Ensure ModuleLinker materializes complete comdat groups

Summary:
The module linker lazy links some "discardable if unused" global
values (e.g. linkonce), materializing and linking them only
if they are referenced in the module. If a comdat group contains a
linkonce member that is not referenced, however, it would not be
materialized and linked, leading to an incomplete comdat group.

If there are other object files not part of the same LTO link that also
define and use that comdat group, the linker may select the incomplete
group leading to link time unsats.

To solve this, whenever a global value body is linked, make sure we
materialize any other members of the same comdat group that are not yet
materialized. This ensures they are in the lazy link list and get linked
as well.

Added new test and adjusted old test to remove parts that didn't
make sense with fix.

Reviewers: rafael

Subscribers: dexonsmith, davidxl, llvm-commits

Differential Revision: http://reviews.llvm.org/D14516

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252647 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Linker/LinkModules.cpp b/lib/Linker/LinkModules.cpp
index 8968377..71af9d0 100644 (file)
--- a/lib/Linker/LinkModules.cpp
+++ b/lib/Linker/LinkModules.cpp
@@ -509,6 +509,8 @@ private:
       ComdatsChosen;
   bool getComdatResult(const Comdat *SrcC, Comdat::SelectionKind &SK,
                        bool &LinkFromSrc);
+  // Keep track of the global value members of each comdat in source.
+  DenseMap<const Comdat *, std::vector<GlobalValue *>> ComdatMembers;
 
   /// Given a global in the source module, return the global in the
   /// destination module that is being linked to, if any.
@@ -1386,6 +1388,7 @@ bool ModuleLinker::linkGlobalValueProto(GlobalValue *SGV) {
     std::tie(SK, LinkFromSrc) = ComdatsChosen[SC];
     C = DstM->getOrInsertComdat(SC->getName());
     C->setSelectionKind(SK);
+    ComdatMembers[SC].push_back(SGV);
   } else if (DGV) {
     if (shouldLinkFromSource(LinkFromSrc, *DGV, *SGV))
       return true;
@@ -1589,6 +1592,19 @@ void ModuleLinker::linkAliasBody(GlobalAlias &Dst, GlobalAlias &Src) {
 bool ModuleLinker::linkGlobalValueBody(GlobalValue &Src) {
   Value *Dst = ValueMap[&Src];
   assert(Dst);
+  if (const Comdat *SC = Src.getComdat()) {
+    // To ensure that we don't generate an incomplete comdat group,
+    // we must materialize and map in any other members that are not
+    // yet materialized in Dst, which also ensures their definitions
+    // are linked in. Otherwise, linkonce and other lazy linked GVs will
+    // not be materialized if they aren't referenced.
+    for (auto *SGV : ComdatMembers[SC]) {
+      if (ValueMap[SGV])
+        continue;
+      Value *NewV = ValMaterializer.materializeValueFor(SGV);
+      ValueMap[SGV] = NewV;
+    }
+  }
   if (shouldInternalizeLinkedSymbols())
     if (auto *DGV = dyn_cast<GlobalValue>(Dst))
       DGV->setLinkage(GlobalValue::InternalLinkage);

diff --git a/test/Linker/Inputs/only-needed-named-metadata.ll b/test/Linker/Inputs/only-needed-named-metadata.ll
index 9515169..fa7bc2e 100644 (file)
--- a/test/Linker/Inputs/only-needed-named-metadata.ll
+++ b/test/Linker/Inputs/only-needed-named-metadata.ll
@@ -2,11 +2,8 @@
 
 declare i32 @foo()
 
-declare void @c1_a()
-
 define void @bar() {
        load i32, i32* @X
        call i32 @foo()
-       call void @c1_a()
        ret void
 }

diff --git a/test/Linker/comdat_group.ll b/test/Linker/comdat_group.ll
new file mode 100644 (file)
index 0000000..efaa94f
--- /dev/null
+++ b/test/Linker/comdat_group.ll
@@ -0,0 +1,20 @@
+; RUN: llvm-as -function-summary %s -o %t.bc
+
+; Ensure complete comdat group is materialized
+; RUN: llvm-link %t.bc -S | FileCheck %s
+; CHECK: $linkoncecomdat = comdat any
+; CHECK: @linkoncecomdat = linkonce global i32 2
+; CHECK: @linkoncecomdat_unref_var = linkonce global i32 2, comdat($linkoncecomdat)
+; CHECK: define linkonce void @linkoncecomdat_unref_func() comdat($linkoncecomdat)
+
+$linkoncecomdat = comdat any
+@linkoncecomdat = linkonce global i32 2, comdat($linkoncecomdat)
+@linkoncecomdat_unref_var = linkonce global i32 2, comdat($linkoncecomdat)
+define linkonce void @linkoncecomdat_unref_func() comdat($linkoncecomdat) {
+  ret void
+}
+; Reference one member of comdat so that comdat is generated.
+define void @ref_linkoncecomdat() {
+  load i32, i32* @linkoncecomdat, align 4
+  ret void
+}

diff --git a/test/Linker/only-needed-named-metadata.ll b/test/Linker/only-needed-named-metadata.ll
index 36b4259..a0df5bf 100644 (file)
--- a/test/Linker/only-needed-named-metadata.ll
+++ b/test/Linker/only-needed-named-metadata.ll
@@ -25,17 +25,6 @@
 ; ONLYNEEDED-NOT:@globalfunc1()
 ; ONLYNEEDED-NOT:@analias
 ; ONLYNEEDED-NOT:@globalfunc2()
-; ONLYNEEDED-NOT:@c1_c
-; ONLYNEEDED-NOT:@c1()
-
-$c1 = comdat any
-@c1_c = global i32 0, comdat($c1)
-define void @c1() comdat {
-  ret void
-}
-define void @c1_a() comdat($c1) {
-  ret void
-}
 
 @X = global i32 5
 @U = global i32 6
@@ -64,7 +53,7 @@ entry:
   ret void
 }
 
-!llvm.named = !{!0, !1, !2, !3, !4, !5, !6, !7}
+!llvm.named = !{!0, !1, !2, !3, !4, !5, !6}
 !0 = !{i32 ()* @unused}
 !1 = !{i32* @U}
 !2 = !{i32 ()* @unused_linkonce}
@@ -72,4 +61,3 @@ entry:
 !4 = !{void (...)* @weakalias}
 !5 = !{void (...)* @analias}
 !6 = !{void (...)* @linkoncealias}
-!7 = !{void ()* @c1}