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Represent runtime preemption in the IR.

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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
This commit is contained in:
Sean Fertile
2017-10-26 15:00:26 +00:00
parent 2232243863
commit c70d28bff5
20 changed files with 1262 additions and 49 deletions
Download Patch File Download Diff File Expand all files Collapse all files

29
llvm/lib/Bitcode/Reader/BitcodeReader.cpp
View File

@@ -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();