[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_32:
   case R_386_GOT32:
   case R_386_GOT32X:
   case R_386_GOTOFF:
   case R_386_GOTPC:

test/ELF/i386-pc8-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
+# RUN: llvm-readobj -r %t1.o | FileCheck %s --check-prefix=OBJRELOC
+
+## Show that we have R_386_PC8 relocation with addend = 0xFF(-1).
+# OBJDISASM:      Disassembly of section .text:
+# OBJDISASM-NEXT: .text:
+# OBJDISASM-NEXT:  0: eb ff jmp -1 <.text+0x1>
+# OBJRELOC:       Relocations [
+# OBJRELOC-NEXT:   Section {{.*}} .rel.text {
+# OBJRELOC-NEXT:     0x1 R_386_PC8 .foo 0x0
+# OBJRELOC-NEXT:   }
+# OBJRELOC-NEXT: ]
+
+## 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
+# DISASM:      Disassembly of section .text:
+# DISASM-NEXT: .text:
+# DISASM-NEXT:    11000: eb 04 jmp 4 <foo>
+# DISASM:      Disassembly of section .foo:
+# DISASM-NEXT: foo:
+# DISASM-NEXT:    11006: 90 nop
+
+# CHECK:      Contents of section .text:
+# CHECK-NEXT:  0000 15253748
+
+.code16
+
+.byte  0xeb   # short (2-byte) jump
+.byte  foo-1f
+
+1:
+nop
+nop
+nop
+nop
+
+.section ".foo", "ax"
+foo:
+ nop