[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

1 files changed, 12 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
index 60bd6def6ce..9764944dc33 100644
--- a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -462,12 +462,14 @@ void PassManagerBuilder::populateModulePassManager(
 
     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 @@ void PassManagerBuilder::populateModulePassManager(
     // 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 @@ void PassManagerBuilder::populateModulePassManager(
 
   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) {