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

llvm-svn: 316668

1 files changed, 25 insertions, 4 deletions

diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
index 68b36ee..c227226 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/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();