update docs of memcpy/memmove/memset re: alignment and len=0

[RalfJung]

Summary

Fix https://bugs.llvm.org/show_bug.cgi?id=38583: Describe how memcpy/memmove/memset behave when len=0. Also fix some fallout from when the alignment parameter was replaced by an attribute.

[kristina]

Please make sure you have llvm-commits added as a subscriber when creating patches in the future (I'm not sure why Herald sometimes doesn't do it). The patch that changed this was rL322965, and I fixed one of the related regressions, and was meaning to update the docs but never did, so LGTM and I'm happy signing off on this.

[kristina]

@RalfJung Do you need someone to commit this? I can do it if you'd like.

[RalfJung]

Please make sure you have llvm-commits added as a subscriber when creating patches in the future

Thanks, I'll try to remember. It might be worth clarifying at https://llvm.org/docs/Phabricator.html#phabricator-request-review-web that "adding a reviewer" and "subscribing someone" is *not* the same thing. As someone who has not used Phabricator ever before, I figured these would just be different terms for the same thing; I had no idea there are separate lists of "reviewers" and "subscribers" (llvm-commits is, after all, mentioned in the bullet point that is all about adding reviewers).

was meaning to update the docs but never did

Notice that this does not just fix the fallout from that patch. The remarks about what happens when "len" is 0 are new, and based on the feedback I received at https://bugs.llvm.org/show_bug.cgi?id=38583.

I also just found a typo in my patch, so now I will have to figure out how to update a patch here.

[RalfJung]

Fix typo.

[RalfJung]

I fixed the typo.

Do you need someone to commit this? I can do it if you'd like.

I guess I do? Not sure what the usual process is. I don't have any commit rights myself.

llvm/docs/LangRef.rst

``sret``
     loads and stores to the structure may be assumed by the callee not
     to trap and to be properly aligned. This is not a valid attribute
     for return values.
 
 .. _attr_align:
 
 ``align <n>``
     This indicates that the pointer value may be assumed by the optimizer to
-    have the specified alignment.
+    have the specified alignment.  If the pointer value does not have the
+    specified assignment, behavior is undefined.
 
     Note that this attribute has additional semantics when combined with the
-    ``byval`` attribute.
+    ``byval`` attribute, which are documented there.
 
 .. _noalias:
 
 ``noalias``
     This indicates that objects accessed via pointer values
     :ref:`based <pointeraliasing>` on the argument or return value are not also
     accessed, during the execution of the function, via pointer values not
     *based* on the argument or return value. The attribute on a return value

 very cleanly specified and it is unwise to depend on it.
 
 Semantics:
 """"""""""
 
 The '``llvm.memcpy.*``' intrinsics copy a block of memory from the
 source location to the destination location, which are not allowed to
 overlap. It copies "len" bytes of memory over. If the argument is known
-to be aligned to some boundary, this can be specified as the fourth
-argument, otherwise it should be set to 0 or 1 (both meaning no alignment).
+to be aligned to some boundary, this can be specified as an attribute on
+the argument.
+
+If "len" is 0, the pointers may be NULL or dangling. However, they must still
+be appropriately aligned.
 
 .. _int_memmove:
 
 '``llvm.memmove``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Syntax:
 """""""

 not very cleanly specified and it is unwise to depend on it.
 
 Semantics:
 """"""""""
 
 The '``llvm.memmove.*``' intrinsics copy a block of memory from the
 source location to the destination location, which may overlap. It
 copies "len" bytes of memory over. If the argument is known to be
-aligned to some boundary, this can be specified as the fourth argument,
-otherwise it should be set to 0 or 1 (both meaning no alignment).
+aligned to some boundary, this can be specified as an attribute on
+the argument.
+
+If "len" is 0, the pointers may be NULL or dangling. However, they must still
+be appropriately aligned.
 
 .. _int_memset:
 
 '``llvm.memset.*``' Intrinsics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Syntax:
 """""""

 If the ``isvolatile`` parameter is ``true``, the ``llvm.memset`` call is
 a :ref:`volatile operation <volatile>`. The detailed access behavior is not
 very cleanly specified and it is unwise to depend on it.
 
 Semantics:
 """"""""""
 
 The '``llvm.memset.*``' intrinsics fill "len" bytes of memory starting
-at the destination location.
+at the destination location. If the argument is known to be
+aligned to some boundary, this can be specified as an attribute on
+the argument.
+
+If "len" is 0, the pointers may be NULL or dangling. However, they must still
+be appropriately aligned.
 
 '``llvm.sqrt.*``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Syntax:
 """""""
 
 This is an overloaded intrinsic. You can use ``llvm.sqrt`` on any