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, 36 insertions, 8 deletions

diff --git a/lib/Bitcode/Reader/BitcodeReader.cpp b/lib/Bitcode/Reader/BitcodeReader.cpp
index 68b36eef5eb..c2272260f44 100644
--- a/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -911,6 +911,14 @@ getDecodedDLLStorageClass(unsigned Val) {
   }
 }
 
+static bool getDecodedDSOLocal(unsigned Val) {
+  switch(Val) {
+  default: // Map unknown values to preemptable.
+  case 0:  return false;
+  case 1:  return true;
+  }
+}
+
 static GlobalVariable::ThreadLocalMode getDecodedThreadLocalMode(unsigned Val) {
   switch (Val) {
     case 0: return GlobalVariable::NotThreadLocal;
@@ -2803,7 +2811,7 @@ Error BitcodeReader::parseComdatRecord(ArrayRef<uint64_t> Record) {
 Error BitcodeReader::parseGlobalVarRecord(ArrayRef<uint64_t> Record) {
   // v1: [pointer type, isconst, initid, linkage, alignment, section,
   // visibility, threadlocal, unnamed_addr, externally_initialized,
-  // dllstorageclass, comdat, attributes] (name in VST)
+  // dllstorageclass, comdat, attributes, preemption specifier] (name in VST)
   // v2: [strtab_offset, strtab_size, v1]
   StringRef Name;
   std::tie(Name, Record) = readNameFromStrtab(Record);
@@ -2888,13 +2896,18 @@ Error BitcodeReader::parseGlobalVarRecord(ArrayRef<uint64_t> Record) {
     auto AS = getAttributes(Record[12]).getFnAttributes();
     NewGV->setAttributes(AS);
   }
+
+  if (Record.size() > 13) {
+    NewGV->setDSOLocal(getDecodedDSOLocal(Record[13]));
+  }
+
   return Error::success();
 }
 
 Error BitcodeReader::parseFunctionRecord(ArrayRef<uint64_t> Record) {
   // v1: [type, callingconv, isproto, linkage, paramattr, alignment, section,
   // visibility, gc, unnamed_addr, prologuedata, dllstorageclass, comdat,
-  // prefixdata] (name in VST)
+  // prefixdata,  personalityfn, preemption specifier] (name in VST)
   // v2: [strtab_offset, strtab_size, v1]
   StringRef Name;
   std::tie(Name, Record) = readNameFromStrtab(Record);
@@ -2968,6 +2981,10 @@ Error BitcodeReader::parseFunctionRecord(ArrayRef<uint64_t> Record) {
   if (Record.size() > 14 && Record[14] != 0)
     FunctionPersonalityFns.push_back(std::make_pair(Func, Record[14] - 1));
 
+  if (Record.size() > 15) {
+    Func->setDSOLocal(getDecodedDSOLocal(Record[15]));
+  }
+
   ValueList.push_back(Func);
 
   // If this is a function with a body, remember the prototype we are
@@ -2984,9 +3001,11 @@ Error BitcodeReader::parseGlobalIndirectSymbolRecord(
     unsigned BitCode, ArrayRef<uint64_t> Record) {
   // v1 ALIAS_OLD: [alias type, aliasee val#, linkage] (name in VST)
   // v1 ALIAS: [alias type, addrspace, aliasee val#, linkage, visibility,
-  // dllstorageclass] (name in VST)
+  // dllstorageclass, threadlocal, unnamed_addr,
+  // preemption specifier] (name in VST)
   // v1 IFUNC: [alias type, addrspace, aliasee val#, linkage,
-  // visibility, dllstorageclass] (name in VST)
+  // visibility, dllstorageclass, threadlocal, unnamed_addr,
+  // preemption specifier] (name in VST)
   // v2: [strtab_offset, strtab_size, v1]
   StringRef Name;
   std::tie(Name, Record) = readNameFromStrtab(Record);
@@ -3036,6 +3055,8 @@ Error BitcodeReader::parseGlobalIndirectSymbolRecord(
     NewGA->setThreadLocalMode(getDecodedThreadLocalMode(Record[OpNum++]));
   if (OpNum != Record.size())
     NewGA->setUnnamedAddr(getDecodedUnnamedAddrType(Record[OpNum++]));
+  if (OpNum != Record.size())
+    NewGA->setDSOLocal(getDecodedDSOLocal(Record[OpNum++]));
   ValueList.push_back(NewGA);
   IndirectSymbolInits.push_back(std::make_pair(NewGA, Val));
   return Error::success();
diff --git a/lib/Bitcode/Writer/BitcodeWriter.cpp b/lib/Bitcode/Writer/BitcodeWriter.cpp
index de154ba6be7..1e491aa066e 100644
--- a/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -1197,7 +1197,7 @@ void ModuleBitcodeWriter::writeModuleInfo() {
     // GLOBALVAR: [strtab offset, strtab size, type, isconst, initid,
     //             linkage, alignment, section, visibility, threadlocal,
     //             unnamed_addr, externally_initialized, dllstorageclass,
-    //             comdat, attributes]
+    //             comdat, attributes, DSO_Local]
     Vals.push_back(addToStrtab(GV.getName()));
     Vals.push_back(GV.getName().size());
     Vals.push_back(VE.getTypeID(GV.getValueType()));
@@ -1213,7 +1213,8 @@ void ModuleBitcodeWriter::writeModuleInfo() {
         GV.isExternallyInitialized() ||
         GV.getDLLStorageClass() != GlobalValue::DefaultStorageClass ||
         GV.hasComdat() ||
-        GV.hasAttributes()) {
+        GV.hasAttributes() ||
+        GV.isDSOLocal()) {
       Vals.push_back(getEncodedVisibility(GV));
       Vals.push_back(getEncodedThreadLocalMode(GV));
       Vals.push_back(getEncodedUnnamedAddr(GV));
@@ -1223,6 +1224,8 @@ void ModuleBitcodeWriter::writeModuleInfo() {
 
       auto AL = GV.getAttributesAsList(AttributeList::FunctionIndex);
       Vals.push_back(VE.getAttributeListID(AL));
+
+      Vals.push_back(GV.isDSOLocal());
     } else {
       AbbrevToUse = SimpleGVarAbbrev;
     }
@@ -1236,7 +1239,7 @@ void ModuleBitcodeWriter::writeModuleInfo() {
     // FUNCTION:  [strtab offset, strtab size, type, callingconv, isproto,
     //             linkage, paramattrs, alignment, section, visibility, gc,
     //             unnamed_addr, prologuedata, dllstorageclass, comdat,
-    //             prefixdata, personalityfn]
+    //             prefixdata, personalityfn, DSO_Local]
     Vals.push_back(addToStrtab(F.getName()));
     Vals.push_back(F.getName().size());
     Vals.push_back(VE.getTypeID(F.getFunctionType()));
@@ -1258,6 +1261,7 @@ void ModuleBitcodeWriter::writeModuleInfo() {
     Vals.push_back(
         F.hasPersonalityFn() ? (VE.getValueID(F.getPersonalityFn()) + 1) : 0);
 
+    Vals.push_back(F.isDSOLocal());
     unsigned AbbrevToUse = 0;
     Stream.EmitRecord(bitc::MODULE_CODE_FUNCTION, Vals, AbbrevToUse);
     Vals.clear();
@@ -1266,7 +1270,8 @@ void ModuleBitcodeWriter::writeModuleInfo() {
   // Emit the alias information.
   for (const GlobalAlias &A : M.aliases()) {
     // ALIAS: [strtab offset, strtab size, alias type, aliasee val#, linkage,
-    //         visibility, dllstorageclass, threadlocal, unnamed_addr]
+    //         visibility, dllstorageclass, threadlocal, unnamed_addr,
+    //         DSO_Local]
     Vals.push_back(addToStrtab(A.getName()));
     Vals.push_back(A.getName().size());
     Vals.push_back(VE.getTypeID(A.getValueType()));
@@ -1277,6 +1282,8 @@ void ModuleBitcodeWriter::writeModuleInfo() {
     Vals.push_back(getEncodedDLLStorageClass(A));
     Vals.push_back(getEncodedThreadLocalMode(A));
     Vals.push_back(getEncodedUnnamedAddr(A));
+    Vals.push_back(A.isDSOLocal());
+
     unsigned AbbrevToUse = 0;
     Stream.EmitRecord(bitc::MODULE_CODE_ALIAS, Vals, AbbrevToUse);
     Vals.clear();