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Allow alias to point to an arbitrary ConstantExpr.

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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
This commit is contained in:
Rafael Espindola
2014-06-03 02:41:57 +00:00
parent 27bd9b361b
commit 2d21b25393
42 changed files with 267 additions and 316 deletions
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30
docs/LangRef.rst
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@@ -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:
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.
* 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.
* No global value in the expression can be a declaration, since that
would require a relocation, which is not possible.
.. _namedmetadatastructure: