[Linker/Mover] Implement merging of PIC flags

[davide]

Summary

We always pick the smallest. I think the ordering is Small < Default < Large, but I might have gotten that wrong, so .. comments welcome.
This fixes an LTO issue found while linking the FreeBSD base system.

[davide]

clang-format.

[• rafael]

Sorry, but I think you hit an area that needs quite a bit of redesign :-(

We have Reloc::Model, PICLevel and PIELevel.

Medium term I would like to have have the "assumeDsoLocalBit" in each GV. With that we would need only one pic level of NotPic, SmallPic or BigPIc.

For a shorter term I think what we can do is:

• Change Reloc::Model to Static, PIC_, DynamicNoPIC, PIE.
• Delete PIELevel.
• Change PICLevel to just NotPIC, SmallPIC, BigPic

Right now we cannot really merge because "Default" means different things to different things to different targets.

I would suggest starting by changing PicLevel. After that we can change Reloc::Model and drop PIELevel.

lib/IR/Module.cpp

497

Not sure if Warning is correct.

Maybe we need a "custom" or "minimum"?

lib/IR/Module.cpp

PICLevel::Level Module::getPICLevel() const {
   if (!Val)
     return PICLevel::Default;
 
   return static_cast<PICLevel::Level>(
       cast<ConstantInt>(Val->getValue())->getZExtValue());
 }
 
 void Module::setPICLevel(PICLevel::Level PL) {
-  addModuleFlag(ModFlagBehavior::Error, "PIC Level", PL);
+  addModuleFlag(ModFlagBehavior::Warning, "PIC Level", PL);
 }
 
 PIELevel::Level Module::getPIELevel() const {
   auto *Val = cast_or_null<ConstantAsMetadata>(getModuleFlag("PIE Level"));
 
   if (!Val)
     return PIELevel::Default;
 

lib/Linker/IRMover.cpp

   // If vendor is apple, pick the triple with the larger version number.
   if (SrcTriple.getVendor() == Triple::Apple)
     if (DstTriple.isOSVersionLT(SrcTriple))
       return SrcTriple.str();
 
   return DstTriple.str();
 }
 
+static void mergePICLevel(std::unique_ptr<Module> &SrcM, Module &DstM) {
+  auto *SrcMD =
+      cast_or_null<ConstantAsMetadata>(SrcM->getModuleFlag("PIC Level"));
+  auto *DstMD =
+      cast_or_null<ConstantAsMetadata>(DstM.getModuleFlag("PIC Level"));
+  if (!SrcMD || !DstMD)
+    return;
+  PICLevel::Level SrcL = SrcM->getPICLevel();
+  PICLevel::Level DstL = DstM.getPICLevel();
+  if (SrcL == DstL)
+    return;
+  if (SrcL == PICLevel::Small)
+    return DstM.setPICLevel(PICLevel::Small);
+  if (DstL == PICLevel::Small)
+    return SrcM->setPICLevel(PICLevel::Small);
+  if (SrcL < DstL)
+    return DstM.setPICLevel(SrcL);
+  return SrcM->setPICLevel(DstL);
+}
+
 Error IRLinker::run() {
   // Ensure metadata materialized before value mapping.
   if (SrcM->getMaterializer())
     if (std::error_code EC = SrcM->getMaterializer()->materializeMetadata())
       return errorCodeToError(EC);
 
   // Inherit the target data from the source module if the destination module
   // doesn't have one already.

   DoneLinkingBodies = true;
   Mapper.addFlags(RF_NullMapMissingGlobalValues);
 
   // Remap all of the named MDNodes in Src into the DstM module. We do this
   // after linking GlobalValues so that MDNodes that reference GlobalValues
   // are properly remapped.
   linkNamedMDNodes();
 
+  mergePICLevel(SrcM, DstM);
+
   // Merge the module flags into the DstM module.
   return linkModuleFlagsMetadata();
 }
 
 IRMover::StructTypeKeyInfo::KeyTy::KeyTy(ArrayRef<Type *> E, bool P)
     : ETypes(E), IsPacked(P) {}
 
 IRMover::StructTypeKeyInfo::KeyTy::KeyTy(const StructType *ST)

test/Linker/Inputs/module-flags-pic-2-b.ll

-!0 = !{ i32 1, !"PIC Level", i32 2 }
+!0 = !{ i32 2, !"PIC Level", i32 2 }
 
 !llvm.module.flags = !{!0}

test/Linker/module-flags-pic-1-a.ll

 ; RUN: llvm-link %s %p/Inputs/module-flags-pic-1-b.ll -S -o - | FileCheck %s
 
 ; test linking modules with specified and default PIC levels
 
-!0 = !{ i32 1, !"PIC Level", i32 1 }
+!0 = !{ i32 2, !"PIC Level", i32 1 }
 
 !llvm.module.flags = !{!0}
 ; CHECK: !llvm.module.flags = !{!0}
-; CHECK: !0 = !{i32 1, !"PIC Level", i32 1}
+; CHECK: !0 = !{i32 2, !"PIC Level", i32 1}

test/Linker/module-flags-pic-2-a.ll

-; RUN: not llvm-link %s %p/Inputs/module-flags-pic-2-b.ll -S -o - 2> %t
-; RUN: FileCheck --check-prefix=CHECK-ERRORS < %t %s
+; RUN: llvm-link %s %p/Inputs/module-flags-pic-2-b.ll -S -o - 2> %t | FileCheck %s \
+; RUN:   --check-prefix=MERGE
+; RUN: FileCheck < %t %s
 
 ; test linking modules with two different PIC levels
 
-!0 = !{ i32 1, !"PIC Level", i32 1 }
+!0 = !{ i32 2, !"PIC Level", i32 1 }
 
 !llvm.module.flags = !{!0}
 
-; CHECK-ERRORS: ERROR: linking module flags 'PIC Level': IDs have conflicting values
+; MERGE: source_filename = "llvm-link"
+; MERGE: !llvm.module.flags = !{!0}
+; MERGE: !0 = !{i32 2, !"PIC Level", i32 1}
+
+; CHECK: WARNING: linking module flags 'PIC Level': IDs have conflicting values