[LoongArch] Emit R_LARCH_64_PCREL relocation for FK_Data_8 when IsPCRel is true

[xen0n]

Straightforward enough!

[MaskRay]

Some nits about the test

llvm/test/MC/LoongArch/Relocations/sub-expr.s

8

Better to make sx and sy SHF_ALLOC by using "a"

13

Changing these from inline comments to non-inline comments should be cleaner.

Add -NEXT and surround them with a pair of CHECK: .rela .data { and CHECK-NEXT: } to test that there is no additional relocation.

[SixWeining]

Address @MaskRay's comments by polishing the test.

llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchELFObjectWriter.cpp

case FK_Data_1:
     Ctx.reportError(Fixup.getLoc(), "1-byte data relocations not supported");
     return ELF::R_LARCH_NONE;
   case FK_Data_2:
     Ctx.reportError(Fixup.getLoc(), "2-byte data relocations not supported");
     return ELF::R_LARCH_NONE;
   case FK_Data_4:
     return IsPCRel ? ELF::R_LARCH_32_PCREL : ELF::R_LARCH_32;
   case FK_Data_8:
-    return ELF::R_LARCH_64;
+    return IsPCRel ? ELF::R_LARCH_64_PCREL : ELF::R_LARCH_64;
   case LoongArch::fixup_loongarch_b16:
     return ELF::R_LARCH_B16;
   case LoongArch::fixup_loongarch_b21:
     return ELF::R_LARCH_B21;
   case LoongArch::fixup_loongarch_b26:
     return ELF::R_LARCH_B26;
   case LoongArch::fixup_loongarch_abs_hi20:
     return ELF::R_LARCH_ABS_HI20;

llvm/test/MC/LoongArch/Relocations/sub-expr.s

+# RUN: llvm-mc --filetype=obj --triple=loongarch64 %s -o %t
+# RUN: llvm-readobj -r %t | FileCheck %s
+
+## Check that subtraction expressions emit R_LARCH_32_PCREL and R_LARCH_64_PCREL relocations.
+
+## TODO: 1- or 2-byte data relocations are not supported for now.
+
+.section sx
+x:
+nop
+
+.data
+.8byte x-.   # CHECK: 0x0 R_LARCH_64_PCREL sx 0x0
+.8byte y-.   # CHECK: 0x8 R_LARCH_64_PCREL sy 0x0
+.4byte x-.   # CHECK: 0x10 R_LARCH_32_PCREL sx 0x0
+.4byte y-.   # CHECK: 0x14 R_LARCH_32_PCREL sy 0x0
+
+.section sy
+y: