Allow alias to point to an arbitrary ConstantExpr.

This  patch changes GlobalAlias to point to an arbitrary ConstantExpr and it is
up to MC (or the system assembler) to decide if that expression is valid or not.

This reduces our ability to diagnose invalid uses and how early we can spot
them, but it also lets us do things like

@test5 = alias inttoptr(i32 sub (i32 ptrtoint (i32* @test2 to i32),
                                 i32 ptrtoint (i32* @bar to i32)) to i32*)

An important implication of this patch is that the notion of aliased global
doesn't exist any more. The alias has to encode the information needed to
access it in its metadata (linkage, visibility, type, etc).

Another consequence to notice is that getSection has to return a "const char *".
It could return a NullTerminatedStringRef if there was such a thing, but when
that was proposed the decision was to just uses "const char*" for that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210062 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 20 insertions, 10 deletions

diff --git a/docs/LangRef.rst b/docs/LangRef.rst
index 650012e7f9..d76fc0d3af 100644
--- a/docs/LangRef.rst
+++ b/docs/LangRef.rst
@@ -679,11 +679,15 @@ Syntax::
 Aliases
 -------
 
-Aliases act as "second name" for the aliasee value (which can be either
-function, global variable, another alias or bitcast of global value).
+Aliases, unlike function or variables, don't create any new data. They
+are just a new symbol and metadata for an existing position.
+
+Aliases have a name and an aliasee that is either a global value or a
+constant expression.
+
 Aliases may have an optional :ref:`linkage type <linkage>`, an optional
-:ref:`visibility style <visibility>`, and an optional :ref:`DLL storage class
-<dllstorageclass>`.
+:ref:`visibility style <visibility>`, an optional :ref:`DLL storage class
+<dllstorageclass>` and an optional :ref:`tls model <tls_model>`.
 
 Syntax::
 
@@ -691,17 +695,23 @@ Syntax::
 
 The linkage must be one of ``private``, ``internal``, ``linkonce``, ``weak``,
 ``linkonce_odr``, ``weak_odr``, ``external``. Note that some system linkers
-might not correctly handle dropping a weak symbol that is aliased by a non-weak
-alias.
+might not correctly handle dropping a weak symbol that is aliased.
 
 Alias that are not ``unnamed_addr`` are guaranteed to have the same address as
 the aliasee.
 
-The aliasee must be a definition.
+Since aliases are only a second name, some restrictions apply, of which
+some can only be checked when producing an object file:
+
+* The expression defining the aliasee must be computable at assembly
+  time. Since it is just a name, no relocations can be used.
+
+* No alias in the expression can be weak as the possibility of the
+  intermediate alias being overridden cannot be represented in an
+  object file.
 
-Aliases are not allowed to point to aliases with linkages that can be
-overridden. Since they are only a second name, the possibility of the
-intermediate alias being overridden cannot be represented in an object file.
+* No global value in the expression can be a declaration, since that
+  would require a relocation, which is not possible.
 
 .. _namedmetadatastructure: