Represent runtime preemption in the IR.

Currently we do not represent runtime preemption in the IR, which has several
drawbacks:

  1) The semantics of GlobalValues differ depending on the object file format
     you are targeting (as well as the relocation-model and -fPIE value).
  2) We have no way of disabling inlining of run time interposable functions,
     since in the IR we only know if a function is link-time interposable.
     Because of this llvm cannot support elf-interposition semantics.
  3) In LTO builds of executables we will have extra knowledge that a symbol
     resolved to a local definition and can't be preemptable, but have no way to
     propagate that knowledge through the compiler.

This patch adds preemptability specifiers to the IR with the following meaning:

dso_local --> means the compiler may assume the symbol will resolve to a
 definition within the current linkage unit and the symbol may be accessed
 directly even if the definition is not within this compilation unit.

dso_preemptable --> means that the compiler must assume the GlobalValue may be
replaced with a definition from outside the current linkage unit at runtime.

To ease transitioning dso_preemptable is treated as a 'default' in that
low-level codegen will still do the same checks it did previously to see if a
symbol should be accessed indirectly. Eventually when IR producers emit the
specifiers on all Globalvalues we can change dso_preemptable to mean 'always
access indirectly', and remove the current logic.

Differential Revision: https://reviews.llvm.org/D20217

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316668 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 42 insertions, 7 deletions

diff --git a/docs/BitCodeFormat.rst b/docs/BitCodeFormat.rst
index 6ee3842c8d9..98a3156e082 100644
--- a/docs/BitCodeFormat.rst
+++ b/docs/BitCodeFormat.rst
@@ -681,7 +681,7 @@ for each library name referenced.
 MODULE_CODE_GLOBALVAR Record
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-``[GLOBALVAR, strtab offset, strtab size, pointer type, isconst, initid, linkage, alignment, section, visibility, threadlocal, unnamed_addr, externally_initialized, dllstorageclass, comdat]``
+``[GLOBALVAR, strtab offset, strtab size, pointer type, isconst, initid, linkage, alignment, section, visibility, threadlocal, unnamed_addr, externally_initialized, dllstorageclass, comdat, attributes, preemptionspecifier]``
 
 The ``GLOBALVAR`` record (code 7) marks the declaration or definition of a
 global variable. The operand fields are:
@@ -761,12 +761,21 @@ global variable. The operand fields are:
 
 * *comdat*: An encoding of the COMDAT of this function
 
+* *attributes*: If nonzero, the 1-based index into the table of AttributeLists.
+
+.. _bcpreemptionspecifier:
+
+* *preemptionspecifier*: If present, an encoding of the runtime preemption specifier of this variable:
+
+  * ``dso_preemptable``: code 0
+  * ``dso_local``: code 1
+
 .. _FUNCTION:
 
 MODULE_CODE_FUNCTION Record
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-``[FUNCTION, strtab offset, strtab size, type, callingconv, isproto, linkage, paramattr, alignment, section, visibility, gc, prologuedata, dllstorageclass, comdat, prefixdata, personalityfn]``
+``[FUNCTION, strtab offset, strtab size, type, callingconv, isproto, linkage, paramattr, alignment, section, visibility, gc, prologuedata, dllstorageclass, comdat, prefixdata, personalityfn, preemptionspecifier]``
 
 The ``FUNCTION`` record (code 8) marks the declaration or definition of a
 function. The operand fields are:
@@ -828,10 +837,12 @@ function. The operand fields are:
 * *personalityfn*: If non-zero, the value index of the personality function for this function,
   plus 1.
 
+* *preemptionspecifier*: If present, an encoding of the :ref:`runtime preemption specifier<bcpreemptionspecifier>`  of this function.
+ 
 MODULE_CODE_ALIAS Record
 ^^^^^^^^^^^^^^^^^^^^^^^^
 
-``[ALIAS, strtab offset, strtab size, alias type, aliasee val#, linkage, visibility, dllstorageclass, threadlocal, unnamed_addr]``
+``[ALIAS, strtab offset, strtab size, alias type, aliasee val#, linkage, visibility, dllstorageclass, threadlocal, unnamed_addr, preemptionspecifier]``
 
 The ``ALIAS`` record (code 9) marks the definition of an alias. The operand
 fields are
@@ -856,6 +867,8 @@ fields are
 * *unnamed_addr*: If present, an encoding of the
   :ref:`unnamed_addr<bcunnamedaddr>` attribute of this alias
 
+* *preemptionspecifier*: If present, an encoding of the :ref:`runtime preemption specifier<bcpreemptionspecifier>`  of this alias.
+
 .. _MODULE_CODE_GCNAME:
 
 MODULE_CODE_GCNAME Record
diff --git a/docs/LangRef.rst b/docs/LangRef.rst
index 99a2ffa40d6..9d910568bd5 100644
--- a/docs/LangRef.rst
+++ b/docs/LangRef.rst
@@ -527,6 +527,24 @@ the alias is accessed. It will not have any effect in the aliasee.
 For platforms without linker support of ELF TLS model, the -femulated-tls
 flag can be used to generate GCC compatible emulated TLS code.
 
+.. _runtime_preemption_model:
+
+Runtime Preemption Specifiers
+-----------------------------
+
+Global variables, functions and aliases may have an optional runtime preemption
+specifier. If a preemption specifier isn't given explicitly, then a
+symbol is assumed to be ``dso_preemptable``.
+
+``dso_preemptable``
+    Indicates that the function or variable may be replaced by a symbol from
+    outside the linkage unit at runtime.
+
+``dso_local``
+    The compiler may assume that a function or variable marked as ``dso_local``
+    will resolve to a symbol within the same linkage unit. Direct access will 
+    be generated even if the definition is not within this compilation unit.
+
 .. _namedtypes:
 
 Structure Types
@@ -650,6 +668,7 @@ iterate over them as an array, alignment padding would break this
 iteration. The maximum alignment is ``1 << 29``.
 
 Globals can also have a :ref:`DLL storage class <dllstorageclass>`,
+an optional :ref:`runtime preemption specifier <runtime_preemption_model>`,
 an optional :ref:`global attributes <glattrs>` and
 an optional list of attached :ref:`metadata <metadata>`.
 
@@ -658,7 +677,8 @@ Variables and aliases can have a
 
 Syntax::
 
-      @<GlobalVarName> = [Linkage] [Visibility] [DLLStorageClass] [ThreadLocal]
+      @<GlobalVarName> = [Linkage] [PreemptionSpecifier] [Visibility]
+                         [DLLStorageClass] [ThreadLocal]
                          [(unnamed_addr|local_unnamed_addr)] [AddrSpace]
                          [ExternallyInitialized]
                          <global | constant> <Type> [<InitializerConstant>]
@@ -691,7 +711,8 @@ Functions
 ---------
 
 LLVM function definitions consist of the "``define``" keyword, an
-optional :ref:`linkage type <linkage>`, an optional :ref:`visibility
+optional :ref:`linkage type <linkage>`, an optional :ref:`runtime preemption
+specifier <runtime_preemption_model>`,  an optional :ref:`visibility
 style <visibility>`, an optional :ref:`DLL storage class <dllstorageclass>`,
 an optional :ref:`calling convention <callingconv>`,
 an optional ``unnamed_addr`` attribute, a return type, an optional
@@ -750,7 +771,7 @@ not be significant within the module.
 
 Syntax::
 
-    define [linkage] [visibility] [DLLStorageClass]
+    define [linkage] [PreemptionSpecifier] [visibility] [DLLStorageClass]
            [cconv] [ret attrs]
            <ResultType> @<FunctionName> ([argument list])
            [(unnamed_addr|local_unnamed_addr)] [fn Attrs] [section "name"]
@@ -777,12 +798,13 @@ 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:`runtime preemption specifier <runtime_preemption_model>`, an optional
 :ref:`visibility style <visibility>`, an optional :ref:`DLL storage class
 <dllstorageclass>` and an optional :ref:`tls model <tls_model>`.
 
 Syntax::
 
-    @<Name> = [Linkage] [Visibility] [DLLStorageClass] [ThreadLocal] [(unnamed_addr|local_unnamed_addr)] alias <AliaseeTy>, <AliaseeTy>* @<Aliasee>
+    @<Name> = [Linkage] [PreemptionSpecifier] [Visibility] [DLLStorageClass] [ThreadLocal] [(unnamed_addr|local_unnamed_addr)] alias <AliaseeTy>, <AliaseeTy>* @<Aliasee>
 
 The linkage must be one of ``private``, ``internal``, ``linkonce``, ``weak``,
 ``linkonce_odr``, ``weak_odr``, ``external``. Note that some system linkers