[BOLT] Testcase to repro R_X86_64_REX_GOTPCRELX bug

[rafauler]

Summary

Add a new testcase that reproduces a bug when BOLTing current
trunk LLD bootstrapped with trunk clang. This makes it official
that we do not support this transformation but are working on
it. When the support is ready, XFAIL should be removed.

[Amir]

LGTM with one comment

bolt/test/X86/gotpcrelx.s

4

Can we also check the presence of R_X86_64_REX_GOTPCRELX relocation in the input binary?

[yota9]

LGTM
AFAIU this might be the first time where the skipRelocationProcess that I'm using for arm might be used for x86

[rafauler]

Thanks, I appreciate the suggestions. Adding check for GOTPCRELX.

[maksfb]

Thanks, Rafael, for adding the test case.

bolt/test/X86/gotpcrelx.s

+# This reproduces a bug with misinterpreting the gotpcrelx reloc
+
+# Here we use llvm-mc -relax-relocations to produce R_X86_64_REX_GOTPCRELX
+# and ld.lld to consume it and optimize it, transforming a CMP <mem, reg>
+# into CMP <imm, reg>.
+# Then we check that BOLT updates correctly the imm operand that references
+# a function address. Currently XFAIL as we do not support it.
+
+# REQUIRES: system-linux
+# XFAIL: *
+
+# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-linux \
+# RUN:   -relax-relocations %s -o %t.o
+# RUN: llvm-strip --strip-unneeded %t.o
+# RUN: ld.lld %t.o -o %t.exe -q
+# RUN: llvm-readobj -r %t.exe | FileCheck --check-prefix=READOBJ %s
+# RUN: llvm-bolt %t.exe -relocs -o %t.out -lite=0
+# RUN: llvm-objdump -d --no-show-raw-insn --print-imm-hex \
+# RUN:   %t.out | FileCheck --check-prefix=DISASM %s
+
+# Check that R_X86_64_REX_GOTPCRELX is present in the input binary
+# READOBJ: 0x[[#%X,]] R_X86_64_REX_GOTPCRELX foo 0x[[#%X,]]
+
+# DISASM:      Disassembly of section .text:
+# DISASM-EMPTY:
+# DISASM-NEXT: <_start>:
+# DISASM-NEXT:                 leaq  0x[[#%x,ADDR:]], %rax
+# DISASM-NEXT:                 cmpq  0x[[#ADDR]], %rax
+
+  .text
+  .globl _start
+  .type _start, %function
+_start:
+  .cfi_startproc
+  leaq foo, %rax
+  cmpq foo@GOTPCREL(%rip), %rax
+  je  b
+c:
+  mov $1, %rdi
+  callq foo
+b:
+  xorq %rdi, %rdi
+  callq foo
+  ret
+  .cfi_endproc
+  .size _start, .-_start
+
+  .globl foo
+  .type foo, %function
+foo:
+  .cfi_startproc
+  ret
+  .cfi_endproc
+  .size foo, .-foo