[DWARF][RISCV] Add support for RISC-V relocations needed for debug info

When code relaxation is enabled many RISC-V fixups are not resolved but
instead relocations are emitted. This happens even for DWARF debug
sections. Therefore, to properly support the parsing of DWARF debug info
we need to be able to resolve RISC-V relocations. This patch adds:

* Support for RISC-V relocations in RelocationResolver
* DWARF support for two relocations per object file offset
* DWARF changes to support relocations in more DIE fields

The two relocations per offset change is needed because some RISC-V
relocations (used for label differences) come in pairs.

Relocations can also be emitted for DWARF fields where relocations were
not yet evaluated. Adding relocation support for some of these fields is
essencial. On the other hand, LLVM currently emits RISC-V relocations
for fixups that could be safely evaluated, since they can never be
affected by code relaxations. This patch also adds relocation support
for the fields affected by those extraneous relocations (the DWARF unit
entry Length, and the DWARF debug line entry TotalLength and
PrologueLength), for testing purposes.

Differential Revision: https://reviews.llvm.org/D62062
Patch by Luís Marques.

llvm-svn: 366402

lld/ELF/DWARF.cpp

@@ -110,7 +110,8 @@ LLDDwarfObj<ELFT>::findAux(const InputSectionBase &sec, uint64_t pos,
   DataRefImpl d;
   d.p = getAddend<ELFT>(rel);
   return RelocAddrEntry{secIndex, RelocationRef(d, nullptr),
-                        LLDRelocationResolver<RelTy>::resolve, val};
+                        val,      Optional<object::RelocationRef>(),
+                        0,        LLDRelocationResolver<RelTy>::resolve};
 }
 
 template <class ELFT>

llvm/include/llvm/DebugInfo/DWARF/DWARFRelocMap.h

@@ -20,8 +20,10 @@ namespace llvm {
 struct RelocAddrEntry {
   uint64_t SectionIndex;
   object::RelocationRef Reloc;
-  object::RelocationResolver Resolver;
   uint64_t SymbolValue;
+  Optional<object::RelocationRef> Reloc2;
+  uint64_t SymbolValue2;
+  object::RelocationResolver Resolver;
 };
 
 /// In place of applying the relocations to the data we've read from disk we use

llvm/lib/DebugInfo/DWARF/DWARFContext.cpp

@@ -1651,9 +1651,25 @@ class DWARFObjInMemory final : public DWARFObject {
         //
         // TODO Don't store Resolver in every RelocAddrEntry.
         if (Supports && Supports(Reloc.getType())) {
-          Map->try_emplace(Reloc.getOffset(),
-                           RelocAddrEntry{SymInfoOrErr->SectionIndex, Reloc,
-                                          Resolver, SymInfoOrErr->Address});
+          auto I = Map->try_emplace(
+              Reloc.getOffset(),
+              RelocAddrEntry{SymInfoOrErr->SectionIndex, Reloc,
+                             SymInfoOrErr->Address,
+                             Optional<object::RelocationRef>(), 0, Resolver});
+          // If we didn't successfully insert that's because we already had a
+          // relocation for that offset. Store it as a second relocation in the
+          // same RelocAddrEntry instead.
+          if (!I.second) {
+            RelocAddrEntry &entry = I.first->getSecond();
+            if (entry.Reloc2) {
+              ErrorPolicy EP = HandleError(createError(
+                  "At most two relocations per offset are supported"));
+              if (EP == ErrorPolicy::Halt)
+                return;
+            }
+            entry.Reloc2 = Reloc;
+            entry.SymbolValue2 = SymInfoOrErr->Address;
+          }
         } else {
           SmallString<32> Type;
           Reloc.getTypeName(Type);

llvm/lib/DebugInfo/DWARF/DWARFDataExtractor.cpp

@@ -24,7 +24,10 @@ uint64_t DWARFDataExtractor::getRelocatedValue(uint32_t Size, uint32_t *Off,
     return A;
   if (SecNdx)
     *SecNdx = E->SectionIndex;
-  return E->Resolver(E->Reloc, E->SymbolValue, A);
+  uint64_t R = E->Resolver(E->Reloc, E->SymbolValue, A);
+  if (E->Reloc2)
+    R = E->Resolver(*E->Reloc2, E->SymbolValue2, R);
+  return R;
 }
 
 Optional<uint64_t>

llvm/lib/DebugInfo/DWARF/DWARFDebugLine.cpp

@@ -300,7 +300,7 @@ Error DWARFDebugLine::Prologue::parse(const DWARFDataExtractor &DebugLineData,
   const uint64_t PrologueOffset = *OffsetPtr;
 
   clear();
-  TotalLength = DebugLineData.getU32(OffsetPtr);
+  TotalLength = DebugLineData.getRelocatedValue(4, OffsetPtr);
   if (TotalLength == UINT32_MAX) {
     FormParams.Format = dwarf::DWARF64;
     TotalLength = DebugLineData.getU64(OffsetPtr);
@@ -325,7 +325,8 @@ Error DWARFDebugLine::Prologue::parse(const DWARFDataExtractor &DebugLineData,
     SegSelectorSize = DebugLineData.getU8(OffsetPtr);
   }
 
-  PrologueLength = DebugLineData.getUnsigned(OffsetPtr, sizeofPrologueLength());
+  PrologueLength =
+      DebugLineData.getRelocatedValue(sizeofPrologueLength(), OffsetPtr);
   const uint64_t EndPrologueOffset = PrologueLength + *OffsetPtr;
   MinInstLength = DebugLineData.getU8(OffsetPtr);
   if (getVersion() >= 4)
@@ -754,7 +755,7 @@ Error DWARFDebugLine::LineTable::parse(
         // requires the use of DW_LNS_advance_pc. Such assemblers, however,
         // can use DW_LNS_fixed_advance_pc instead, sacrificing compression.
         {
-          uint16_t PCOffset = DebugLineData.getU16(OffsetPtr);
+          uint16_t PCOffset = DebugLineData.getRelocatedValue(2, OffsetPtr);
           State.Row.Address.Address += PCOffset;
           if (OS)
             *OS

llvm/lib/DebugInfo/DWARF/DWARFFormValue.cpp

@@ -299,7 +299,7 @@ bool DWARFFormValue::extractValue(const DWARFDataExtractor &Data,
     case DW_FORM_data8:
     case DW_FORM_ref8:
     case DW_FORM_ref_sup8:
-      Value.uval = Data.getU64(OffsetPtr);
+      Value.uval = Data.getRelocatedValue(8, OffsetPtr);
       break;
     case DW_FORM_data16:
       // Treat this like a 16-byte block.

llvm/lib/DebugInfo/DWARF/DWARFListTable.cpp

@@ -25,7 +25,7 @@ Error DWARFListTableHeader::extract(DWARFDataExtractor Data,
                        "%s table length at offset 0x%" PRIx32,
                        SectionName.data(), *OffsetPtr);
   // TODO: Add support for DWARF64.
-  HeaderData.Length = Data.getU32(OffsetPtr);
+  HeaderData.Length = Data.getRelocatedValue(4, OffsetPtr);
   if (HeaderData.Length == 0xffffffffu)
     return createStringError(errc::not_supported,
                        "DWARF64 is not supported in %s at offset 0x%" PRIx32,
@@ -73,7 +73,7 @@ Error DWARFListTableHeader::extract(DWARFDataExtractor Data,
         SectionName.data(), HeaderOffset, HeaderData.OffsetEntryCount);
   Data.setAddressSize(HeaderData.AddrSize);
   for (uint32_t I = 0; I < HeaderData.OffsetEntryCount; ++I)
-    Offsets.push_back(Data.getU32(OffsetPtr));
+    Offsets.push_back(Data.getRelocatedValue(4, OffsetPtr));
   return Error::success();
 }
 

llvm/lib/DebugInfo/DWARF/DWARFUnit.cpp

@@ -241,7 +241,7 @@ bool DWARFUnitHeader::extract(DWARFContext &Context,
   IndexEntry = Entry;
   if (!IndexEntry && Index)
     IndexEntry = Index->getFromOffset(*offset_ptr);
-  Length = debug_info.getU32(offset_ptr);
+  Length = debug_info.getRelocatedValue(4, offset_ptr);
   FormParams.Format = DWARF32;
   unsigned SizeOfLength = 4;
   if (Length == 0xffffffff) {

llvm/lib/Object/RelocationResolver.cpp

@@ -330,6 +330,55 @@ static uint64_t resolveHexagon(RelocationRef R, uint64_t S, uint64_t A) {
   llvm_unreachable("Invalid relocation type");
 }
 
+static bool supportsRISCV(uint64_t Type) {
+  switch (Type) {
+  case ELF::R_RISCV_NONE:
+  case ELF::R_RISCV_32:
+  case ELF::R_RISCV_64:
+  case ELF::R_RISCV_ADD8:
+  case ELF::R_RISCV_SUB8:
+  case ELF::R_RISCV_ADD16:
+  case ELF::R_RISCV_SUB16:
+  case ELF::R_RISCV_ADD32:
+  case ELF::R_RISCV_SUB32:
+  case ELF::R_RISCV_ADD64:
+  case ELF::R_RISCV_SUB64:
+    return true;
+  default:
+    return false;
+  }
+}
+
+static uint64_t resolveRISCV(RelocationRef R, uint64_t S, uint64_t A) {
+  int64_t RA = getELFAddend(R);
+  switch (R.getType()) {
+  case ELF::R_RISCV_NONE:
+    return A;
+  case ELF::R_RISCV_32:
+    return (S + RA) & 0xFFFFFFFF;
+  case ELF::R_RISCV_64:
+    return S + RA;
+  case ELF::R_RISCV_ADD8:
+    return (A + (S + RA)) & 0xFF;
+  case ELF::R_RISCV_SUB8:
+    return (A - (S + RA)) & 0xFF;
+  case ELF::R_RISCV_ADD16:
+    return (A + (S + RA)) & 0xFFFF;
+  case ELF::R_RISCV_SUB16:
+    return (A - (S + RA)) & 0xFFFF;
+  case ELF::R_RISCV_ADD32:
+    return (A + (S + RA)) & 0xFFFFFFFF;
+  case ELF::R_RISCV_SUB32:
+    return (A - (S + RA)) & 0xFFFFFFFF;
+  case ELF::R_RISCV_ADD64:
+    return (A + (S + RA));
+  case ELF::R_RISCV_SUB64:
+    return (A - (S + RA));
+  default:
+    llvm_unreachable("Invalid relocation type");
+  }
+}
+
 static bool supportsCOFFX86(uint64_t Type) {
   switch (Type) {
   case COFF::IMAGE_REL_I386_SECREL:
@@ -449,6 +498,8 @@ getRelocationResolver(const ObjectFile &Obj) {
         return {supportsSparc64, resolveSparc64};
       case Triple::amdgcn:
         return {supportsAmdgpu, resolveAmdgpu};
+      case Triple::riscv64:
+        return {supportsRISCV, resolveRISCV};
       default:
         return {nullptr, nullptr};
       }
@@ -477,6 +528,8 @@ getRelocationResolver(const ObjectFile &Obj) {
       return {supportsSparc32, resolveSparc32};
     case Triple::hexagon:
       return {supportsHexagon, resolveHexagon};
+    case Triple::riscv32:
+      return {supportsRISCV, resolveRISCV};
     default:
       return {nullptr, nullptr};
     }

llvm/test/DebugInfo/RISCV/dwarf-riscv-relocs.ll

@@ -0,0 +1,105 @@
+; RUN: llc -filetype=obj -mtriple=riscv32 -mattr=+relax %s -o %t.o
+; RUN: llvm-readobj -r %t.o | FileCheck -check-prefix=READOBJ-RELOCS %s
+; RUN: llvm-objdump --source %t.o | FileCheck -check-prefix=OBJDUMP-SOURCE %s
+; RUN: llvm-dwarfdump --debug-info --debug-line %t.o | \
+; RUN:     FileCheck -check-prefix=DWARF-DUMP %s
+
+; Check that we actually have relocations, otherwise this is kind of pointless.
+; READOBJ-RELOCS:  Section (8) .rela.debug_info {
+; READOBJ-RELOCS-NEXT:    0x0 R_RISCV_ADD32 - 0x0
+; READOBJ-RELOCS-NEXT:    0x0 R_RISCV_SUB32 - 0x0
+; READOBJ-RELOCS:  Section (11) .rela.debug_addr {
+; READOBJ-RELOCS-NEXT:    0x0 R_RISCV_ADD32 - 0x0
+; READOBJ-RELOCS-NEXT:    0x0 R_RISCV_SUB32 - 0x0
+; READOBJ-RELOCS:  Section (17) .rela.debug_line {
+; READOBJ-RELOCS-NEXT:    0x0 R_RISCV_ADD32 - 0xFFFFFFFC
+; READOBJ-RELOCS-NEXT:    0x0 R_RISCV_SUB32 .Lline_table_start0 0x0
+
+; Check that we can print the source, even with relocations.
+; OBJDUMP-SOURCE: Disassembly of section .text:
+; OBJDUMP-SOURCE-EMPTY:
+; OBJDUMP-SOURCE-NEXT: 00000000 main:
+; OBJDUMP-SOURCE: ; {
+; OBJDUMP-SOURCE: ; return 0;
+
+; Check that we correctly dump the DWARF info, even with relocations.
+; DWARF-DUMP: DW_AT_name        ("dwarf-riscv-relocs.c")
+; DWARF-DUMP: DW_AT_comp_dir    (".")
+; DWARF-DUMP: DW_AT_name      ("main")
+; DWARF-DUMP: DW_AT_decl_file ("./dwarf-riscv-relocs.c")
+; DWARF-DUMP: DW_AT_decl_line (1)
+; DWARF-DUMP: DW_AT_type      (0x00000032 "int")
+; DWARF-DUMP: DW_AT_name      ("int")
+; DWARF-DUMP: DW_AT_encoding  (DW_ATE_signed)
+; DWARF-DUMP: DW_AT_byte_size (0x04)
+
+; DWARF-DUMP: .debug_line contents:
+; DWARF-DUMP-NEXT: debug_line[0x00000000]
+; DWARF-DUMP-NEXT: Line table prologue:
+; DWARF-DUMP-NEXT:     total_length: 0x0000005f
+; DWARF-DUMP-NEXT:          version: 5
+; DWARF-DUMP-NEXT:     address_size: 4
+; DWARF-DUMP-NEXT:  seg_select_size: 0
+; DWARF-DUMP-NEXT:  prologue_length: 0x0000003e
+; DWARF-DUMP-NEXT:  min_inst_length: 1
+; DWARF-DUMP-NEXT: max_ops_per_inst: 1
+; DWARF-DUMP-NEXT:  default_is_stmt: 1
+; DWARF-DUMP-NEXT:        line_base: -5
+; DWARF-DUMP-NEXT:       line_range: 14
+; DWARF-DUMP-NEXT:      opcode_base: 13
+; DWARF-DUMP-NEXT: standard_opcode_lengths[DW_LNS_copy] = 0
+; DWARF-DUMP-NEXT: standard_opcode_lengths[DW_LNS_advance_pc] = 1
+; DWARF-DUMP-NEXT: standard_opcode_lengths[DW_LNS_advance_line] = 1
+; DWARF-DUMP-NEXT: standard_opcode_lengths[DW_LNS_set_file] = 1
+; DWARF-DUMP-NEXT: standard_opcode_lengths[DW_LNS_set_column] = 1
+; DWARF-DUMP-NEXT: standard_opcode_lengths[DW_LNS_negate_stmt] = 0
+; DWARF-DUMP-NEXT: standard_opcode_lengths[DW_LNS_set_basic_block] = 0
+; DWARF-DUMP-NEXT: standard_opcode_lengths[DW_LNS_const_add_pc] = 0
+; DWARF-DUMP-NEXT: standard_opcode_lengths[DW_LNS_fixed_advance_pc] = 1
+; DWARF-DUMP-NEXT: standard_opcode_lengths[DW_LNS_set_prologue_end] = 0
+; DWARF-DUMP-NEXT: standard_opcode_lengths[DW_LNS_set_epilogue_begin] = 0
+; DWARF-DUMP-NEXT: standard_opcode_lengths[DW_LNS_set_isa] = 1
+; DWARF-DUMP-NEXT: include_directories[  0] = "."
+; DWARF-DUMP-NEXT: file_names[  0]:
+; DWARF-DUMP-NEXT:            name: "dwarf-riscv-relocs.c"
+; DWARF-DUMP-NEXT:       dir_index: 0
+; DWARF-DUMP-NEXT:    md5_checksum: 05ab89f5481bc9f2d037e7886641e919
+; DWARF-DUMP-NEXT:          source: "int main()\n{\n    return 0;\n}\n"
+; DWARF-DUMP-EMPTY:
+; DWARF-DUMP-NEXT: Address            Line   Column File   ISA Discriminator Flags
+; DWARF-DUMP-NEXT: ------------------ ------ ------ ------ --- ------------- -------------
+; DWARF-DUMP-NEXT: 0x0000000000000000      2      0      0   0             0  is_stmt
+; DWARF-DUMP-NEXT: 0x0000000000000014      3      5      0   0             0  is_stmt prologue_end
+; DWARF-DUMP-NEXT: 0x0000000000000028      3      5      0   0             0  is_stmt end_sequence
+
+; ModuleID = 'dwarf-riscv-relocs.c'
+source_filename = "dwarf-riscv-relocs.c"
+target datalayout = "e-m:e-p:32:32-i64:64-n32-S128"
+target triple = "riscv32"
+
+; Function Attrs: noinline nounwind optnone
+define dso_local i32 @main() #0 !dbg !7 {
+entry:
+  %retval = alloca i32, align 4
+  store i32 0, i32* %retval, align 4
+  ret i32 0, !dbg !11
+}
+
+attributes #0 = { noinline nounwind optnone "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-features"="+relax" "unsafe-fp-math"="false" "use-soft-float"="false" }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "dwarf-riscv-relocs.c", directory: ".", checksumkind: CSK_MD5, checksum: "05ab89f5481bc9f2d037e7886641e919", source: "int main()\0A{\0A    return 0;\0A}\0A")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 5}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang"}
+!7 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 1, type: !8, scopeLine: 2, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !2)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !DILocation(line: 3, column: 5, scope: !7)