[ELF] - Use SignExtend when reading R_386_PC8, R_386_PC16 addends.

[grimar]

Summary

Previously we did not do that. For example, for R_386_PC8,
0xFF addend was not treated as 0xFFFFFFFF(-1),
but was 0x000000FF.

Recently added checks for R_386_PC8/R_386_PC16 failed because of calculation
overflow as a result.

[grimar]

• Addressed comments, simplified testcase.

[ruiu]

This does the same thing as https://reviews.llvm.org/D29493. Please merge the two patches.

[grimar]

• Merged with D29493 (R_386_PC16 part)
• Addressed review comments.

[grimar]

• Addressed review comments.

[ruiu]

Don't you need to sign-extend not only these two but all operands that have signed values, do you?

ELF/Target.cpp

 }
 
 uint64_t X86TargetInfo::getImplicitAddend(const uint8_t *Buf,
                                           uint32_t Type) const {
   switch (Type) {
   default:
     return 0;
   case R_386_8:
-  case R_386_PC8:
     return *Buf;
+  case R_386_PC8:
+    return SignExtend64<8>(*Buf);
   case R_386_16:
-  case R_386_PC16:
     return read16le(Buf);
+  case R_386_PC16:
+    return SignExtend64<16>(read16le(Buf));
   case R_386_32:
   case R_386_GOT32:
   case R_386_GOT32X:
   case R_386_GOTOFF:
   case R_386_GOTPC:
   case R_386_PC32:
   case R_386_PLT32:
   case R_386_TLS_LE:

test/ELF/i386-pc8-pc16-addend.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=i386-pc-linux-gnu %s -o %t1.o
+# RUN: llvm-objdump -d %t1.o | FileCheck %s --check-prefix=OBJDISASM
+
+## Show that we have R_386_PC8 relocation with addend = 0xFF(-1),
+## and R_386_PC16 relocation with addend = 0xFEFF(-2)
+## we do sign extend for addends values in the code.
+# OBJDISASM:      Disassembly of section .text:
+# OBJDISASM-NEXT: .text:
+# OBJDISASM-NEXT:  0: ff fe
+# OBJDISASM-NEXT:  2: ff
+
+## Check that there is no link error and relocation applied correctly.
+# RUN: ld.lld %t1.o -o %t.out
+# RUN: llvm-objdump -d %t.out | FileCheck %s --check-prefix=DISASM
+# RUN: llvm-readobj -symbols %t.out | FileCheck %s --check-prefix=SYM
+# DISASM:      Disassembly of section .text:
+# DISASM-NEXT: .text:
+# DISASM-NEXT:   11000: 02 00
+# DISASM-NEXT:   11002: 00
+## 0x11003 - 0x11000 + addend(-1) = 0x02
+## 0x11003 - 0x11001 + addend(-2) = 0x0000
+# SYM:     Symbol {
+# SYM:       Name: und
+# SYM-NEXT:  Value: 0x11003
+
+.code16
+.byte und-.-1
+.word und-.-2
+
+.section .und, "ax"
+und: