Respect alignment of nested bitfields

tools/clang/test/CodeGen/packed-nest-unpacked.c contains this test:

struct XBitfield {
  unsigned b1 : 10;
  unsigned b2 : 12;
  unsigned b3 : 10;
};
struct YBitfield {
  char x;
  struct XBitfield y;
} __attribute((packed));
struct YBitfield gbitfield;

unsigned test7() {
  // CHECK: @test7
  // CHECK: load i32, i32* getelementptr inbounds (%struct.YBitfield, %struct.YBitfield* @gbitfield, i32 0, i32 1, i32 0), align 4
  return gbitfield.y.b2;
}

The "align 4" is actually wrong.  Accessing all of "gbitfield.y" as a single
i32 is of course possible, but that still doesn't make it 4-byte aligned as
it remains packed at offset 1 in the surrounding gbitfield object.

This alignment was changed by commit r169489, which also introduced changes
to bitfield access code in CGExpr.cpp.  Code before that change used to take
into account *both* the alignment of the field to be accessed within the
current struct, *and* the alignment of that outer struct itself; this logic
was removed by the above commit.

Neglecting to consider both values can cause incorrect code to be generated
(I've seen an unaligned access crash on SystemZ due to this bug).

In order to always use the best known alignment value, this patch removes
the CGBitFieldInfo::StorageAlignment member and replaces it with a
StorageOffset member specifying the offset from the start of the surrounding
struct to the bitfield's underlying storage.  This offset can then be combined
with the best-known alignment for a bitfield access lvalue to determine the
alignment to use when accessing the bitfield's storage.

Differential Revision: http://reviews.llvm.org/D11034


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@241916 91177308-0d34-0410-b5e6-96231b3b80d8

Author: uweigand

lib/CodeGen/CGAtomic.cpp

@@ -93,6 +93,7 @@ namespace {
         BFI = OrigBFI;
         BFI.Offset = Offset;
         BFI.StorageSize = AtomicSizeInBits;
+        BFI.StorageOffset += OffsetInChars;
         LVal = LValue::MakeBitfield(Addr, BFI, lvalue.getType(),
                                     lvalue.getAlignment());
         LVal.setTBAAInfo(lvalue.getTBAAInfo());

lib/CodeGen/CGClass.cpp

@@ -911,32 +911,22 @@ namespace {
         return;
       }
 
-      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));
+        FirstByteOffset = CGF.getContext().toBits(BFInfo.StorageOffset);
       } else {
-        Alignment = CGF.getContext().getDeclAlign(FirstField);
         FirstByteOffset = FirstFieldOffset;
       }
 
-      assert((CGF.getContext().toCharUnitsFromBits(FirstByteOffset) %
-              Alignment) == 0 && "Bad field alignment.");
-
       CharUnits MemcpySize = getMemcpySize(FirstByteOffset);
       QualType RecordTy = CGF.getContext().getTypeDeclType(ClassDecl);
       llvm::Value *ThisPtr = CGF.LoadCXXThis();
@@ -946,6 +936,9 @@ namespace {
       LValue SrcLV = CGF.MakeNaturalAlignAddrLValue(SrcPtr, RecordTy);
       LValue Src = CGF.EmitLValueForFieldInitialization(SrcLV, FirstField);
 
+      CharUnits Offset = CGF.getContext().toCharUnitsFromBits(FirstByteOffset);
+      CharUnits Alignment = DestLV.getAlignment().alignmentAtOffset(Offset);
+
       emitMemcpyIR(Dest.isBitField() ? Dest.getBitFieldAddr() : Dest.getAddress(),
                    Src.isBitField() ? Src.getBitFieldAddr() : Src.getAddress(),
                    MemcpySize, Alignment);

lib/CodeGen/CGExpr.cpp

@@ -1356,14 +1356,15 @@ RValue CodeGenFunction::EmitLoadOfLValue(LValue LV, SourceLocation Loc) {
 
 RValue CodeGenFunction::EmitLoadOfBitfieldLValue(LValue LV) {
   const CGBitFieldInfo &Info = LV.getBitFieldInfo();
+  CharUnits Align = LV.getAlignment().alignmentAtOffset(Info.StorageOffset);
 
   // Get the output type.
   llvm::Type *ResLTy = ConvertType(LV.getType());
 
   llvm::Value *Ptr = LV.getBitFieldAddr();
-  llvm::Value *Val = Builder.CreateLoad(Ptr, LV.isVolatileQualified(),
-                                        "bf.load");
-  cast<llvm::LoadInst>(Val)->setAlignment(Info.StorageAlignment);
+  llvm::Value *Val = Builder.CreateAlignedLoad(Ptr, Align.getQuantity(),
+                                               LV.isVolatileQualified(),
+                                               "bf.load");
 
   if (Info.IsSigned) {
     assert(static_cast<unsigned>(Info.Offset + Info.Size) <= Info.StorageSize);
@@ -1559,6 +1560,7 @@ void CodeGenFunction::EmitStoreThroughLValue(RValue Src, LValue Dst,
 void CodeGenFunction::EmitStoreThroughBitfieldLValue(RValue Src, LValue Dst,
                                                      llvm::Value **Result) {
   const CGBitFieldInfo &Info = Dst.getBitFieldInfo();
+  CharUnits Align = Dst.getAlignment().alignmentAtOffset(Info.StorageOffset);
   llvm::Type *ResLTy = ConvertTypeForMem(Dst.getType());
   llvm::Value *Ptr = Dst.getBitFieldAddr();
 
@@ -1575,9 +1577,9 @@ void CodeGenFunction::EmitStoreThroughBitfieldLValue(RValue Src, LValue Dst,
   // and mask together with source before storing.
   if (Info.StorageSize != Info.Size) {
     assert(Info.StorageSize > Info.Size && "Invalid bitfield size.");
-    llvm::Value *Val = Builder.CreateLoad(Ptr, Dst.isVolatileQualified(),
-                                          "bf.load");
-    cast<llvm::LoadInst>(Val)->setAlignment(Info.StorageAlignment);
+    llvm::Value *Val = Builder.CreateAlignedLoad(Ptr, Align.getQuantity(),
+                                                 Dst.isVolatileQualified(),
+                                                 "bf.load");
 
     // Mask the source value as needed.
     if (!hasBooleanRepresentation(Dst.getType()))
@@ -1603,9 +1605,8 @@ void CodeGenFunction::EmitStoreThroughBitfieldLValue(RValue Src, LValue Dst,
   }
 
   // Write the new value back out.
-  llvm::StoreInst *Store = Builder.CreateStore(SrcVal, Ptr,
-                                               Dst.isVolatileQualified());
-  Store->setAlignment(Info.StorageAlignment);
+  Builder.CreateAlignedStore(SrcVal, Ptr, Align.getQuantity(),
+                             Dst.isVolatileQualified());
 
   // Return the new value of the bit-field, if requested.
   if (Result) {

lib/CodeGen/CGObjCRuntime.cpp

@@ -134,7 +134,7 @@ LValue CGObjCRuntime::EmitValueForIvarAtOffset(CodeGen::CodeGenFunction &CGF,
   CGBitFieldInfo *Info = new (CGF.CGM.getContext()) CGBitFieldInfo(
     CGBitFieldInfo::MakeInfo(CGF.CGM.getTypes(), Ivar, BitOffset, BitFieldSize,
                              CGF.CGM.getContext().toBits(StorageSize),
-                             Alignment.getQuantity()));
+                             CharUnits::fromQuantity(0)));
 
   V = CGF.Builder.CreateBitCast(V,
                                 llvm::Type::getIntNPtrTy(CGF.getLLVMContext(),

lib/CodeGen/CGRecordLayout.h

@@ -78,16 +78,16 @@ struct CGBitFieldInfo {
   /// bitfield.
   unsigned StorageSize;
 
-  /// The alignment which should be used when accessing the bitfield.
-  unsigned StorageAlignment;
+  /// The offset of the bitfield storage from the start of the struct.
+  CharUnits StorageOffset;
 
   CGBitFieldInfo()
-      : Offset(), Size(), IsSigned(), StorageSize(), StorageAlignment() {}
+      : Offset(), Size(), IsSigned(), StorageSize(), StorageOffset() {}
 
   CGBitFieldInfo(unsigned Offset, unsigned Size, bool IsSigned,
-                 unsigned StorageSize, unsigned StorageAlignment)
+                 unsigned StorageSize, CharUnits StorageOffset)
       : Offset(Offset), Size(Size), IsSigned(IsSigned),
-        StorageSize(StorageSize), StorageAlignment(StorageAlignment) {}
+        StorageSize(StorageSize), StorageOffset(StorageOffset) {}
 
   void print(raw_ostream &OS) const;
   void dump() const;
@@ -99,7 +99,7 @@ struct CGBitFieldInfo {
                                  const FieldDecl *FD,
                                  uint64_t Offset, uint64_t Size,
                                  uint64_t StorageSize,
-                                 uint64_t StorageAlignment);
+                                 CharUnits StorageOffset);
 };
 
 /// CGRecordLayout - This class handles struct and union layout info while

lib/CodeGen/CGRecordLayoutBuilder.cpp

@@ -228,11 +228,7 @@ void CGRecordLowering::setBitFieldInfo(
   Info.Offset = (unsigned)(getFieldBitOffset(FD) - Context.toBits(StartOffset));
   Info.Size = FD->getBitWidthValue(Context);
   Info.StorageSize = (unsigned)DataLayout.getTypeAllocSizeInBits(StorageType);
-  // Here we calculate the actual storage alignment of the bits.  E.g if we've
-  // got an alignment >= 2 and the bitfield starts at offset 6 we've got an
-  // alignment of 2.
-  Info.StorageAlignment =
-      Layout.getAlignment().alignmentAtOffset(StartOffset).getQuantity();
+  Info.StorageOffset = StartOffset;
   if (Info.Size > Info.StorageSize)
     Info.Size = Info.StorageSize;
   // Reverse the bit offsets for big endian machines. Because we represent
@@ -651,7 +647,7 @@ CGBitFieldInfo CGBitFieldInfo::MakeInfo(CodeGenTypes &Types,
                                         const FieldDecl *FD,
                                         uint64_t Offset, uint64_t Size,
                                         uint64_t StorageSize,
-                                        uint64_t StorageAlignment) {
+                                        CharUnits StorageOffset) {
   // This function is vestigial from CGRecordLayoutBuilder days but is still 
   // used in GCObjCRuntime.cpp.  That usage has a "fixme" attached to it that
   // when addressed will allow for the removal of this function.
@@ -683,7 +679,7 @@ CGBitFieldInfo CGBitFieldInfo::MakeInfo(CodeGenTypes &Types,
     Offset = StorageSize - (Offset + Size);
   }
 
-  return CGBitFieldInfo(Offset, Size, IsSigned, StorageSize, StorageAlignment);
+  return CGBitFieldInfo(Offset, Size, IsSigned, StorageSize, StorageOffset);
 }
 
 CGRecordLayout *CodeGenTypes::ComputeRecordLayout(const RecordDecl *D,
@@ -856,7 +852,7 @@ void CGBitFieldInfo::print(raw_ostream &OS) const {
      << " Size:" << Size
      << " IsSigned:" << IsSigned
      << " StorageSize:" << StorageSize
-     << " StorageAlignment:" << StorageAlignment << ">";
+     << " StorageOffset:" << StorageOffset.getQuantity() << ">";
 }
 
 void CGBitFieldInfo::dump() const {

test/CodeGen/bitfield-2.c

@@ -14,7 +14,7 @@
 // CHECK-RECORD:   LLVMType:%struct.s0 = type { [3 x i8] }
 // CHECK-RECORD:   IsZeroInitializable:1
 // CHECK-RECORD:   BitFields:[
-// CHECK-RECORD:     <CGBitFieldInfo Offset:0 Size:24 IsSigned:1 StorageSize:24 StorageAlignment:1>
+// CHECK-RECORD:     <CGBitFieldInfo Offset:0 Size:24 IsSigned:1 StorageSize:24 StorageOffset:0>
 struct __attribute((packed)) s0 {
   int f0 : 24;
 };
@@ -54,8 +54,8 @@ unsigned long long test_0() {
 // CHECK-RECORD:   LLVMType:%struct.s1 = type { [3 x i8] }
 // CHECK-RECORD:   IsZeroInitializable:1
 // CHECK-RECORD:   BitFields:[
-// CHECK-RECORD:     <CGBitFieldInfo Offset:0 Size:10 IsSigned:1 StorageSize:24 StorageAlignment:1>
-// CHECK-RECORD:     <CGBitFieldInfo Offset:10 Size:10 IsSigned:1 StorageSize:24 StorageAlignment:1>
+// CHECK-RECORD:     <CGBitFieldInfo Offset:0 Size:10 IsSigned:1 StorageSize:24 StorageOffset:0>
+// CHECK-RECORD:     <CGBitFieldInfo Offset:10 Size:10 IsSigned:1 StorageSize:24 StorageOffset:0>
 
 #pragma pack(push)
 #pragma pack(1)
@@ -102,7 +102,7 @@ unsigned long long test_1() {
 // CHECK-RECORD:   LLVMType:%union.u2 = type { i8 }
 // CHECK-RECORD:   IsZeroInitializable:1
 // CHECK-RECORD:   BitFields:[
-// CHECK-RECORD:     <CGBitFieldInfo Offset:0 Size:3 IsSigned:0 StorageSize:8 StorageAlignment:1>
+// CHECK-RECORD:     <CGBitFieldInfo Offset:0 Size:3 IsSigned:0 StorageSize:8 StorageOffset:0>
 
 union __attribute__((packed)) u2 {
   unsigned long long f0 : 3;
@@ -274,8 +274,8 @@ _Bool test_6() {
 // CHECK-RECORD:   LLVMType:%struct.s7 = type { i32, i32, i32, i8, i32, [12 x i8] }
 // CHECK-RECORD:   IsZeroInitializable:1
 // CHECK-RECORD:   BitFields:[
-// CHECK-RECORD:     <CGBitFieldInfo Offset:0 Size:5 IsSigned:1 StorageSize:8 StorageAlignment:4>
-// CHECK-RECORD:     <CGBitFieldInfo Offset:0 Size:29 IsSigned:1 StorageSize:32 StorageAlignment:16>
+// CHECK-RECORD:     <CGBitFieldInfo Offset:0 Size:5 IsSigned:1 StorageSize:8 StorageOffset:12>
+// CHECK-RECORD:     <CGBitFieldInfo Offset:0 Size:29 IsSigned:1 StorageSize:32 StorageOffset:16>
 
 struct __attribute__((aligned(16))) s7 {
   int a, b, c;

test/CodeGen/packed-nest-unpacked.c

@@ -60,6 +60,35 @@ struct YBitfield gbitfield;
 
 unsigned test7() {
   // CHECK: @test7
-  // CHECK: load i32, i32* getelementptr inbounds (%struct.YBitfield, %struct.YBitfield* @gbitfield, i32 0, i32 1, i32 0), align 4
+  // CHECK: load i32, i32* getelementptr inbounds (%struct.YBitfield, %struct.YBitfield* @gbitfield, i32 0, i32 1, i32 0), align 1
   return gbitfield.y.b2;
 }
+
+void test8(unsigned x) {
+  // CHECK: @test8
+  // CHECK: load i32, i32* getelementptr inbounds (%struct.YBitfield, %struct.YBitfield* @gbitfield, i32 0, i32 1, i32 0), align 1
+  // CHECK: store i32 {{.*}}, i32* getelementptr inbounds (%struct.YBitfield, %struct.YBitfield* @gbitfield, i32 0, i32 1, i32 0), align 1
+  gbitfield.y.b2 = x;
+}
+
+struct TBitfield
+{
+  long a;
+  char b;
+  unsigned c:15;
+};
+struct TBitfield tbitfield;
+
+unsigned test9() {
+  // CHECK: @test9
+  // CHECK: load i16, i16* getelementptr inbounds (%struct.TBitfield, %struct.TBitfield* @tbitfield, i32 0, i32 2), align 1
+  return tbitfield.c;
+}
+
+void test10(unsigned x) {
+  // CHECK: @test10
+  // CHECK: load i16, i16* getelementptr inbounds (%struct.TBitfield, %struct.TBitfield* @tbitfield, i32 0, i32 2), align 1
+  // CHECK: store i16 {{.*}}, i16* getelementptr inbounds (%struct.TBitfield, %struct.TBitfield* @tbitfield, i32 0, i32 2), align 1
+  tbitfield.c = x;
+}
+