));
+    if (Builder.isMicrosoftCXXABI() &&
+        Builder.VFPtrOffset != CharUnits::fromQuantity(-1) &&
+        Builder.NonVirtualAlignment.getQuantity() > PtrWidth.getQuantity()) {
+      Builder.resetWithTargetAlignment(Builder.NonVirtualAlignment);
+      Builder.Layout(RD);
+    }
+
     // FIXME: This is not always correct. See the part about bitfields at
     // http://www.codesourcery.com/public/cxx-abi/abi.html#POD for more info.
     // FIXME: IsPODForThePurposeOfLayout should be stored in the record layout.




________________________________
 From: Don Williamson <don.williamson at yahoo.com>
To: cfe-dev <cfe-dev at cs.uiuc.edu> 
Sent: Thursday, August 30, 2012 12:07 PM
Subject: Re: [cfe-dev] double/virtual offsets for MSVC broken in 3.1
 

The change appears to stem from:

http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/AST/RecordLayoutBuilder.cpp?r1=143800&r2=144072&diff_format=h
Revision 144072, Modified Mon Nov 7 22:01:03 2011 CST (9 months, 3 weeks ago) by rjmccall 
Fix the layout of vb-tables and vf-tables in the MS C++ ABI. Based on work by Dmitry Sokolov!

Trying to figure out what's different...



________________________________
 From: Don Williamson <don.williamson at yahoo.com>
To: cfe-dev <cfe-dev at cs.uiuc.edu> 
Sent: Thursday, August 30, 2012 11:26 AM
Subject: [cfe-dev] double/virtual offsets for MSVC broken in 3.1
 

Hi,

last year I submitted a patch (commit by somebody else) to solve cases like this:

// The addition of a double anywhere in this struct forces the vtable ptr to occupy 4 bytes + 4 bytes padding in MSVC
struct DoubleInPolymorphicStruct
{
virtual void Empty() { }
int a;
double b;
};


The offset of a in MSVC is 8 bytes, but clang in MSVC offset calculation mode puts it at 4. The initial posts are here:

http://lists.cs.uiuc.edu/pipermail/cfe-dev/2011-September/016944.html
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2011-September/016970.html

These changes did not make it into 3.0 but as far as I could tell, did make it into the trunk. However, I've just sync'ed up to the 3.1 release and this is still a problem.

I'm about to go back and take a look and what's broken but it'll take me a couple of hours to settle back into the code again.

Meanwhile, could anybody help with any ideas as to what may have changed?

Thanks!
- Don
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