[ThinLTO] Loosen up variable importing correctness checks

[tejohnson]

Summary

After importing variables, we do some checking to ensure that variables
marked read or write only, which have been marked exported (e.g.
because a referencing function has been exported), are on at least one
module's imports list. This is because the read or write only variables
will be internalized, so we need a copy any any module that references
it.

This checking is overly conservative in the case of linkonce_odr or
other linkage types where there can already be a duplicate copy in
existence in the importing module, which therefore wouldn't need to
import it. Loosen up the checking for these linkage types.

Fixes https://github.com/llvm/llvm-project/issues/62468.

[int3]

Thank you! :D

llvm/lib/Transforms/IPO/FunctionImport.cpp

checkVariableImport(const ModuleSummaryIndex &Index,
   for (auto &ImportPerModule : ImportLists)
     for (auto &ExportPerModule : ImportPerModule.second)
       FlattenedImports.insert(ExportPerModule.second.begin(),
                               ExportPerModule.second.end());
 
   // Checks that all GUIDs of read/writeonly vars we see in export lists
   // are also in the import lists. Otherwise we my face linker undefs,
   // because readonly and writeonly vars are internalized in their
-  // source modules.
-  auto IsReadOrWriteOnlyVar = [&](StringRef ModulePath, const ValueInfo &VI) {
+  // source modules. The exception would be if it has a linkage type indicating
+  // that there may have been a copy existing in the importing module (e.g.
+  // linkonce_odr). In that case we cannot accurately do this checking.
+  auto IsReadOrWriteOnlyVarNeedingImporting = [&](StringRef ModulePath,
+                                                  const ValueInfo &VI) {
     auto *GVS = dyn_cast_or_null<GlobalVarSummary>(
         Index.findSummaryInModule(VI, ModulePath));
-    return GVS && (Index.isReadOnly(GVS) || Index.isWriteOnly(GVS));
+    return GVS && (Index.isReadOnly(GVS) || Index.isWriteOnly(GVS)) &&
+           !(GVS->linkage() == GlobalValue::AvailableExternallyLinkage ||
+             GVS->linkage() == GlobalValue::WeakODRLinkage ||
+             GVS->linkage() == GlobalValue::LinkOnceODRLinkage);
   };
 
   for (auto &ExportPerModule : ExportLists)
     for (auto &VI : ExportPerModule.second)
       if (!FlattenedImports.count(VI.getGUID()) &&
-          IsReadOrWriteOnlyVar(ExportPerModule.first(), VI))
+          IsReadOrWriteOnlyVarNeedingImporting(ExportPerModule.first(), VI))
         return false;
 
   return true;
 }
 #endif
 
 /// Compute all the import and export for every module using the Index.
 void llvm::ComputeCrossModuleImport(

llvm/test/ThinLTO/X86/check_var_import_odr.ll

+;; Test to ensure that we don't assert when checking variable importing
+;; correctness for read only variables where the importing module contains
+;; a linkonce_odr copy of the variable. In that case we do not need to
+;; import the read only variable even when it has been exported from its
+;; original module (in this case when we decided to import @bar).
+
+; RUN: split-file %s %t
+; RUN: opt -module-summary %t/foo.ll -o %t/foo.o
+; RUN: opt -module-summary %t/bar.ll -o %t/bar.o
+; RUN: llvm-lto2 run %t/foo.o %t/bar.o -r=%t/foo.o,foo,pl -r=%t/foo.o,bar,l -r=%t/foo.o,qux,pl -r=%t/bar.o,bar,pl -r=%t/bar.o,qux, -o %t.out -save-temps
+; RUN: llvm-dis %t.out.1.3.import.bc -o - | FileCheck %s
+
+;; Check that we have internalized @qux (since it is read only), and imported @bar.
+; CHECK: @qux = internal global i32 0
+; CHECK: define available_externally hidden void @bar()
+
+;--- foo.ll
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@qux = linkonce_odr hidden global i32 0
+
+define linkonce_odr hidden void @foo() {
+  call void @bar()
+  ret void
+}
+
+declare hidden void @bar()
+
+;--- bar.ll
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@qux = linkonce_odr hidden global i32 0
+
+define hidden void @bar() {
+  %1 = load i32, i32* @qux, align 8
+  ret void
+}