LTO: Export functions referenced by the CFI jump table.

[pcc]

Summary

If the LowerTypeTests pass decides to add a function to a jump
table for CFI, it will add its name to the set cfiFunctionDefs,
which among other things will cause the function to be renamed in
the ThinLTO backend.

One other thing that we must do with such functions is to not
internalize them, because the jump table in the full LTO object will
contain a reference to the actual function body in the ThinLTO object.

This patch handles that by ensuring that we export any functions
whose names appear in the cfiFunctionDefs set.

Fixes PR33831.

[tejohnson]

LGTM

llvm/lib/LTO/LTO.cpp

1079 ↗

(On Diff #107238)

Not for this patch, but as an aside - can dropLLVMMangingEscape be called unconditionally from within GlobalValue::getGUID, instead of callers needing to know when it must be invoked? I see that GlobalValue::getGlobalIdentifier also unconditionally does this (although not via dropLLVMMangingEscape - it should probably be refactored to just invoke that helper).

llvm/test/LTO/Resolution/X86/export-jumptable.ll

1 ↗

(On Diff #107238)

Please document what this test is doing and looking for.

[pcc]

llvm/lib/LTO/LTO.cpp

1079 ↗

(On Diff #107238)

Maybe once we figure out what to do about GUIDs in general... as you know I'm not sure if we should be using them outside of PGO.

llvm/trunk/lib/LTO/LTO.cpp

for (auto &Res : GlobalResolutions) {
         continue;
       auto GUID = GlobalValue::getGUID(
           GlobalValue::dropLLVMManglingEscape(Res.second.IRName));
       // Mark exported unless index-based analysis determined it to be dead.
       if (ThinLTO.CombinedIndex.isGUIDLive(GUID))
         ExportedGUIDs.insert(GUID);
     }
 
+    // Any functions referenced by the jump table in the regular LTO object must
+    // be exported.
+    for (auto &Def : ThinLTO.CombinedIndex.cfiFunctionDefs())
+      ExportedGUIDs.insert(
+          GlobalValue::getGUID(GlobalValue::dropLLVMManglingEscape(Def)));
+
     auto isExported = [&](StringRef ModuleIdentifier, GlobalValue::GUID GUID) {
       const auto &ExportList = ExportLists.find(ModuleIdentifier);
       return (ExportList != ExportLists.end() &&
               ExportList->second.count(GUID)) ||
              ExportedGUIDs.count(GUID);
     };
     thinLTOInternalizeAndPromoteInIndex(ThinLTO.CombinedIndex, isExported);
   }

llvm/trunk/test/LTO/Resolution/X86/export-jumptable.ll

+; Test that we do not internalize functions that appear in the CFI jump table in
+; the full LTO object file; any such functions will be referenced by the jump
+; table.
+
+; RUN: opt -thinlto-bc -o %t %s
+; RUN: llvm-lto2 run -o %t2 -r %t,f1,p -r %t,f2,p -r %t,_start,px %t -save-temps
+; RUN: llvm-dis %t2.1.2.internalize.bc -o - | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK: define void @f1()
+define void @f1() !type !0 {
+  ret void
+}
+
+; CHECK: define internal void @f2()
+define void @f2() !type !1 {
+  ret void
+}
+
+define i1 @_start(i8* %p) {
+  %1 = call i1 @llvm.type.test(i8* %p, metadata !"typeid1")
+  call void @f1()
+  call void @f2()
+  ret i1 %1
+}
+
+declare i1 @llvm.type.test(i8*, metadata)
+
+!0 = !{i64 0, !"typeid1"}
+!1 = !{i64 0, !"typeid2"}