[ARM] Support .reloc *, R_ARM_NONE, *

R_ARM_NONE can be used to create references among sections. When
--gc-sections is used, the referenced section will be retained if the
origin section is retained.

Add a generic MCFixupKind FK_NONE as this kind of no-op relocation is
ubiquitous on ELF and COFF, and probably available on many other binary
formats. See D62014.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D61992

llvm-svn: 360980

3 files changed, 21 insertions, 1 deletions

diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
index b846ca0699b..538bc2594c3 100644
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
@@ -46,6 +46,13 @@ public:
 };
 } // end anonymous namespace
 
+Optional<MCFixupKind> ARMAsmBackend::getFixupKind(StringRef Name) const {
+  if (STI.getTargetTriple().isOSBinFormatELF() && Name == "R_ARM_NONE")
+    return FK_NONE;
+
+  return MCAsmBackend::getFixupKind(Name);
+}
+
 const MCFixupKindInfo &ARMAsmBackend::getFixupKindInfo(MCFixupKind Kind) const {
   const static MCFixupKindInfo InfosLE[ARM::NumTargetFixupKinds] = {
       // This table *must* be in the order that the fixup_* kinds are defined in
@@ -383,6 +390,7 @@ unsigned ARMAsmBackend::adjustFixupValue(const MCAssembler &Asm,
   default:
     Ctx.reportError(Fixup.getLoc(), "bad relocation fixup type");
     return 0;
+  case FK_NONE:
   case FK_Data_1:
   case FK_Data_2:
   case FK_Data_4:
@@ -761,7 +769,9 @@ bool ARMAsmBackend::shouldForceRelocation(const MCAssembler &Asm,
   const MCSymbolRefExpr *A = Target.getSymA();
   const MCSymbol *Sym = A ? &A->getSymbol() : nullptr;
   const unsigned FixupKind = Fixup.getKind() ;
-  if ((unsigned)Fixup.getKind() == ARM::fixup_arm_thumb_bl) {
+  if (FixupKind == FK_NONE)
+    return true;
+  if (FixupKind == ARM::fixup_arm_thumb_bl) {
     assert(Sym && "How did we resolve this?");
 
     // If the symbol is external the linker will handle it.
@@ -803,6 +813,9 @@ static unsigned getFixupKindNumBytes(unsigned Kind) {
   default:
     llvm_unreachable("Unknown fixup kind!");
 
+  case FK_NONE:
+    return 0;
+
   case FK_Data_1:
   case ARM::fixup_arm_thumb_bcc:
   case ARM::fixup_arm_thumb_cp:
@@ -857,6 +870,9 @@ static unsigned getFixupKindContainerSizeBytes(unsigned Kind) {
   default:
     llvm_unreachable("Unknown fixup kind!");
 
+  case FK_NONE:
+    return 0;
+
   case FK_Data_1:
     return 1;
   case FK_Data_2:
diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.h b/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.h
index 52abc2f20e0..67722a5e5b6 100644
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.h
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.h
@@ -37,6 +37,8 @@ public:
   // different.
   bool hasNOP() const { return STI.getFeatureBits()[ARM::HasV6T2Ops]; }
 
+  Optional<MCFixupKind> getFixupKind(StringRef Name) const override;
+
   const MCFixupKindInfo &getFixupKindInfo(MCFixupKind Kind) const override;
 
   bool shouldForceRelocation(const MCAssembler &Asm, const MCFixup &Fixup,
diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
index a93202b8202..1d51a9696cd 100644
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
@@ -143,6 +143,8 @@ unsigned ARMELFObjectWriter::GetRelocTypeInner(const MCValue &Target,
   default:
     Ctx.reportFatalError(Fixup.getLoc(), "unsupported relocation on symbol");
     return ELF::R_ARM_NONE;
+  case FK_NONE:
+    return ELF::R_ARM_NONE;
   case FK_Data_1:
     switch (Modifier) {
     default: