[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

include/llvm/MC/MCFixup.h

@@ -20,7 +20,8 @@ class MCExpr;
 
 /// Extensible enumeration to represent the type of a fixup.
 enum MCFixupKind {
-  FK_Data_1 = 0, ///< A one-byte fixup.
+  FK_NONE = 0,   ///< A no-op fixup.
+  FK_Data_1,     ///< A one-byte fixup.
   FK_Data_2,     ///< A two-byte fixup.
   FK_Data_4,     ///< A four-byte fixup.
   FK_Data_8,     ///< A eight-byte fixup.

lib/MC/MCAsmBackend.cpp

@@ -64,6 +64,7 @@ Optional<MCFixupKind> MCAsmBackend::getFixupKind(StringRef Name) const {
 
 const MCFixupKindInfo &MCAsmBackend::getFixupKindInfo(MCFixupKind Kind) const {
   static const MCFixupKindInfo Builtins[] = {
+      {"FK_NONE", 0, 0, 0},
       {"FK_Data_1", 0, 8, 0},
       {"FK_Data_2", 0, 16, 0},
       {"FK_Data_4", 0, 32, 0},

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:

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,

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:

lib/Target/Mips/MCTargetDesc/MipsAsmBackend.cpp

@@ -302,7 +302,7 @@ void MipsAsmBackend::applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
 
 Optional<MCFixupKind> MipsAsmBackend::getFixupKind(StringRef Name) const {
   return StringSwitch<Optional<MCFixupKind>>(Name)
-      .Case("R_MIPS_NONE", (MCFixupKind)Mips::fixup_Mips_NONE)
+      .Case("R_MIPS_NONE", FK_NONE)
       .Case("R_MIPS_32", FK_Data_4)
       .Case("R_MIPS_GOT_PAGE", (MCFixupKind)Mips::fixup_Mips_GOT_PAGE)
       .Case("R_MIPS_CALL_HI16", (MCFixupKind)Mips::fixup_Mips_CALL_HI16)
@@ -350,7 +350,6 @@ getFixupKindInfo(MCFixupKind Kind) const {
     // MipsFixupKinds.h.
     //
     // name                    offset  bits  flags
-    { "fixup_Mips_NONE",         0,      0,   0 },
     { "fixup_Mips_16",           0,     16,   0 },
     { "fixup_Mips_32",           0,     32,   0 },
     { "fixup_Mips_REL32",        0,     32,   0 },
@@ -430,7 +429,6 @@ getFixupKindInfo(MCFixupKind Kind) const {
     // MipsFixupKinds.h.
     //
     // name                    offset  bits  flags
-    { "fixup_Mips_NONE",         0,      0,   0 },
     { "fixup_Mips_16",          16,     16,   0 },
     { "fixup_Mips_32",           0,     32,   0 },
     { "fixup_Mips_REL32",        0,     32,   0 },

lib/Target/Mips/MCTargetDesc/MipsELFObjectWriter.cpp

@@ -222,7 +222,7 @@ unsigned MipsELFObjectWriter::getRelocType(MCContext &Ctx,
   unsigned Kind = (unsigned)Fixup.getKind();
 
   switch (Kind) {
-  case Mips::fixup_Mips_NONE:
+  case FK_NONE:
     return ELF::R_MIPS_NONE;
   case FK_Data_1:
     Ctx.reportError(Fixup.getLoc(),

lib/Target/Mips/MCTargetDesc/MipsFixupKinds.h

@@ -22,11 +22,8 @@ namespace Mips {
   // in MipsAsmBackend.cpp.
   //
   enum Fixups {
-    // Branch fixups resulting in R_MIPS_NONE.
-    fixup_Mips_NONE = FirstTargetFixupKind,
-
     // Branch fixups resulting in R_MIPS_16.
-    fixup_Mips_16,
+    fixup_Mips_16 = FirstTargetFixupKind,
 
     // Pure 32 bit data fixup resulting in - R_MIPS_32.
     fixup_Mips_32,

test/MC/ARM/reloc-directive.s

@@ -0,0 +1,37 @@
+# RUN: llvm-mc -triple=armv7-linux-gnueabi %s | FileCheck --check-prefix=PRINT %s
+# RUN: llvm-mc -triple=armv7eb-linux-gnueabi %s | FileCheck --check-prefix=PRINT %s
+
+# RUN: llvm-mc -filetype=obj -triple=armv7-linux-gnueabi %s -o %t
+# RUN: llvm-readobj -r %t | FileCheck %s
+# RUN: llvm-readelf -x .data %t | FileCheck --check-prefix=HEX %s
+
+# RUN: llvm-mc -filetype=obj -triple=armv7eb-linux-gnueabi %s -o %t
+# RUN: llvm-readobj -r %t | FileCheck %s
+# RUN: llvm-readelf -x .data %t | FileCheck --check-prefix=HEX %s
+
+.text
+  bx lr
+  nop
+  nop
+  .reloc 8, R_ARM_NONE, .data
+  .reloc 4, R_ARM_NONE, foo+4
+  .reloc 0, R_ARM_NONE, 8
+
+.data
+.globl foo
+foo:
+  .word 0
+  .word 0
+  .word 0
+
+# PRINT: .reloc 8, R_ARM_NONE, .data
+# PRINT: .reloc 4, R_ARM_NONE, foo+4
+# PRINT: .reloc 0, R_ARM_NONE, 8
+
+# ARM relocations use the Elf32_Rel format. Addends are neither stored in the
+# relocation entries nor applied in the referenced locations.
+# CHECK:      0x8 R_ARM_NONE .data 0x0
+# CHECK-NEXT: 0x4 R_ARM_NONE foo 0x0
+# CHECK-NEXT: 0x0 R_ARM_NONE - 0x0
+
+# HEX: 0x00000000 00000000 00000000