[ThinLTO] Handle chains of aliases
At -O0, globalopt is not run during the compile step, and we can have a
chain of an alias having an immediate aliasee of another alias. The
summaries are constructed assuming aliases in a canonical form
(flattened chains), and as a result only the base object but no
intermediate aliases were preserved.
Fix by adding a pass that canonicalize aliases, which ensures each
alias is a direct alias of the base object.
Reviewers: pcc, davidxl
Subscribers: mehdi_amini, inglorion, eraman, steven_wu, dexonsmith, arphaman, llvm-commits
Differential Revision: https://reviews.llvm.org/D54507
llvm-svn: 350423
diff --git a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
index 60bd6de..9764944 100644
--- a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -462,12 +462,14 @@
addExtensionsToPM(EP_EnabledOnOptLevel0, MPM);
- // Rename anon globals to be able to export them in the summary.
- // This has to be done after we add the extensions to the pass manager
- // as there could be passes (e.g. Adddress sanitizer) which introduce
- // new unnamed globals.
- if (PrepareForLTO || PrepareForThinLTO)
+ if (PrepareForLTO || PrepareForThinLTO) {
+ MPM.add(createCanonicalizeAliasesPass());
+ // Rename anon globals to be able to export them in the summary.
+ // This has to be done after we add the extensions to the pass manager
+ // as there could be passes (e.g. Adddress sanitizer) which introduce
+ // new unnamed globals.
MPM.add(createNameAnonGlobalPass());
+ }
return;
}
@@ -585,6 +587,7 @@
// Ensure we perform any last passes, but do so before renaming anonymous
// globals in case the passes add any.
addExtensionsToPM(EP_OptimizerLast, MPM);
+ MPM.add(createCanonicalizeAliasesPass());
// Rename anon globals to be able to export them in the summary.
MPM.add(createNameAnonGlobalPass());
return;
@@ -744,9 +747,11 @@
addExtensionsToPM(EP_OptimizerLast, MPM);
- // Rename anon globals to be able to handle them in the summary
- if (PrepareForLTO)
+ if (PrepareForLTO) {
+ MPM.add(createCanonicalizeAliasesPass());
+ // Rename anon globals to be able to handle them in the summary
MPM.add(createNameAnonGlobalPass());
+ }
}
void PassManagerBuilder::addLTOOptimizationPasses(legacy::PassManagerBase &PM) {
diff --git a/llvm/lib/Transforms/Utils/CMakeLists.txt b/llvm/lib/Transforms/Utils/CMakeLists.txt
index eb3bad7..cb3dc17 100644
--- a/llvm/lib/Transforms/Utils/CMakeLists.txt
+++ b/llvm/lib/Transforms/Utils/CMakeLists.txt
@@ -6,6 +6,7 @@
BuildLibCalls.cpp
BypassSlowDivision.cpp
CallPromotionUtils.cpp
+ CanonicalizeAliases.cpp
CloneFunction.cpp
CloneModule.cpp
CodeExtractor.cpp
diff --git a/llvm/lib/Transforms/Utils/CanonicalizeAliases.cpp b/llvm/lib/Transforms/Utils/CanonicalizeAliases.cpp
new file mode 100644
index 0000000..cf41fd2
--- /dev/null
+++ b/llvm/lib/Transforms/Utils/CanonicalizeAliases.cpp
@@ -0,0 +1,105 @@
+//===- CanonicalizeAliases.cpp - ThinLTO Support: Canonicalize Aliases ----===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// Currently this file implements partial alias canonicalization, to
+// flatten chains of aliases (also done by GlobalOpt, but not on for
+// O0 compiles). E.g.
+// @a = alias i8, i8 *@b
+// @b = alias i8, i8 *@g
+//
+// will be converted to:
+// @a = alias i8, i8 *@g <-- @a is now an alias to base object @g
+// @b = alias i8, i8 *@g
+//
+// Eventually this file will implement full alias canonicalation, so that
+// all aliasees are private anonymous values. E.g.
+// @a = alias i8, i8 *@g
+// @g = global i8 0
+//
+// will be converted to:
+// @0 = private global
+// @a = alias i8, i8* @0
+// @g = alias i8, i8* @0
+//
+// This simplifies optimization and ThinLTO linking of the original symbols.
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Utils/CanonicalizeAliases.h"
+
+#include "llvm/IR/Operator.h"
+#include "llvm/IR/ValueHandle.h"
+
+using namespace llvm;
+
+namespace {
+
+static Constant *canonicalizeAlias(Constant *C, bool &Changed) {
+ if (auto *GA = dyn_cast<GlobalAlias>(C)) {
+ auto *NewAliasee = canonicalizeAlias(GA->getAliasee(), Changed);
+ if (NewAliasee != GA->getAliasee()) {
+ GA->setAliasee(NewAliasee);
+ Changed = true;
+ }
+ return NewAliasee;
+ }
+
+ auto *CE = dyn_cast<ConstantExpr>(C);
+ if (!CE)
+ return C;
+
+ std::vector<Constant *> Ops;
+ for (Use &U : CE->operands())
+ Ops.push_back(canonicalizeAlias(cast<Constant>(U), Changed));
+ return CE->getWithOperands(Ops);
+}
+
+/// Convert aliases to canonical form.
+static bool canonicalizeAliases(Module &M) {
+ bool Changed = false;
+ for (auto &GA : M.aliases())
+ canonicalizeAlias(&GA, Changed);
+ return Changed;
+}
+
+// Legacy pass that canonicalizes aliases.
+class CanonicalizeAliasesLegacyPass : public ModulePass {
+
+public:
+ /// Pass identification, replacement for typeid
+ static char ID;
+
+ /// Specify pass name for debug output
+ StringRef getPassName() const override { return "Canonicalize Aliases"; }
+
+ explicit CanonicalizeAliasesLegacyPass() : ModulePass(ID) {}
+
+ bool runOnModule(Module &M) override { return canonicalizeAliases(M); }
+};
+char CanonicalizeAliasesLegacyPass::ID = 0;
+
+} // anonymous namespace
+
+PreservedAnalyses CanonicalizeAliasesPass::run(Module &M,
+ ModuleAnalysisManager &AM) {
+ if (!canonicalizeAliases(M))
+ return PreservedAnalyses::all();
+
+ return PreservedAnalyses::none();
+}
+
+INITIALIZE_PASS_BEGIN(CanonicalizeAliasesLegacyPass, "canonicalize-aliases",
+ "Canonicalize aliases", false, false)
+INITIALIZE_PASS_END(CanonicalizeAliasesLegacyPass, "canonicalize-aliases",
+ "Canonicalize aliases", false, false)
+
+namespace llvm {
+ModulePass *createCanonicalizeAliasesPass() {
+ return new CanonicalizeAliasesLegacyPass();
+}
+} // namespace llvm
diff --git a/llvm/lib/Transforms/Utils/Utils.cpp b/llvm/lib/Transforms/Utils/Utils.cpp
index afd842f..95416de 100644
--- a/llvm/lib/Transforms/Utils/Utils.cpp
+++ b/llvm/lib/Transforms/Utils/Utils.cpp
@@ -26,6 +26,7 @@
void llvm::initializeTransformUtils(PassRegistry &Registry) {
initializeAddDiscriminatorsLegacyPassPass(Registry);
initializeBreakCriticalEdgesPass(Registry);
+ initializeCanonicalizeAliasesLegacyPassPass(Registry);
initializeInstNamerPass(Registry);
initializeLCSSAWrapperPassPass(Registry);
initializeLibCallsShrinkWrapLegacyPassPass(Registry);