Relax GOTPCREL relocations for tail jmp instructions

[tmsriram]

Summary

R_X86_64_GOTPCREL is not relaxed if opcode is a 64-bit tail jmp.

foo.c
extern int foo();

int main() {

return foo();

}

$ clang -O2 -fno-plt -mrelax-relocations=yes foo.cc -c
$ objdump -dr foo.o
0000000000000000 <main>:

   0:	ff 25 00 00 00 00    	jmpq   *0x0(%rip)        # 6 <main+0x6>
			2: R_X86_64_GOTPCREL	foo-0x4

Adding --save-temps to the above command-line will generate R_X86_64_GOTPCRELX

This is because in the former case, opcode is JMP64m but in the latter case it is a TAILJMPm64.

[rnk]

lgtm

lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp

unsigned FixupKind = [=]() {
       switch (Opcode) {
       default:
         return X86::reloc_riprel_4byte;
       case X86::MOV64rm:
         assert(Rex);
         return X86::reloc_riprel_4byte_movq_load;
       case X86::CALL64m:
       case X86::JMP64m:
+      case X86::TAILJMPm64:
       case X86::TEST64mr:
       case X86::ADC64rm:
       case X86::ADD64rm:
       case X86::AND64rm:
       case X86::CMP64rm:
       case X86::OR64rm:
       case X86::SBB64rm:
       case X86::SUB64rm:

test/CodeGen/X86/tailjmp_gotpcrel_relax_relocation.ll

+; RUN: llc -filetype=obj -relax-elf-relocations=true -mtriple=x86_64-linux-gnu -o - %s |  llvm-objdump - -d -r | FileCheck %s
+
+; CHECK: jmpq *(%rip)
+; CHECK-NEXT: R_X86_64_GOTPCRELX
+
+define i32 @main() {
+entry:
+  %call = tail call i32 @foo()
+  ret i32 %call
+}
+
+; Function Attrs: nonlazybind
+declare i32 @foo() #1
+
+attributes #1 = { nonlazybind }