[RISCV] Fix builtin fixup sizes (alternate approach)

[asb]

Summary

This is a different approach to fixing the problem described in D46746. RISCVAsmBackend currently depends on the getSize helper function returning the number of bytes a fixup may change (note: some other backends have a similar helper named getFixupNumKindBytes). As noted in that review, this doesn't return the correct size for FK_Data_1, FK_Data_2, or FK_Data_8 meaning that too few bytes will be written in the case of FK_Data_8, and there's the potential of writing outside the Data array for the smaller fixups.

D46746 extends getSize to recognise some of the builtin fixup types. Rather than having a function that needs to be kept up to date as new builtin or target-specific fixups are added, We can calculate an appropriate bound on the number of bytes that might be touched using Info.TargetSize and Info.TargetOffset.

This seems trivial, but posting for review in case there's something obvious I'm missing...

[shiva0217]

LGTM.

[simoncook]

This LGTM.

llvm/trunk/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp

case RISCV::fixup_riscv_rvc_branch: {
     Value = (Bit8 << 12) | (Bit4_3 << 10) | (Bit7_6 << 5) | (Bit2_1 << 3) |
             (Bit5 << 2);
     return Value;
   }
 
   }
 }
 
-static unsigned getSize(unsigned Kind) {
-  switch (Kind) {
-  default:
-    return 4;
-  case RISCV::fixup_riscv_rvc_jump:
-  case RISCV::fixup_riscv_rvc_branch:
-    return 2;
-  }
-}
-
 void RISCVAsmBackend::applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
                                  const MCValue &Target,
                                  MutableArrayRef<char> Data, uint64_t Value,
                                  bool IsResolved) const {
   MCContext &Ctx = Asm.getContext();
   MCFixupKindInfo Info = getFixupKindInfo(Fixup.getKind());
   if (!Value)
     return; // Doesn't change encoding.
   // Apply any target-specific value adjustments.
   Value = adjustFixupValue(Fixup, Value, Ctx);
 
   // Shift the value into position.
   Value <<= Info.TargetOffset;
 
   unsigned Offset = Fixup.getOffset();
-  unsigned FullSize = getSize(Fixup.getKind());
+  unsigned NumBytes = alignTo(Info.TargetSize + Info.TargetOffset, 8) / 8;
 
-#ifndef NDEBUG
-  unsigned NumBytes = (Info.TargetSize + 7) / 8;
   assert(Offset + NumBytes <= Data.size() && "Invalid fixup offset!");
-#endif
 
   // For each byte of the fragment that the fixup touches, mask in the
   // bits from the fixup value.
-  for (unsigned i = 0; i != FullSize; ++i) {
+  for (unsigned i = 0; i != NumBytes; ++i) {
     Data[Offset + i] |= uint8_t((Value >> (i * 8)) & 0xff);
   }
 }
 
 std::unique_ptr<MCObjectTargetWriter>
 RISCVAsmBackend::createObjectTargetWriter() const {
   return createRISCVELFObjectWriter(OSABI, Is64Bit);
 }