Avoid merging globals residing in different comdats (#172835)

The global-merge pass may merge globals residing in different comdat
groups, which may later confuse linker if section GC is being used. In
particular this may happen when merging instrumentation profiler
counters when their corresponding __llvm_prf_cnts sections are being
merged into a single one and moved out of a comdat group, containing
__llvm_prf_cnts and __llvm_prf_data sections. After that __llvm_prf_data
section is becoming orphaned and is garbage-collected when --gc-sections
linker flag is used.

Author: eleviant

llvm/lib/CodeGen/GlobalMerge.cpp

@@ -562,6 +562,7 @@ bool GlobalMergeImpl::doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
 
     MergedGV->setAlignment(MaxAlign);
     MergedGV->setSection(Globals[i]->getSection());
+    MergedGV->setComdat(Globals[i]->getComdat());
 
     LLVM_DEBUG(dbgs() << "MergedGV:  " << *MergedGV << "\n");
 
@@ -677,7 +678,8 @@ bool GlobalMergeImpl::run(Module &M) {
   IsMachO = M.getTargetTriple().isOSBinFormatMachO();
 
   auto &DL = M.getDataLayout();
-  MapVector<std::pair<unsigned, StringRef>, SmallVector<GlobalVariable *, 0>>
+  MapVector<std::tuple<unsigned, StringRef, Comdat *>,
+            SmallVector<GlobalVariable *, 0>>
       Globals, ConstGlobals, BSSGlobals;
   bool Changed = false;
   setMustKeepGlobalVariables(M);
@@ -735,28 +737,28 @@ bool GlobalMergeImpl::run(Module &M) {
     if (CanMerge) {
       if (TM &&
           TargetLoweringObjectFile::getKindForGlobal(&GV, *TM).isBSS())
-        BSSGlobals[{AddressSpace, Section}].push_back(&GV);
+        BSSGlobals[{AddressSpace, Section, GV.getComdat()}].push_back(&GV);
       else if (GV.isConstant())
-        ConstGlobals[{AddressSpace, Section}].push_back(&GV);
+        ConstGlobals[{AddressSpace, Section, GV.getComdat()}].push_back(&GV);
       else
-        Globals[{AddressSpace, Section}].push_back(&GV);
+        Globals[{AddressSpace, Section, GV.getComdat()}].push_back(&GV);
     }
     LLVM_DEBUG(dbgs() << "GV " << (CanMerge ? "" : "not ") << "to merge: " << GV
                       << "\n");
   }
 
   for (auto &P : Globals)
     if (P.second.size() > 1)
-      Changed |= doMerge(P.second, M, false, P.first.first);
+      Changed |= doMerge(P.second, M, false, std::get<0>(P.first));
 
   for (auto &P : BSSGlobals)
     if (P.second.size() > 1)
-      Changed |= doMerge(P.second, M, false, P.first.first);
+      Changed |= doMerge(P.second, M, false, std::get<0>(P.first));
 
   if (Opt.MergeConstantGlobals)
     for (auto &P : ConstGlobals)
       if (P.second.size() > 1)
-        Changed |= doMerge(P.second, M, true, P.first.first);
+        Changed |= doMerge(P.second, M, true, std::get<0>(P.first));
 
   return Changed;
 }

llvm/test/CodeGen/AArch64/global-merge-profile-sections.ll

@@ -0,0 +1,11 @@
+; RUN: llc -mtriple=aarch64-linux-gnu -aarch64-enable-global-merge  -global-merge-group-by-use=false < %s | FileCheck %s
+; CHECK-NOT: _MergedGlobals
+
+$__profc_begin = comdat nodeduplicate
+$__profc_end = comdat nodeduplicate
+
+@__profc_begin = private global [2 x i64] zeroinitializer, section "__llvm_prf_cnts", comdat, align 8
+@__profd_begin = private global { i64, i64, i64, i64, ptr, ptr, i32, [3 x i16], i32 } { i64 -1301828029439649651, i64 172590168, i64 sub (i64 ptrtoint (ptr @__profc_begin to i64), i64 ptrtoint (ptr @__profd_begin to i64)), i64 0, ptr null, ptr null, i32 2, [3 x i16] zeroinitializer, i32 0 }, section "__llvm_prf_data", comdat($__profc_begin), align 8
+@__profc_end = private global [2 x i64] zeroinitializer, section "__llvm_prf_cnts", comdat, align 8
+@__profd_end = private global { i64, i64, i64, i64, ptr, ptr, i32, [3 x i16], i32 } { i64 3274037854792712831, i64 172590168, i64 sub (i64 ptrtoint (ptr @__profc_end to i64), i64 ptrtoint (ptr @__profd_end to i64)), i64 0, ptr null, ptr null, i32 2, [3 x i16] zeroinitializer, i32 0 }, section "__llvm_prf_data", comdat($__profc_end), align 8
+

llvm/test/Transforms/GlobalMerge/global-merge-comdat.ll

@@ -0,0 +1,7 @@
+; RUN: opt -global-merge -global-merge-max-offset=16 -global-merge-group-by-use=false %s -S -o - | FileCheck %s
+; CHECK: @_MergedGlobals = private global <{ i64, i64 }> zeroinitializer, section "__foo", comdat($__foo), align 8
+
+$__foo = comdat nodeduplicate
+
+@__bar = private global i64 0, section "__foo", comdat($__foo), align 8
+@__baz = private global i64 0, section "__foo", comdat($__foo), align 8