[DataLayout] Introduce DataLayout::getAddressSize(AS)

llvm/docs/LangRef.rst

@@ -3147,14 +3147,21 @@ as follows:
 ``A<address space>``
     Specifies the address space of objects created by '``alloca``'.
     Defaults to the default address space of 0.
-``p[n]:<size>:<abi>[:<pref>][:<idx>]``
-    This specifies the *size* of a pointer and its ``<abi>`` and
-    ``<pref>``\erred alignments for address space ``n``.
-    The fourth parameter ``<idx>`` is the size of the
-    index that used for address calculation, which must be less than or equal
-    to the pointer size. If not
-    specified, the default index size is equal to the pointer size. All sizes
-    are in bits. The address space, ``n``, is optional, and if not specified,
+``p[n]:<size>:<abi>[:<pref>[:<idx>]]``
+    This specifies the properties of a pointer in address space ``n``.
+    The ``<size>`` parameter specifies the size of the bitwise representation.
+    For :ref:`non-integral pointers <nointptrtype>` the representation size may
+    be larger than the address width of the underlying address space (e.g. to
+    accommodate additional metadata).
+    The alignment requirements are specified via the ``<abi>`` and
+    ``<pref>``\erred alignments parameters.
+    The fourth parameter ``<idx>`` is the size of the index that used for
+    address calculations such as :ref:`getelementptr <i_getelementptr>`.
+    It must be less than or equal to the pointer size. If not specified, the
+    default index size is equal to the pointer size.
+    The index size also specifies the width of addresses in this address space.
+    All sizes are in bits.
+    The address space, ``n``, is optional, and if not specified,
     denotes the default address space 0. The value of ``n`` must be
     in the range [1,2^24).
 ``i<size>:<abi>[:<pref>]``
@@ -4266,6 +4273,16 @@ address spaces defined in the :ref:`datalayout string<langref_datalayout>`.
 the default globals address space and ``addrspace("P")`` the program address
 space.
 
+The representation of pointers can be different for each address space and does
+not necessarily need to be a plain integer address (e.g. for
+:ref:`non-integral pointers <nointptrtype>`). In addition to a representation
+bits size, pointers in each address space also have an index size which defines
+the bitwidth of indexing operations as well as the size of `integer addresses`
+in this address space. For example, CHERI capabilities are twice the size of the
+underlying addresses to accommodate for additional metadata such as bounds and
+permissions: on a 32-bit system the bitwidth of the pointer representation size
+is 64, but the underlying address width remains 32 bits.
+
 The default address space is number zero.
 
 The semantics of non-zero address spaces are target-specific. Memory

llvm/include/llvm/IR/DataLayout.h

@@ -324,16 +324,38 @@ class DataLayout {
   /// the backends/clients are updated.
   Align getPointerPrefAlignment(unsigned AS = 0) const;
 
-  /// Layout pointer size in bytes, rounded up to a whole
-  /// number of bytes.
+  /// The pointer representation size in bytes, rounded up to a whole number of
+  /// bytes. The difference between this function and getPointerAddressSize() is
+  /// this one returns the size of the entire pointer type (this includes
+  /// metadata bits for fat pointers) and the latter only returns the number of
+  /// address bits.
+  /// \sa DataLayout::getPointerAddressSizeInBits
   /// FIXME: The defaults need to be removed once all of
   /// the backends/clients are updated.
   unsigned getPointerSize(unsigned AS = 0) const;
 
-  // Index size in bytes used for address calculation,
-  /// rounded up to a whole number of bytes.
+  /// The index size in bytes used for address calculation, rounded up to a
+  /// whole number of bytes. This not only defines the size used in
+  /// getelementptr operations, but also the size of addresses in this \p AS.
+  /// For example, a 64-bit CHERI-enabled target has 128-bit pointers of which
+  /// only 64 are used to represent the address and the remaining ones are used
+  /// for metadata such as bounds and access permissions. In this case
+  /// getPointerSize() returns 16, but getIndexSize() returns 8.
+  /// To help with code understanding, the alias getPointerAddressSize() can be
+  /// used instead of getIndexSize() to clarify that an address width is needed.
   unsigned getIndexSize(unsigned AS) const;
 
+  /// The integral size of a pointer in a given address space in bytes, which
+  /// is defined to be the same as getIndexSize(). This exists as a separate
+  /// function to make it clearer when reading code that the size of an address
+  /// is being requested. While targets exist where index size and the
+  /// underlying address width are not identical (e.g. AMDGPU fat pointers with
+  /// 48-bit addresses and 32-bit offsets indexing), there is currently no need
+  /// to differentiate these properties in LLVM.
+  /// \sa DataLayout::getIndexSize
+  /// \sa DataLayout::getPointerAddressSizeInBits
+  unsigned getPointerAddressSize(unsigned AS) const { return getIndexSize(AS); }
+
   /// Return the address spaces containing non-integral pointers.  Pointers in
   /// this address space don't have a well-defined bitwise representation.
   SmallVector<unsigned, 8> getNonIntegralAddressSpaces() const {
@@ -358,29 +380,53 @@ class DataLayout {
     return PTy && isNonIntegralPointerType(PTy);
   }
 
-  /// Layout pointer size, in bits
+  /// The size in bits of the pointer representation in a given address space.
+  /// This is not necessarily the same as the integer address of a pointer (e.g.
+  /// for fat pointers).
+  /// \sa DataLayout::getPointerAddressSizeInBits()
   /// FIXME: The defaults need to be removed once all of
   /// the backends/clients are updated.
   unsigned getPointerSizeInBits(unsigned AS = 0) const {
     return getPointerSpec(AS).BitWidth;
   }
 
-  /// Size in bits of index used for address calculation in getelementptr.
+  /// The size in bits of indices used for address calculation in getelementptr
+  /// and for addresses in the given AS. See getIndexSize() for more
+  /// information.
+  /// \sa DataLayout::getPointerAddressSizeInBits()
   unsigned getIndexSizeInBits(unsigned AS) const {
     return getPointerSpec(AS).IndexBitWidth;
   }
 
-  /// Layout pointer size, in bits, based on the type.  If this function is
+  /// The size in bits of an address in for the given AS. This is defined to
+  /// return the same value as getIndexSizeInBits() since there is currently no
+  /// target that requires these two properties to have different values. See
+  /// getIndexSize() for more information.
+  /// \sa DataLayout::getIndexSizeInBits()
+  unsigned getPointerAddressSizeInBits(unsigned AS) const {
+    return getIndexSizeInBits(AS);
+  }
+
+  /// The pointer representation size in bits for this type. If this function is
   /// called with a pointer type, then the type size of the pointer is returned.
   /// If this function is called with a vector of pointers, then the type size
   /// of the pointer is returned.  This should only be called with a pointer or
   /// vector of pointers.
   unsigned getPointerTypeSizeInBits(Type *) const;
 
-  /// Layout size of the index used in GEP calculation.
+  /// The size in bits of the index used in GEP calculation for this type.
   /// The function should be called with pointer or vector of pointers type.
+  /// This is defined to return the same value as getPointerAddressSizeInBits(),
+  /// but separate functions exist for code clarity.
   unsigned getIndexTypeSizeInBits(Type *Ty) const;
 
+  /// The size in bits of an address for this type.
+  /// This is defined to return the same value as getIndexTypeSizeInBits(),
+  /// but separate functions exist for code clarity.
+  unsigned getPointerAddressSizeInBits(Type *Ty) const {
+    return getIndexTypeSizeInBits(Ty);
+  }
+
   unsigned getPointerTypeSize(Type *Ty) const {
     return getPointerTypeSizeInBits(Ty) / 8;
   }