[PATCH] D57600: update docs of memcpy/memmove/memset re: alignment and len=0

[Ralf via Phabricator via llvm-commits]

RalfJung created this revision.
RalfJung added reviewers: llvm-commits, eli.friedman, dneilson.
Herald added a project: LLVM.

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.


Repository:
  rL LLVM

https://reviews.llvm.org/D57600

Files:
  llvm/docs/LangRef.rst


Index: llvm/docs/LangRef.rst
===================================================================
--- llvm/docs/LangRef.rst
+++ llvm/docs/LangRef.rst
@@ -1071,10 +1071,11 @@
 
 ``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:
 
@@ -11234,8 +11235,11 @@
 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:
 
@@ -11289,8 +11293,11 @@
 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:
 
@@ -11340,7 +11347,12 @@
 """"""""""
 
 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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^


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