[MC] Support relocation offset of sym+constant form for PendingFixup

[StephenFan]

Summary

This patch add support relocation offset of sym+constant(like foo+4) form for pending fixup.

In the past, llvm-mc ignored the constant in sym+constant form, for foo+4, 4 would be ignored. And test case

.text
  ret
  nop
  nop
  .reloc foo+4, R_RISCV_32, 6

.data
.globl foo
foo:
  .word 0
  .word 0
  .word 0

when run llvm-mc -filetype=obj -triple=riscv64 %s | llvm-readobj -r
The output is

Relocations [
  Section (3) .rela.text {
    0x0 R_RISCV_32 - 0x6
  }
]

After applying this patch, the output is

Relocations [
  Section (3) .rela.text {
    0x4 R_RISCV_32 - 0x6
  }
]

[luismarques]

LGTM

llvm/lib/MC/MCObjectStreamer.cpp

 void MCObjectStreamer::resolvePendingFixups() {
   for (PendingMCFixup &PendingFixup : PendingFixups) {
     if (!PendingFixup.Sym || PendingFixup.Sym->isUndefined ()) {
       getContext().reportError(PendingFixup.Fixup.getLoc(),
                                "unresolved relocation offset");
       continue;
     }
     flushPendingLabels(PendingFixup.DF, PendingFixup.DF->getContents().size());
-    PendingFixup.Fixup.setOffset(PendingFixup.Sym->getOffset());
+    PendingFixup.Fixup.setOffset(PendingFixup.Sym->getOffset() +
+                                 PendingFixup.Fixup.getOffset());
     PendingFixup.DF->getFixups().push_back(PendingFixup.Fixup);
   }
   PendingFixups.clear();
 }
 
 // As a compile-time optimization, avoid allocating and evaluating an MCExpr
 // tree for (Hi - Lo) when Hi and Lo are offsets into the same fragment.
 static Optional<uint64_t> absoluteSymbolDiff(const MCSymbol *Hi,
if (Error != None)
       return Error;
 
     DF->getFixups().push_back(
         MCFixup::create(SymbolOffset + OffsetVal.getConstant(),
                         Expr, Kind, Loc));
     return None;
   }
 
-  PendingFixups.emplace_back(&SRE.getSymbol(), DF,
-                             MCFixup::create(-1, Expr, Kind, Loc));
+  PendingFixups.emplace_back(
+      &SRE.getSymbol(), DF,
+      MCFixup::create(OffsetVal.getConstant(), Expr, Kind, Loc));
   return None;
 }
 
 void MCObjectStreamer::emitFill(const MCExpr &NumBytes, uint64_t FillValue,
                                 SMLoc Loc) {
   MCDataFragment *DF = getOrCreateDataFragment();
   flushPendingLabels(DF, DF->getContents().size());
 

llvm/test/MC/RISCV/reloc-directive.s

 # CHECK-NEXT: 0x4 R_RISCV_NONE foo 0x4
 # CHECK-NEXT: 0x0 R_RISCV_NONE - 0x8
 # CHECK-NEXT: 0x0 R_RISCV_32 .data 0x2
 # CHECK-NEXT: 0x0 R_RISCV_SET32 foo 0x3
 # CHECK-NEXT: 0x0 R_RISCV_32_PCREL - 0x5
 # CHECK-NEXT: 0x0 R_RISCV_NONE - 0x9
 # CHECK-NEXT: 0x0 R_RISCV_32 - 0x9
 # CHECK-NEXT: 0x0 R_RISCV_64 - 0x9
+# CHECK-NEXT: 0x4 R_RISCV_32 - 0x6
 .text
   ret
   nop
   nop
   .reloc 8, R_RISCV_NONE, .data
   .reloc 4, R_RISCV_NONE, foo+4
   .reloc 0, R_RISCV_NONE, 8
 
   .reloc 0, R_RISCV_32, .data+2
   .reloc 0, R_RISCV_SET32, foo+3
   .reloc 0, R_RISCV_32_PCREL, 5
 
   .reloc 0, BFD_RELOC_NONE, 9
   .reloc 0, BFD_RELOC_32, 9
   .reloc 0, BFD_RELOC_64, 9
 
+  .reloc foo+4, R_RISCV_32, 6
+
 .data
 .globl foo
 foo:
   .word 0
   .word 0
   .word 0