[JITLink] Add R_RISCV_SUB6 relocation

[StephenFan]

Summary

Add R_RISCV_SUB6 relocation

[fourdim]

Hi, StephenFan, I have a question in the inline comment.
Would you be so kind to explain it to us?

llvm/lib/ExecutionEngine/JITLink/ELF_riscv.cpp

365

Can we safely assumed that, here, the Value is 6 bits?
What if (*FixupPtr & 0xc0) == 0b01000000 and Value == 0b11111111?

[jrtc27]

llvm/lib/ExecutionEngine/JITLink/ELF_riscv.cpp

363

You need to mask off the upper 2 bits of the value at the address

365

LLD will truncate, not sure if BFD will truncate or error

[StephenFan]

llvm/lib/ExecutionEngine/JITLink/ELF_riscv.cpp

365

You are right, I need to mask off the upper 2 bits of the value at the address and truncate the Value

[StephenFan]

mask off the upper 2 bits of the value at the address and truncate the Value

[fourdim]

Please update this file as well and clang format the codes.
llvm/lib/ExecutionEngine/JITLink/riscv.cpp

[StephenFan]

Address @fourdim 's comments

[fourdim]

LGTM

llvm/include/llvm/ExecutionEngine/JITLink/riscv.h

enum EdgeKind_riscv : Edge::Kind {
   R_RISCV_SUB16,
 
   /// 8 bits label subtraction
   ///
   /// Fixup expression
   ///   Fixup <- (Target - *{1}Fixup - Addend)
   R_RISCV_SUB8,
 
+  /// 6 bits label subtraction
+  ///
+  /// Fixup expression
+  ///   Fixup <- (Target - *{1}Fixup - Addend)
+  R_RISCV_SUB6,
+
   /// Local label assignment
   ///
   /// Fixup expression:
   ///   Fixup <- (Target + Addend)
   R_RISCV_SET6,
 
   /// Local label assignment
   ///

llvm/lib/ExecutionEngine/JITLink/ELF_riscv.cpp

Error applyFixup(LinkGraph &G, Block &B, const Edge &E) const {
     }
     case R_RISCV_SUB8: {
       int64_t Value =
           *(reinterpret_cast<const uint8_t *>(FixupAddress.getValue())) -
           E.getTarget().getAddress().getValue() - E.getAddend();
       *FixupPtr = static_cast<uint8_t>(Value);
       break;
     }
+    case R_RISCV_SUB6: {
+      int64_t Value =
+          *(reinterpret_cast<const uint8_t *>(FixupAddress.getValue())) & 0x3f -
+          E.getTarget().getAddress().getValue() - E.getAddend();
+      *FixupPtr = (*FixupPtr & 0xc0) | (static_cast<uint8_t>(Value) & 0x3f);
+      break;
+    }
     case R_RISCV_SET6: {
       int64_t Value = (E.getTarget().getAddress() + E.getAddend()).getValue();
       uint32_t RawData = *(little32_t *)FixupPtr;
       int64_t Word6 = Value & 0x3f;
       *(little32_t *)FixupPtr = (RawData & 0xffffffc0) | Word6;
       break;
     }
     case R_RISCV_SET8: {
getRelocationKind(const uint32_t Type) {
     case ELF::R_RISCV_SUB64:
       return EdgeKind_riscv::R_RISCV_SUB64;
     case ELF::R_RISCV_SUB32:
       return EdgeKind_riscv::R_RISCV_SUB32;
     case ELF::R_RISCV_SUB16:
       return EdgeKind_riscv::R_RISCV_SUB16;
     case ELF::R_RISCV_SUB8:
       return EdgeKind_riscv::R_RISCV_SUB8;
+    case ELF::R_RISCV_SUB6:
+      return EdgeKind_riscv::R_RISCV_SUB6;
     case ELF::R_RISCV_SET6:
       return EdgeKind_riscv::R_RISCV_SET6;
     case ELF::R_RISCV_SET8:
       return EdgeKind_riscv::R_RISCV_SET8;
     case ELF::R_RISCV_SET16:
       return EdgeKind_riscv::R_RISCV_SET16;
     case ELF::R_RISCV_SET32:
       return EdgeKind_riscv::R_RISCV_SET32;

llvm/test/ExecutionEngine/JITLink/RISCV/riscv_reloc_add.s

 # RUN: rm -rf %t && mkdir -p %t
 # RUN: llvm-mc -triple=riscv64 -filetype=obj -o %t/riscv64_reloc_add.o %s
 # RUN: llvm-mc -triple=riscv32 -filetype=obj -o %t/riscv32_reloc_add.o %s
 # RUN: llvm-jitlink -noexec -check %s %t/riscv64_reloc_add.o
 # RUN: llvm-jitlink -noexec -check %s %t/riscv32_reloc_add.o
 
 # jitlink-check: *{8}(named_data) = 0x8
 # jitlink-check: *{4}(named_data+8) = 0x8
 # jitlink-check: *{2}(named_data+12) = 0x8
 # jitlink-check: *{1}(named_data+14) = 0x8
+# jitlink-check: *{1}(named_data+15) = 0x8
 
 .global main
 main:
 .L0:
 # Referencing named_data symbol to avoid the following .rodata section be skipped.
 # This instruction will be expand to two instructions (auipc + ld).
   lw a0, named_data
 .L1:
 
 .section ".rodata","",@progbits
 .type named_data,@object
 named_data:
+.reloc named_data+15, R_RISCV_SUB6, .L0
 .dword .L1 - .L0
 .word .L1 - .L0
 .half .L1 - .L0
 .byte .L1 - .L0
-.size named_data, 15
+.byte 0x8
+.size named_data, 16