r219524 - CodeGen: FieldMemcpyizer didn't handle copies starting inside bitfields

[David Majnemer]

Author: majnemer
Date: Fri Oct 10 13:57:10 2014
New Revision: 219524

URL: http://llvm.org/viewvc/llvm-project?rev=219524&view=rev
Log:
CodeGen: FieldMemcpyizer didn't handle copies starting inside bitfields

It's possible to construct cases where the first field we are trying to
copy is in the middle of an IR field.  In some complicated cases, we
would fail to use an appropriate offset inside the object.  Earlier
builds of clang seemed to miscompile the code by copying an insufficient
number of bytes.  Up until now, we would assert: the copying offset was
insufficiently aligned.

This fixes PR21232.

Modified:
    cfe/trunk/lib/CodeGen/CGClass.cpp
    cfe/trunk/test/CodeGenCXX/pod-member-memcpys.cpp

Modified: cfe/trunk/lib/CodeGen/CGClass.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGClass.cpp?rev=219524&r1=219523&r2=219524&view=diff
==============================================================================
--- cfe/trunk/lib/CodeGen/CGClass.cpp (original)
+++ cfe/trunk/lib/CodeGen/CGClass.cpp Fri Oct 10 13:57:10 2014
@@ -796,13 +796,13 @@ namespace {
         addNextField(F);
     }
 
-    CharUnits getMemcpySize() const {
+    CharUnits getMemcpySize(uint64_t FirstByteOffset) const {
       unsigned LastFieldSize =
         LastField->isBitField() ?
           LastField->getBitWidthValue(CGF.getContext()) :
           CGF.getContext().getTypeSize(LastField->getType()); 
       uint64_t MemcpySizeBits =
-        LastFieldOffset + LastFieldSize - FirstFieldOffset +
+        LastFieldOffset + LastFieldSize - FirstByteOffset +
         CGF.getContext().getCharWidth() - 1;
       CharUnits MemcpySize =
         CGF.getContext().toCharUnitsFromBits(MemcpySizeBits);
@@ -818,19 +818,31 @@ namespace {
 
       CharUnits Alignment;
 
+      uint64_t FirstByteOffset;
       if (FirstField->isBitField()) {
         const CGRecordLayout &RL =
           CGF.getTypes().getCGRecordLayout(FirstField->getParent());
         const CGBitFieldInfo &BFInfo = RL.getBitFieldInfo(FirstField);
         Alignment = CharUnits::fromQuantity(BFInfo.StorageAlignment);
+        // FirstFieldOffset is not appropriate for bitfields,
+        // it won't tell us what the storage offset should be and thus might not
+        // be properly aligned.
+        //
+        // Instead calculate the storage offset using the offset of the field in
+        // the struct type.
+        const llvm::DataLayout &DL = CGF.CGM.getDataLayout();
+        FirstByteOffset =
+            DL.getStructLayout(RL.getLLVMType())
+                ->getElementOffsetInBits(RL.getLLVMFieldNo(FirstField));
       } else {
         Alignment = CGF.getContext().getDeclAlign(FirstField);
+        FirstByteOffset = FirstFieldOffset;
       }
 
-      assert((CGF.getContext().toCharUnitsFromBits(FirstFieldOffset) %
+      assert((CGF.getContext().toCharUnitsFromBits(FirstByteOffset) %
               Alignment) == 0 && "Bad field alignment.");
 
-      CharUnits MemcpySize = getMemcpySize();
+      CharUnits MemcpySize = getMemcpySize(FirstByteOffset);
       QualType RecordTy = CGF.getContext().getTypeDeclType(ClassDecl);
       llvm::Value *ThisPtr = CGF.LoadCXXThis();
       LValue DestLV = CGF.MakeNaturalAlignAddrLValue(ThisPtr, RecordTy);

Modified: cfe/trunk/test/CodeGenCXX/pod-member-memcpys.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/CodeGenCXX/pod-member-memcpys.cpp?rev=219524&r1=219523&r2=219524&view=diff
==============================================================================
--- cfe/trunk/test/CodeGenCXX/pod-member-memcpys.cpp (original)
+++ cfe/trunk/test/CodeGenCXX/pod-member-memcpys.cpp Fri Oct 10 13:57:10 2014
@@ -67,6 +67,13 @@ struct BitfieldMember2 {
   NonPOD np;
 };
 
+struct BitfieldMember3 {
+  virtual void f();
+  int   : 8;
+  int x : 1;
+  int y;
+};
+
 struct InnerClassMember {
   struct {
     int a, b, c, d;
@@ -174,6 +181,7 @@ CALL_AO(PackedMembers)
 
 CALL_CC(PackedMembers)
 CALL_CC(BitfieldMember2)
+CALL_CC(BitfieldMember3)
 CALL_CC(ReferenceMember)
 CALL_CC(InnerClassMember)
 CALL_CC(BitfieldMember)
@@ -243,6 +251,11 @@ CALL_CC(Basic)
 // CHECK: call void @llvm.memcpy.p0i8.p0i8.i64({{.*}}i64 16, i32 8{{.*}})
 // CHECK: ret void
 
+// BitfieldMember3 copy-constructor:
+// CHECK-LABEL: define linkonce_odr void @_ZN15BitfieldMember3C2ERKS_(%struct.BitfieldMember3* %this, %struct.BitfieldMember3* dereferenceable({{[0-9]+}}))
+// CHECK: call void @llvm.memcpy.p0i8.p0i8.i64(i8* %3, i8* %4, i64 8, i32 8, i1 false)
+// CHECK: ret void
+
 // BitfieldMember2 copy-constructor:
 // CHECK-2-LABEL: define linkonce_odr void @_ZN15BitfieldMember2C2ERKS_(%struct.BitfieldMember2* %this, %struct.BitfieldMember2* dereferenceable({{[0-9]+}}))
 // CHECK-2: call void @llvm.memcpy.p0i8.p0i8.i64({{.*}}i64 16, i32 4, i1 false)