IRgen/Obj-C: Rewrite Objective-C bit-field access to compute the access strategy

using the same methods as used for normal structures.
 - This fixes problems with reading past the end of the structure and with
   handling straddled bit-field access.

llvm-svn: 112914

clang/lib/CodeGen/CGObjCMac.cpp

@@ -113,11 +113,27 @@ LValue CGObjCRuntime::EmitValueForIvarAtOffset(CodeGen::CodeGenFunction &CGF,
     return LV;
   }
 
-  // We need to compute the bit offset for the bit-field, the offset is to the
-  // byte. Note, there is a subtle invariant here: we can only call this routine
-  // on non-synthesized ivars but we may be called for synthesized ivars.
-  // However, a synthesized ivar can never be a bit-field, so this is safe.
-  uint64_t BitOffset = LookupFieldBitOffset(CGF.CGM, OID, 0, Ivar) % 8;
+  // We need to compute an access strategy for this bit-field. We are given the
+  // offset to the first byte in the bit-field, the sub-byte offset is taken
+  // from the original layout. We reuse the normal bit-field access strategy by
+  // treating this as an access to a struct where the bit-field is in byte 0,
+  // and adjust the containing type size as appropriate.
+  //
+  // FIXME: Note that currently we make a very conservative estimate of the
+  // alignment of the bit-field, because (a) it is not clear what guarantees the
+  // runtime makes us, and (b) we don't have a way to specify that the struct is
+  // at an alignment plus offset.
+  //
+  // Note, there is a subtle invariant here: we can only call this routine on
+  // non-synthesized ivars but we may be called for synthesized ivars.  However,
+  // a synthesized ivar can never be a bit-field, so this is safe.
+  const ASTRecordLayout &RL =
+    CGF.CGM.getContext().getASTObjCInterfaceLayout(OID);
+  uint64_t TypeSizeInBits = RL.getSize();
+  uint64_t FieldBitOffset = LookupFieldBitOffset(CGF.CGM, OID, 0, Ivar);
+  uint64_t BitOffset = FieldBitOffset % 8;
+  uint64_t ContainingTypeAlign = 8;
+  uint64_t ContainingTypeSize = TypeSizeInBits - (FieldBitOffset - BitOffset);
   uint64_t BitFieldSize =
     Ivar->getBitWidth()->EvaluateAsInt(CGF.getContext()).getZExtValue();
 
@@ -127,23 +143,10 @@ LValue CGObjCRuntime::EmitValueForIvarAtOffset(CodeGen::CodeGenFunction &CGF,
   // layout object. However, this is blocked on other cleanups to the
   // Objective-C code, so for now we just live with allocating a bunch of these
   // objects.
+  CGBitFieldInfo *Info = new (CGF.CGM.getContext()) CGBitFieldInfo(
+    CGBitFieldInfo::MakeInfo(CGF.CGM.getTypes(), Ivar, BitOffset, BitFieldSize,
+                             ContainingTypeSize, ContainingTypeAlign));
 
-  // We always construct a single, possibly unaligned, access for this case.
-  CGBitFieldInfo::AccessInfo AI;
-  AI.FieldIndex = 0;
-  AI.FieldByteOffset = 0;
-  AI.FieldBitStart = BitOffset;
-  AI.AccessWidth = CGF.CGM.getContext().getTypeSize(IvarTy);
-  AI.AccessAlignment = 0;
-  AI.TargetBitOffset = 0;
-  AI.TargetBitWidth = BitFieldSize;
-
-  CGBitFieldInfo *Info =
-    new (CGF.CGM.getContext()) CGBitFieldInfo(BitFieldSize, 1, &AI,
-                                              IvarTy->isSignedIntegerType());
-
-  // FIXME: We need to set a very conservative alignment on this, or make sure
-  // that the runtime is doing the right thing.
   return LValue::MakeBitfield(V, *Info,
                               IvarTy.getCVRQualifiers() | CVRQualifiers);
 }

clang/test/CodeGenObjC/bitfield-access.m

@@ -0,0 +1,43 @@
+// RUN: %clang_cc1 -triple i386-apple-darwin10 -emit-llvm -o %t %s
+// RUN: FileCheck -check-prefix=CHECK-I386 < %t %s
+// RUN: %clang_cc1 -triple armv6-apple-darwin10 -target-abi apcs-gnu -emit-llvm -o %t %s
+// RUN: FileCheck -check-prefix=CHECK-ARM < %t %s
+
+@interface I0 { 
+@public
+    unsigned x:15;
+    unsigned y: 1;
+} 
+@end
+
+// Check that we don't try to use an i32 load here, which would reach beyond the
+// end of the structure.
+//
+// CHECK-I386: define i32 @f0(
+// CHECK-I386:   [[t0_0:%.*]] = load i16* {{.*}}, align 1
+// CHECK-I386:   lshr i16 [[t0_0]], 7
+// CHECK-I386: }
+int f0(I0 *a) {
+  return a->y;
+}
+
+// Check that we can handled straddled loads.
+//
+// CHECK-ARM: define i32 @f1(
+// CHECK-ARM:    [[t1_ptr:%.*]] = getelementptr
+// CHECK-ARM:    [[t1_base:%.*]] = bitcast i8* [[t1_ptr]] to i32*
+// CHECK-ARM:    [[t1_0:%.*]] = load i32* [[t1_base]], align 1
+// CHECK-ARM:    lshr i32 [[t1_0]], 1
+// CHECK-ARM:    [[t1_base_2_cast:%.*]] = bitcast i32* %1 to i8*
+// CHECK-ARM:    [[t1_base_2:%.*]] = getelementptr i8* [[t1_base_2_cast]]
+// CHECK-ARM:    [[t1_1:%.*]] = load i8* [[t1_base_2]], align 1
+// CHECK-ARM:    and i8 [[t1_1:%.*]], 1
+// CHECK-ARM: }
+@interface I1 {
+@public
+    unsigned x: 1;
+    unsigned y:32;
+}
+@end
+
+int f1(I1 *a) { return a->y; }