CodeGen: handle llvm.used properly for COFF

[compnerd]

Summary

llvm.used contains a list of pointers to named values which the
compiler, assembler, and linker are required to treat as if there is a
reference that they cannot see. Ensure that the symbols are preserved
by adding an explicit -include reference to the linker command.

[rnk]

Duplicating this in the ARM and X86 AsmPrinters is lame. Maybe we should hoist the COFF-specific handling up to the next level? I'm going to look into doing that for -export flags.

[rnk]

Cool, look at rL322788.

BTW, afterwards this is a great opportunity to fix this bug:

template <typename T> struct Foo { static int x; };
int f();
template <typename T> int Foo<T>::x = f();
template struct Foo<int>;

MSVC emits "/include:?x@?$Foo@H@@2HA" to make sure that x survives dead stripping. Once we have this llvm.used functionality, we have an easy IR-only way to get that /include directive.

[compnerd]

Awesome; I was thinking about doing that as a follow up. I'll hoist this into that new layer. Completely agree on the fixing the emission after this infrastructure is in place. I can't believe that we never implemented this.

[pcc]

This doesn't seem like it will do the right thing if the global has internal linkage. Do we care about that case? I guess we could handle it by emitting dummy ABSOLUTE relocations from a non-COMDAT section to the section or symbol that we want to preserve.

[compnerd]

rebase and emit one entry per line

[compnerd]

ignore internal globals as per IRC conversion with @rnk , @pcc, @smeenai

[smeenai]

Patch is missing context.

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

1459

Add a comment about why internal symbols are ignored?

[compnerd]

context + comment

[rnk]

lgtm

test/CodeGen/X86/coff-no-dead-strip.ll

7

Maybe test a vectorcallcc function to test that we're using the GlobalValue overload of getNameWithPrefix.

13

CHECK-ULP-NOT: " /INCLUDE:_k" maybe?

[compnerd]

test/CodeGen/X86/coff-no-dead-strip.ll

7

Sounds good.

13

Hah, I forgot to adjust the commit. I have that added and tested with and without the user label prefix.

[compnerd]

SVN r323017

include/llvm/CodeGen/TargetLoweringObjectFile.h

virtual const MCExpr *getIndirectSymViaGOTPCRel(const MCSymbol *Sym,
                                                   MachineModuleInfo *MMI,
                                                   MCStreamer &Streamer) const {
     return nullptr;
   }
 
   virtual void emitLinkerFlagsForGlobal(raw_ostream &OS,
                                         const GlobalValue *GV) const {}
 
+  virtual void emitLinkerFlagsForUsed(raw_ostream &OS,
+                                      const GlobalValue *GV) const {}
+
 protected:
   virtual MCSection *SelectSectionForGlobal(const GlobalObject *GO,
                                             SectionKind Kind,
                                             const TargetMachine &TM) const = 0;
 };
 
 } // end namespace llvm
 
 #endif // LLVM_CODEGEN_TARGETLOWERINGOBJECTFILE_H

include/llvm/CodeGen/TargetLoweringObjectFileImpl.h

public:
 
   MCSection *getStaticCtorSection(unsigned Priority,
                                   const MCSymbol *KeySym) const override;
   MCSection *getStaticDtorSection(unsigned Priority,
                                   const MCSymbol *KeySym) const override;
 
   void emitLinkerFlagsForGlobal(raw_ostream &OS,
                                 const GlobalValue *GV) const override;
+
+  void emitLinkerFlagsForUsed(raw_ostream &OS,
+                              const GlobalValue *GV) const override;
 };
 
 class TargetLoweringObjectFileWasm : public TargetLoweringObjectFile {
   mutable unsigned NextUniqueID = 0;
 
 public:
   TargetLoweringObjectFileWasm() = default;
   ~TargetLoweringObjectFileWasm() override = default;

include/llvm/IR/Mangler.h

static void getNameWithPrefix(raw_ostream &OS, const Twine &GVName,
                                 const DataLayout &DL);
   static void getNameWithPrefix(SmallVectorImpl<char> &OutName,
                                 const Twine &GVName, const DataLayout &DL);
 };
 
 void emitLinkerFlagsForGlobalCOFF(raw_ostream &OS, const GlobalValue *GV,
                                   const Triple &TT, Mangler &Mangler);
 
+void emitLinkerFlagsForUsedCOFF(raw_ostream &OS, const GlobalValue *GV,
+                                const Triple &T, Mangler &M);
+
 } // End llvm namespace
 
 #endif

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

   if (!InitTrampolineIntrinsic || InitTrampolineIntrinsic->use_empty())
     if (MCSection *S = MAI->getNonexecutableStackSection(OutContext))
       OutStreamer->SwitchSection(S);
 
   if (TM.getTargetTriple().isOSBinFormatCOFF()) {
     // Emit /EXPORT: flags for each exported global as necessary.
     const auto &TLOF = getObjFileLowering();
     std::string Flags;
+
     for (const GlobalValue &GV : M.global_values()) {
       raw_string_ostream OS(Flags);
       TLOF.emitLinkerFlagsForGlobal(OS, &GV);
       OS.flush();
       if (!Flags.empty()) {
         OutStreamer->SwitchSection(TLOF.getDrectveSection());
         OutStreamer->EmitBytes(Flags);
       }
       Flags.clear();
     }
+
+    // Emit /INCLUDE: flags for each used global as necessary.
+    if (const auto *LU = M.getNamedGlobal("llvm.used")) {
+      assert(LU->hasInitializer() &&
+             "expected llvm.used to have an initializer");
+      assert(isa<ArrayType>(LU->getValueType()) &&
+             "expected llvm.used to be an array type");
+      if (const auto *A = cast<ConstantArray>(LU->getInitializer())) {
+        for (const Value *Op : A->operands()) {
+          const auto *GV =
+              cast<GlobalValue>(Op->stripPointerCastsNoFollowAliases());
+          // Global symbols with internal linkage are not visible to the linker,
+          // and thus would cause an error when the linker tried to preserve the
+          // symbol due to the `/include:` directive.
+          if (GV->hasInternalLinkage())
+            continue;
+
+          raw_string_ostream OS(Flags);
+          TLOF.emitLinkerFlagsForUsed(OS, GV);
+          OS.flush();
+
+          if (!Flags.empty()) {
+            OutStreamer->SwitchSection(TLOF.getDrectveSection());
+            OutStreamer->EmitBytes(Flags);
+          }
+          Flags.clear();
+        }
+      }
+    }
   }
 
   // Allow the target to emit any magic that it wants at the end of the file,
   // after everything else has gone out.
   EmitEndOfAsmFile(M);
 
   MMI = nullptr;
 

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

                                       cast<MCSectionCOFF>(StaticDtorSection));
 }
 
 void TargetLoweringObjectFileCOFF::emitLinkerFlagsForGlobal(
     raw_ostream &OS, const GlobalValue *GV) const {
   emitLinkerFlagsForGlobalCOFF(OS, GV, getTargetTriple(), getMangler());
 }
 
+void TargetLoweringObjectFileCOFF::emitLinkerFlagsForUsed(
+    raw_ostream &OS, const GlobalValue *GV) const {
+  emitLinkerFlagsForUsedCOFF(OS, GV, getTargetTriple(), getMangler());
+}
+
 //===----------------------------------------------------------------------===//
 //                                  Wasm
 //===----------------------------------------------------------------------===//
 
 static const Comdat *getWasmComdat(const GlobalValue *GV) {
   const Comdat *C = GV->getComdat();
   if (!C)
     return nullptr;

lib/IR/Mangler.cpp

void llvm::emitLinkerFlagsForGlobalCOFF(raw_ostream &OS, const GlobalValue *GV,
 
   if (!GV->getValueType()->isFunctionTy()) {
     if (TT.isKnownWindowsMSVCEnvironment())
       OS << ",DATA";
     else
       OS << ",data";
   }
 }
+
+void llvm::emitLinkerFlagsForUsedCOFF(raw_ostream &OS, const GlobalValue *GV,
+                                      const Triple &T, Mangler &M) {
+  if (!T.isKnownWindowsMSVCEnvironment())
+    return;
+
+  OS << " /INCLUDE:";
+  M.getNameWithPrefix(OS, GV, false);
+}
+

test/CodeGen/X86/coff-no-dead-strip.ll

+; RUN: llc -mtriple i686-windows-msvc -filetype asm -o - %s | FileCheck %s -check-prefix CHECK -check-prefix CHECK-ULP
+; RUN: llc -mtriple x86_64-windows-msvc -filetype asm -o - %s | FileCheck %s -check-prefix CHECK -check-prefix CHECK-NOULP
+; RUN: llc -mtriple thumbv7-windows-msvc -filetype asm -o - %s | FileCheck %s -check-prefix -check-prefix CHECK-NOULP
+
+@i = global i32 0
+@j = weak global i32 0
+@k = internal global i32 0
+
[e-mail] = appending global [3 x i8*] [i8* bitcast (i32* @i to i8*), i8* bitcast (i32* @j to i8*), i8* bitcast (i32* @k to i8*)]
+
+; CHECK: .section .drectve
+; CHECK-ULP: .ascii " /INCLUDE:_i"
+; CHECK-ULP: .ascii " /INCLUDE:_j"
+; CHECK-NOULP: .ascii " /INCLUDE:i"
+; CHECK-NOULP: .ascii " /INCLUDE:j"
+