[llvm-objdump][RISCV] Display `@plt' symbols when disassembling .plt section (#147933)

This patch adds dummy symbols for PLT entries for RISC-V 32-bit and
64-bit targets so llvm-objdump can show the function symbol that
corresponds to each PLT entry.

llvm/lib/Object/ELFObjectFile.cpp

@@ -810,6 +810,10 @@ ELFObjectFileBase::getPltEntries(const MCSubtargetInfo &STI) const {
       JumpSlotReloc = ELF::R_HEX_JMP_SLOT;
       GlobDatReloc = ELF::R_HEX_GLOB_DAT;
       break;
+    case Triple::riscv32:
+    case Triple::riscv64:
+      JumpSlotReloc = ELF::R_RISCV_JUMP_SLOT;
+      break;
     default:
       return {};
   }

llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCTargetDesc.cpp

@@ -30,6 +30,7 @@
 #include "llvm/MC/TargetRegistry.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/MathExtras.h"
 #include <bitset>
 
 #define GET_INSTRINFO_MC_DESC
@@ -305,6 +306,47 @@ public:
     }
   }
 
+  /// Returns (PLT virtual address, GOT virtual address) pairs for PLT entries.
+  std::vector<std::pair<uint64_t, uint64_t>>
+  findPltEntries(uint64_t PltSectionVA, ArrayRef<uint8_t> PltContents,
+                 const MCSubtargetInfo &STI) const override {
+    uint32_t LoadInsnOpCode;
+    if (const Triple &T = STI.getTargetTriple(); T.isRISCV64())
+      LoadInsnOpCode = 0x3003; // ld
+    else if (T.isRISCV32())
+      LoadInsnOpCode = 0x2003; // lw
+    else
+      return {};
+
+    constexpr uint64_t FirstEntryAt = 32, EntrySize = 16;
+    if (PltContents.size() < FirstEntryAt + EntrySize)
+      return {};
+
+    std::vector<std::pair<uint64_t, uint64_t>> Results;
+    for (uint64_t EntryStart = FirstEntryAt,
+                  EntryStartEnd = PltContents.size() - EntrySize;
+         EntryStart <= EntryStartEnd; EntryStart += EntrySize) {
+      const uint32_t AuipcInsn =
+          support::endian::read32le(PltContents.data() + EntryStart);
+      const bool IsAuipc = (AuipcInsn & 0x7F) == 0x17;
+      if (!IsAuipc)
+        continue;
+
+      const uint32_t LoadInsn =
+          support::endian::read32le(PltContents.data() + EntryStart + 4);
+      const bool IsLoad = (LoadInsn & 0x707F) == LoadInsnOpCode;
+      if (!IsLoad)
+        continue;
+
+      const uint64_t GotPltSlotVA = PltSectionVA + EntryStart +
+                                    (AuipcInsn & 0xFFFFF000) +
+                                    SignExtend64<12>(LoadInsn >> 20);
+      Results.emplace_back(PltSectionVA + EntryStart, GotPltSlotVA);
+    }
+
+    return Results;
+  }
+
 private:
   static bool maybeReturnAddress(MCRegister Reg) {
     // X1 is used for normal returns, X5 for returns from outlined functions.

llvm/test/tools/llvm-objdump/ELF/RISCV/rv32-plt.test

@@ -0,0 +1,44 @@
+# RUN: yaml2obj %s | llvm-objdump -d --no-show-raw-insn - | FileCheck %s
+
+# CHECK:      00001470 <strcmp@plt>:
+# CHECK-NEXT:     1470:       auipc   t3, 0x2
+# CHECK-NEXT:     1474:       lw      t3, 0xfc(t3)
+# CHECK-NEXT:     1478:       jalr    t1, t3
+# CHECK-NEXT:     147c:       nop
+
+--- !ELF
+FileHeader:
+  Class:           ELFCLASS32
+  Data:            ELFDATA2LSB
+  Type:            ET_DYN
+  Machine:         EM_RISCV
+  Flags:           [ EF_RISCV_RVC, EF_RISCV_FLOAT_ABI_DOUBLE ]
+Sections:
+  - Name:            .rela.plt
+    Type:            SHT_RELA
+    Flags:           [ SHF_ALLOC, SHF_INFO_LINK ]
+    Address:         0x290
+    Link:            .dynsym
+    AddressAlign:    0x4
+    Info:            .got.plt
+    Relocations:
+      - Offset:          0x356C
+        Symbol:          strcmp
+        Type:            R_RISCV_JUMP_SLOT
+  - Name:            .plt
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    Address:         0x1450
+    AddressAlign:    0x10
+    Content:         972300003303C34103AE4311130343FD938243111353230083A2420067000E00172E0000032ECE0F67030E0013000000
+  - Name:            .got.plt
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    Address:         0x3564
+    AddressAlign:    0x4
+    Content:         '000000000000000050140000'
+DynamicSymbols:
+  - Name:            strcmp
+    Type:            STT_FUNC
+    Binding:         STB_GLOBAL
+...

llvm/test/tools/llvm-objdump/ELF/RISCV/rv64-plt.test

@@ -0,0 +1,44 @@
+# RUN: yaml2obj %s | llvm-objdump -d --no-show-raw-insn - | FileCheck %s
+
+# CHECK:      0000000000001630 <strcmp@plt>:
+# CHECK-NEXT:     1630:       auipc   t3, 0x2
+# CHECK-NEXT:     1634:       ld      t3, 0x1b8(t3)
+# CHECK-NEXT:     1638:       jalr    t1, t3
+# CHECK-NEXT:     163c:       nop
+
+--- !ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_DYN
+  Machine:         EM_RISCV
+  Flags:           [ EF_RISCV_RVC, EF_RISCV_FLOAT_ABI_DOUBLE ]
+Sections:
+  - Name:            .rela.plt
+    Type:            SHT_RELA
+    Flags:           [ SHF_ALLOC, SHF_INFO_LINK ]
+    Address:         0x408
+    Link:            .dynsym
+    AddressAlign:    0x8
+    Info:            .got.plt
+    Relocations:
+      - Offset:          0x37E8
+        Symbol:          strcmp
+        Type:            R_RISCV_JUMP_SLOT
+  - Name:            .plt
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    Address:         0x1610
+    AddressAlign:    0x10
+    Content:         972300003303C34103BE831C130343FD9382831C1353130083B2820067000E00172E0000033E8E1B67030E0013000000
+  - Name:            .got.plt
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    Address:         0x37D8
+    AddressAlign:    0x8
+    Content:         '000000000000000000000000000000001016000000000000'
+DynamicSymbols:
+  - Name:            strcmp
+    Type:            STT_FUNC
+    Binding:         STB_GLOBAL
+...