Allow targets to override the use of getKindForGlobal in AsmPrinter to avoid text relocations on PPC64

[joerg]

Summary

getKindForGlobal is currently a static member of TargetLoweringObjectFile and can't be easily customised. One of the more complicated uses is to decide whether a IR constant with relocations can be emitted as constant section.

The default implementation can deal with static vs PIC, but the SYSV ABI for PPC64 is special in that it always requires the PIC rules to be followed. The patch introduces a new method getSectionKindForGlobal with a matching override in the PPC implementation to get the right behavior.

[echristo]

I've been in this code more than anyone, I'll add it to my list.

[hfinkel]

The default implementation can deal with static vs PIC, but the SYSV ABI for PPC64 is special in that it always requires the PIC rules to be followed.

Please demonstrate where the ABI requires this; I'm not sure that your premise is correct. http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi.html says, for example, "One aspect of segment loading differs between executable files and shared objects. Executable file segments may contain absolute code... On the other hand, shared object segments typically contain position-independent code...".

[joerg]

The most explicit statement is http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi.html#CODE-EX This is different from the ELFv2 statements, which explicitly also discusses position dependent code.

[hfinkel]

In D26326#590530, @joerg wrote:

The most explicit statement is http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi.html#CODE-EX This is different from the ELFv2 statements, which explicitly also discusses position dependent code.

Commented in D26290; let's work out the issue there and then come back to this one.

[joerg]

Superseded by PIC default for PPC64.

include/llvm/Target/TargetLoweringObjectFile.h

virtual MCSection *getSectionForConstant(const DataLayout &DL,
                                            const Constant *C,
                                            unsigned &Align) const;
 
   /// Classify the specified global variable into a set of target independent
   /// categories embodied in SectionKind.
   static SectionKind getKindForGlobal(const GlobalObject *GO,
                                       const TargetMachine &TM);
 
+  virtual SectionKind getSectionKindForGlobal(const GlobalObject *GO,
+                                              const TargetMachine &TM) const {
+    return TargetLoweringObjectFile::getKindForGlobal(GO, TM);
+  }
+
   /// This method computes the appropriate section to emit the specified global
   /// variable or function definition. This should not be passed external (or
   /// available externally) globals.
   MCSection *SectionForGlobal(const GlobalObject *GO, SectionKind Kind,
                               const TargetMachine &TM) const;
 
   /// This method computes the appropriate section to emit the specified global
   /// variable or function definition. This should not be passed external (or

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

void AsmPrinter::EmitGlobalVariable(const GlobalVariable *GV) {
   GVSym->redefineIfPossible();
   if (GVSym->isDefined() || GVSym->isVariable())
     report_fatal_error("symbol '" + Twine(GVSym->getName()) +
                        "' is already defined");
 
   if (MAI->hasDotTypeDotSizeDirective())
     OutStreamer->EmitSymbolAttribute(EmittedSym, MCSA_ELF_TypeObject);
 
-  SectionKind GVKind = TargetLoweringObjectFile::getKindForGlobal(GV, TM);
+  const TargetLoweringObjectFile &TLOF = getObjFileLowering();
+  SectionKind GVKind = TLOF.getSectionKindForGlobal(GV, TM);
 
   const DataLayout &DL = GV->getParent()->getDataLayout();
   uint64_t Size = DL.getTypeAllocSize(GV->getType()->getElementType());
 
   // If the alignment is specified, we *must* obey it.  Overaligning a global
   // with a specified alignment is a prompt way to break globals emitted to
   // sections and expected to be contiguous (e.g. ObjC metadata).
   unsigned AlignLog = getGVAlignmentLog2(GV, DL);

lib/Target/PowerPC/PPCTargetObjectFile.h

namespace llvm {
   /// 64-bit PowerPC Linux.
   class PPC64LinuxTargetObjectFile : public TargetLoweringObjectFileELF {
 
     void Initialize(MCContext &Ctx, const TargetMachine &TM) override;
 
     MCSection *SelectSectionForGlobal(const GlobalObject *GO, SectionKind Kind,
                                       const TargetMachine &TM) const override;
 
+    SectionKind getSectionKindForGlobal(const GlobalObject *GO,
+                                        const TargetMachine &TM) const override;
+
+
     /// \brief Describe a TLS variable address within debug info.
     const MCExpr *getDebugThreadLocalSymbol(const MCSymbol *Sym) const override;
   };
 
 }  // end namespace llvm
 
 #endif

lib/Target/PowerPC/PPCTargetObjectFile.cpp

if (Kind.isReadOnly()) {
 
     if (GVar && GVar->isConstant() && GVar->getInitializer()->needsRelocation())
       Kind = SectionKind::getReadOnlyWithRel();
   }
 
   return TargetLoweringObjectFileELF::SelectSectionForGlobal(GO, Kind, TM);
 }
 
+SectionKind PPC64LinuxTargetObjectFile::getSectionKindForGlobal(
+    const GlobalObject *GO, const TargetMachine &TM) const {
+  auto Kind = TargetLoweringObjectFileELF::getSectionKindForGlobal(GO, TM);
+
+  // See in PPC64LinuxTargetObjectFile::SelectSectionForGlobal above.
+  if (Kind.isReadOnly()) {
+    const auto *GVar = dyn_cast<GlobalVariable>(GO);
+
+    if (GVar && GVar->isConstant() && GVar->getInitializer()->needsRelocation())
+      Kind = SectionKind::getReadOnlyWithRel();
+  }
+
+  return Kind;
+}
+
+
 const MCExpr *PPC64LinuxTargetObjectFile::
 getDebugThreadLocalSymbol(const MCSymbol *Sym) const {
   const MCExpr *Expr =
     MCSymbolRefExpr::create(Sym, MCSymbolRefExpr::VK_DTPREL, getContext());
   return MCBinaryExpr::createAdd(Expr,
                                  MCConstantExpr::create(0x8000, getContext()),
                                  getContext());
 }
 

test/CodeGen/PowerPC/func-addr-consts.ll

+; RUN: llc < %s | FileCheck %s
+target datalayout = "E-m:e-i64:64-n32:64"
+target triple = "powerpc64--linux"
+
+@g = internal constant i8* bitcast (void ()* @f to i8*), section "gsection", align 8
+@h = constant i8* bitcast (void ()* @f to i8*), section "hsection", align 8
[e-mail] = appending global [2 x i8*] [i8* bitcast (i8** @g to i8*), i8* bitcast (i8** @h to i8*)], section "llvm.metadata"
+
+; Function Attrs: nounwind uwtable
+define internal void @f() {
+entry:
+  ret void
+}
+
+; CHECK: .section gsection,"aw",@progbits
+; CHECK: .section hsection,"aw",@progbits