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 ↗

(On Diff #130689)

Add a comment about why internal symbols are ignored?

[compnerd]

context + comment

[rnk]

lgtm

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

8

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

14

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

[compnerd]

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

8

Sounds good.

14

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 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,

include/llvm/CodeGen/TargetLoweringObjectFileImpl.h

 
   void emitLinkerFlagsForGlobal(raw_ostream &OS,
                                 const GlobalValue *GV) const override;
+
+  void emitLinkerFlagsForUsed(raw_ostream &OS,
+                              const GlobalValue *GV) const override;
 };
 
 class TargetLoweringObjectFileWasm : public TargetLoweringObjectFile {

include/llvm/IR/Mangler.h

 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/TargetLoweringObjectFileImpl.cpp

   emitLinkerFlagsForGlobalCOFF(OS, GV, getTargetTriple(), getMangler());
 }
 
+void TargetLoweringObjectFileCOFF::emitLinkerFlagsForUsed(
+    raw_ostream &OS, const GlobalValue *GV) const {
+  emitLinkerFlagsForUsedCOFF(OS, GV, getTargetTriple(), getMangler());
+}
+
 //===----------------------------------------------------------------------===//
 //                                  Wasm
 //===----------------------------------------------------------------------===//

lib/IR/Mangler.cpp

       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);
+}
+

lib/Target/ARM/ARMAsmPrinter.cpp

     for (const auto &Alias : M.aliases())
       TLOF.emitLinkerFlagsForGlobal(OS, &Alias);
 
+    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 *IA = dyn_cast<ConstantArray>(LU->getInitializer()))
+        for (Value *Op : IA->operands())
+          TLOF.emitLinkerFlagsForUsed(
+              OS, cast<GlobalValue>(Op->stripPointerCastsNoFollowAliases()));
+    }
+
     OS.flush();
 
     // Output collected flags

lib/Target/X86/X86AsmPrinter.cpp

     for (const auto &Alias : M.aliases())
       TLOFCOFF.emitLinkerFlagsForGlobal(FlagsOS, &Alias);
 
+    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 *IA = dyn_cast<ConstantArray>(LU->getInitializer()))
+        for (Value *Op : IA->operands())
+          TLOFCOFF.emitLinkerFlagsForUsed(
+              FlagsOS,
+              cast<GlobalValue>(Op->stripPointerCastsNoFollowAliases()));
+    }
+
     FlagsOS.flush();
 
     // Output collected flags.

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

+; RUN: llc -mtriple i686-windows-msvc -filetype asm -o - %s | FileCheck %s -check-prefix CHECK-X86
+; RUN: llc -mtriple x86_64-windows-msvc -filetype asm -o - %s | FileCheck %s -check-prefix CHECK-X64
+; RUN: llc -mtriple thumbv7-windows-msvc -filetype asm -o - %s | FileCheck %s -check-prefix CHECK-ARM
+
+@i = global i32 0
+@j = weak global i32 0
+
[e-mail] = appending global [2 x i8*] [i8* bitcast (i32* @i to i8*), i8* bitcast (i32* @j to i8*)]
+
+; CHECK-X86: .ascii " /INCLUDE:_i /INCLUDE:_j"
+; CHECK-X64: .ascii " /INCLUDE:i /INCLUDE:j"
+; CHECK-ARM: .ascii " /INCLUDE:i /INCLUDE:j"
+