[PATCH] D63525: LangRef: Attempt to formulate some rules for addrspacecast

[Matt Arsenault via Phabricator via llvm-commits]

arsenm updated this revision to Diff 205986.
arsenm added a comment.

Add some notes about changing the address space of accesses


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D63525/new/

https://reviews.llvm.org/D63525

Files:
  docs/LangRef.rst


Index: docs/LangRef.rst
===================================================================
--- docs/LangRef.rst
+++ docs/LangRef.rst
@@ -2704,10 +2704,20 @@
 The pointer type is used to specify memory locations. Pointers are
 commonly used to reference objects in memory.
 
-Pointer types may have an optional address space attribute defining the
-numbered address space where the pointed-to object resides. The default
-address space is number zero. The semantics of non-zero address spaces
-are target-specific.
+Pointer types may have an optional address space attribute defining
+the numbered address space where the pointed-to object resides. The
+default address space is number zero. The semantics of non-zero
+address spaces are target-specific. Memory access through an invalid,
+non-dereferencable pointer is undefined behavior in any address
+space. In general pointers with the bit-value 0 are assumed to be
+dereferencable in non-0 address spaces.
+
+For a non-volatile access, if an object can be proven accessible
+through a pointer with a different address space, the access may be
+modified to use that address space. In general, it is not permitted to
+change the address space of a volatile memory access. Different
+address spaces may have different trapping behavior when dereferencing
+an invalid pointer.
 
 Note that LLVM does not permit pointers to void (``void*``) nor does it
 permit pointers to labels (``label*``). Use ``i8*`` instead.
@@ -9689,9 +9699,19 @@
 ``ptrval`` to type ``pty2``. It can be a *no-op cast* or a complex
 value modification, depending on the target and the address space
 pair. Pointer conversions within the same address space must be
-performed with the ``bitcast`` instruction. Note that if the address space
-conversion is legal then both result and operand refer to the same memory
-location.
+performed with the ``bitcast`` instruction. Note that if the address
+space conversion produces a dereferencable result then both result and
+operand refer to the same memory location. The conversion must have no
+side effects, or capture the value of the pointer.
+
+If the source is :ref:`poison <poisonvalues>`, the result is
+:ref:`poison <poisonvalues>`.
+
+If the source is not :ref:`poison <poisonvalues>`, and both source and
+destination address spaces are :ref:`integral pointers
+<nointptrtype>`, and the result pointer is dereferencable, the cast is
+assumed to be reversible (i.e. casting the result back to the original
+address space should yield the original bit pattern).
 
 Example:
 """"""""


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