Tests for object writing can be written in one of two forms. The first consists of using llvm assembly files which are then compiled with llc and the file examined by elf-dump or native instructions are interpreted by llvm-mc and again examined by elf-dump.

elf-dump is a command that is part of the LLVM test infrastructure and outputs the instructions and relocations in an object file for the purposes of testing that instruction encoding, sections, relocations and other ELF-based functionality work correctly.

To provide a simple demonstration of the tests that can be written, the following has been taken from the tests for the x86_64 Linux target. The test consists of two instructions which when encoded should have relocations associated with them.

The CHECK lines of the test look for the section where the encoded instructions will appear and then check that the section's location, flags, etc. are correct. Finally it checks the relocations on the two instructions and ensures that their types, values and addends are correct.

If needed, only the relocation sections can be checked. This is useful where section address may not be known but the relocation symbol identifiers and types are still known.

// RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o - | \
// RUN: elf-dump --dump-section-data | FileCheck  %s

// Test that we produce the correct relocation.

  loope 0                 # R_X86_64_PC8
  jmp -256                # R_X86_64_PC32

// CHECK:      # Section 2
// CHECK-NEXT: (('sh_name', 0x00000001) # '.rela.text'
// CHECK-NEXT:  ('sh_type', 0x00000004)
// CHECK-NEXT:  ('sh_flags', 0x0000000000000000)
// CHECK-NEXT:  ('sh_addr', 0x0000000000000000)
// CHECK-NEXT:  ('sh_offset', 0x00000000000002e8)
// CHECK-NEXT:  ('sh_size', 0x0000000000000030)
// CHECK-NEXT:  ('sh_link', 0x00000006)
// CHECK-NEXT:  ('sh_info', 0x00000001)
// CHECK-NEXT:  ('sh_addralign', 0x0000000000000008)
// CHECK-NEXT:  ('sh_entsize', 0x0000000000000018)
// CHECK-NEXT:  ('_relocations', [
// CHECK-NEXT:   # Relocation 0
// CHECK-NEXT:   (('r_offset', 0x0000000000000001)
// CHECK-NEXT:    ('r_sym', 0x00000000)
// CHECK-NEXT:    ('r_type', 0x0000000f)
// CHECK-NEXT:    ('r_addend', 0x0000000000000000)
// CHECK-NEXT:   ),
// CHECK-NEXT:   # Relocation 1
// CHECK-NEXT:   (('r_offset', 0x0000000000000003)
// CHECK-NEXT:    ('r_sym', 0x00000000)
// CHECK-NEXT:    ('r_type', 0x00000002)
// CHECK-NEXT:    ('r_addend', 0x0000000000000000)
// CHECK-NEXT:   ),
// CHECK-NEXT:  ])
// CHECK-NEXT: ),