ThinLTO: add early "dead-stripping" on the Index
Summary:
Using the linker-supplied list of "preserved" symbols, we can compute
the list of "dead" symbols, i.e. the one that are not reachable from
a "preserved" symbol transitively on the reference graph.
Right now we are using this information to mark these functions as
non-eligible for import.
The impact is two folds:
- Reduction of compile time: we don't import these functions anywhere
or import the function these symbols are calling.
- The limited number of import/export leads to better internalization.
Patch originally by Mehdi Amini.
Reviewers: mehdi_amini, pcc
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D23488
llvm-svn: 291177
diff --git a/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp b/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp
index 058a601..6387bb3 100644
--- a/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp
+++ b/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp
@@ -189,7 +189,8 @@
// Inliner doesn't handle variadic functions.
// FIXME: refactor this to use the same code that inliner is using.
F.isVarArg();
- GlobalValueSummary::GVFlags Flags(F.getLinkage(), NotEligibleForImport);
+ GlobalValueSummary::GVFlags Flags(F.getLinkage(), NotEligibleForImport,
+ /* LiveRoot = */ false);
auto FuncSummary = llvm::make_unique<FunctionSummary>(
Flags, NumInsts, RefEdges.takeVector(), CallGraphEdges.takeVector(),
TypeTests.takeVector());
@@ -205,7 +206,8 @@
SmallPtrSet<const User *, 8> Visited;
findRefEdges(&V, RefEdges, Visited);
bool NonRenamableLocal = isNonRenamableLocal(V);
- GlobalValueSummary::GVFlags Flags(V.getLinkage(), NonRenamableLocal);
+ GlobalValueSummary::GVFlags Flags(V.getLinkage(), NonRenamableLocal,
+ /* LiveRoot = */ false);
auto GVarSummary =
llvm::make_unique<GlobalVarSummary>(Flags, RefEdges.takeVector());
if (NonRenamableLocal)
@@ -217,7 +219,8 @@
computeAliasSummary(ModuleSummaryIndex &Index, const GlobalAlias &A,
DenseSet<GlobalValue::GUID> &CantBePromoted) {
bool NonRenamableLocal = isNonRenamableLocal(A);
- GlobalValueSummary::GVFlags Flags(A.getLinkage(), NonRenamableLocal);
+ GlobalValueSummary::GVFlags Flags(A.getLinkage(), NonRenamableLocal,
+ /* LiveRoot = */ false);
auto AS = llvm::make_unique<AliasSummary>(Flags, ArrayRef<ValueInfo>{});
auto *Aliasee = A.getBaseObject();
auto *AliaseeSummary = Index.getGlobalValueSummary(*Aliasee);
@@ -228,6 +231,16 @@
Index.addGlobalValueSummary(A.getName(), std::move(AS));
}
+// Set LiveRoot flag on entries matching the given value name.
+static void setLiveRoot(ModuleSummaryIndex &Index, StringRef Name) {
+ auto SummaryList =
+ Index.findGlobalValueSummaryList(GlobalValue::getGUID(Name));
+ if (SummaryList == Index.end())
+ return;
+ for (auto &Summary : SummaryList->second)
+ Summary->setLiveRoot();
+}
+
ModuleSummaryIndex llvm::buildModuleSummaryIndex(
const Module &M,
std::function<BlockFrequencyInfo *(const Function &F)> GetBFICallback,
@@ -293,6 +306,15 @@
Summary->setNotEligibleToImport();
}
+ // The linker doesn't know about these LLVM produced values, so we need
+ // to flag them as live in the index to ensure index-based dead value
+ // analysis treats them as live roots of the analysis.
+ setLiveRoot(Index, "llvm.used");
+ setLiveRoot(Index, "llvm.compiler.used");
+ setLiveRoot(Index, "llvm.global_ctors");
+ setLiveRoot(Index, "llvm.global_dtors");
+ setLiveRoot(Index, "llvm.global.annotations");
+
if (!M.getModuleInlineAsm().empty()) {
// Collect the local values defined by module level asm, and set up
// summaries for these symbols so that they can be marked as NoRename,
@@ -316,7 +338,8 @@
return;
assert(GV->isDeclaration() && "Def in module asm already has definition");
GlobalValueSummary::GVFlags GVFlags(GlobalValue::InternalLinkage,
- /* NotEligibleToImport */ true);
+ /* NotEligibleToImport */ true,
+ /* LiveRoot */ true);
CantBePromoted.insert(GlobalValue::getGUID(Name));
// Create the appropriate summary type.
if (isa<Function>(GV)) {