[ELF][RISCV] Treat R_RISCV_{ADD,SET,SUB}* as link-time constants

[MaskRay]

Summary

R_RISCV_{ADD,SET,SUB}* are expected to be used as local label
computation. Add a new RelExpr R_RISCV_ADD to represent them.

R_RISCV_ADD is treated as a link-time constant because otherwise
R_RISCV_{ADD,SET,SUB}* are not allowed in a non-writable section in
-pie/-shared mode.

In glibc Scrt1.o, .rela.eh_frame contains such relocations. With D63076
and this patch, I can run -pie/-shared programs linked against glibc.

Note llvm-mc cannot currently produce R_RISCV_SET* so they are not
tested.

[ruiu]

LGTM

lld/trunk/ELF/Arch/RISCV.cpp

uint32_t RISCV::calcEFlags() const {
   }
 
   return Target;
 }
 
 RelExpr RISCV::getRelExpr(const RelType Type, const Symbol &S,
                           const uint8_t *Loc) const {
   switch (Type) {
+  case R_RISCV_ADD8:
+  case R_RISCV_ADD16:
+  case R_RISCV_ADD32:
+  case R_RISCV_ADD64:
+  case R_RISCV_SET6:
+  case R_RISCV_SET8:
+  case R_RISCV_SET16:
+  case R_RISCV_SET32:
+  case R_RISCV_SUB6:
+  case R_RISCV_SUB8:
+  case R_RISCV_SUB16:
+  case R_RISCV_SUB32:
+  case R_RISCV_SUB64:
+    return R_RISCV_ADD;
   case R_RISCV_JAL:
   case R_RISCV_BRANCH:
   case R_RISCV_CALL:
   case R_RISCV_PCREL_HI20:
   case R_RISCV_RVC_BRANCH:
   case R_RISCV_RVC_JUMP:
   case R_RISCV_32_PCREL:
     return R_PC;

lld/trunk/ELF/InputSection.cpp

 
 static uint64_t getRelocTargetVA(const InputFile *File, RelType Type, int64_t A,
                                  uint64_t P, const Symbol &Sym, RelExpr Expr) {
   switch (Expr) {
   case R_ABS:
   case R_DTPREL:
   case R_RELAX_TLS_LD_TO_LE_ABS:
   case R_RELAX_GOT_PC_NOPIC:
+  case R_RISCV_ADD:
     return Sym.getVA(A);
   case R_ADDEND:
     return A;
   case R_ARM_SBREL:
     return Sym.getVA(A) - getARMStaticBase(Sym);
   case R_GOT:
   case R_RELAX_TLS_GD_TO_IE_ABS:
     return Sym.getGotVA() + A;

lld/trunk/ELF/Relocations.h

enum RelExpr {
   R_MIPS_GOT_OFF32,
   R_MIPS_TLSGD,
   R_MIPS_TLSLD,
   R_PPC32_PLTREL,
   R_PPC64_CALL,
   R_PPC64_CALL_PLT,
   R_PPC64_RELAX_TOC,
   R_PPC64_TOCBASE,
+  R_RISCV_ADD,
   R_RISCV_PC_INDIRECT,
 };
 
 // Architecture-neutral representation of relocation.
 struct Relocation {
   RelExpr Expr;
   RelType Type;
   uint64_t Offset;

lld/trunk/ELF/Relocations.cpp

 static bool isStaticLinkTimeConstant(RelExpr E, RelType Type, const Symbol &Sym,
                                      InputSectionBase &S, uint64_t RelOff) {
   // These expressions always compute a constant
   if (oneof<R_DTPREL, R_GOTPLT, R_GOT_OFF, R_HEXAGON_GOT, R_TLSLD_GOT_OFF,
             R_MIPS_GOT_LOCAL_PAGE, R_MIPS_GOTREL, R_MIPS_GOT_OFF,
             R_MIPS_GOT_OFF32, R_MIPS_GOT_GP_PC, R_MIPS_TLSGD,
             R_AARCH64_GOT_PAGE_PC, R_GOT_PC, R_GOTONLY_PC, R_GOTPLTONLY_PC,
             R_PLT_PC, R_TLSGD_GOT, R_TLSGD_GOTPLT, R_TLSGD_PC, R_PPC32_PLTREL,
-            R_PPC64_CALL_PLT, R_PPC64_RELAX_TOC, R_TLSDESC_CALL, R_TLSDESC_PC,
-            R_AARCH64_TLSDESC_PAGE, R_HINT, R_TLSLD_HINT, R_TLSIE_HINT>(E))
+            R_PPC64_CALL_PLT, R_PPC64_RELAX_TOC, R_RISCV_ADD, R_TLSDESC_CALL,
+            R_TLSDESC_PC, R_AARCH64_TLSDESC_PAGE, R_HINT, R_TLSLD_HINT,
+            R_TLSIE_HINT>(E))
     return true;
 
   // These never do, except if the entire file is position dependent or if
   // only the low bits are used.
   if (E == R_GOT || E == R_PLT || E == R_TLSDESC)
     return Target->usesOnlyLowPageBits(Type) || !Config->Pic;
 
   if (Sym.IsPreemptible)

lld/trunk/test/ELF/riscv-reloc-add.s

+# REQUIRES: riscv
+# RUN: llvm-mc -filetype=obj -triple=riscv32 -mattr=+relax %s -o %t.32.o
+# RUN: ld.lld -pie %t.32.o -o %t.32
+# RUN: llvm-readelf -x .rodata %t.32 | FileCheck --check-prefix=HEX %s
+
+# RUN: llvm-mc -filetype=obj -triple=riscv64 -mattr=+relax %s -o %t.64.o
+# RUN: ld.lld -shared %t.64.o -o %t.64
+# RUN: llvm-readelf -x .rodata %t.64 | FileCheck --check-prefix=HEX %s
+
+# HEX:      section '.rodata':
+# HEX-NEXT: 0x{{[0-9a-f]+}} 04000000 00000000 04000000 040004
+
+## R_RISCV_ADD* and R_RISCV_SUB* are link-time constants, otherwise they are
+## not allowed in -pie/-shared mode.
+
+.global _start
+_start:
+.L0:
+  ret
+.L1:
+
+.rodata
+.dword .L1 - .L0
+.word .L1 - .L0
+.half .L1 - .L0
+.byte .L1 - .L0