[X86][MC] Fix assembling rip-relative addressing + immediate displacements

[thegameg]

Summary

In the rare case where the input contains rip-relative addressing with immediate displacements, *and* the instruction ends with an immediate, we encode the instruction in the wrong way:

movl $12345678, 0x400(%rdi) // all good, no rip-relative addr
movl %eax, 0x400(%rip) // all good, no immediate at the end of the instruction
movl $12345678, 0x400(%rip) // fails, encodes address as 0x3fc(%rip)

• Offset is a label:

movl $12345678, foo(%rip)

we want to account for the size of the immediate (in this case, $12345678, 4 bytes) in the relocation value.

• Offset is an immediate:

movl $12345678, 0x400(%rip)

we should not account for the size of the immediate, assuming the immediate offset is what the user wanted.

[craig.topper]

LGTM

llvm/trunk/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp

unsigned FixupKind = [=]() {
       case X86::XOR64rm:
         return Rex ? X86::reloc_riprel_4byte_relax_rex
                    : X86::reloc_riprel_4byte_relax;
       }
     }();
 
     // rip-relative addressing is actually relative to the *next* instruction.
     // Since an immediate can follow the mod/rm byte for an instruction, this
-    // means that we need to bias the immediate field of the instruction with
-    // the size of the immediate field.  If we have this case, add it into the
+    // means that we need to bias the displacement field of the instruction with
+    // the size of the immediate field. If we have this case, add it into the
     // expression to emit.
-    int ImmSize = X86II::hasImm(TSFlags) ? X86II::getSizeOfImm(TSFlags) : 0;
+    // Note: rip-relative addressing using immediate displacement values should
+    // not be adjusted, assuming it was the user's intent.
+    int ImmSize = !Disp.isImm() && X86II::hasImm(TSFlags)
+                      ? X86II::getSizeOfImm(TSFlags)
+                      : 0;
 
     EmitImmediate(Disp, MI.getLoc(), 4, MCFixupKind(FixupKind),
                   CurByte, OS, Fixups, -ImmSize);
     return;
   }
 
   unsigned BaseRegNo = BaseReg ? GetX86RegNum(Base) : -1U;
 

llvm/trunk/test/MC/X86/x86-64.s

 // CHECK: encoding: [0x66,0xc7,0x05,A,A,A,A,0x0c,0x00]
 // CHECK:    fixup A - offset: 3, value: foo-6, kind: reloc_riprel_4byte
 
 movl  $12, foo(%rip)
 // CHECK: movl  $12, foo(%rip)
 // CHECK: encoding: [0xc7,0x05,A,A,A,A,0x0c,0x00,0x00,0x00]
 // CHECK:    fixup A - offset: 2, value: foo-8, kind: reloc_riprel_4byte
 
+// rdar://37247000
+movl  $12, 1024(%rip)
+// CHECK: movl  $12, 1024(%rip)
+// CHECK: encoding: [0xc7,0x05,0x00,0x04,0x00,0x00,0x0c,0x00,0x00,0x00]
+
 movq  $12, foo(%rip)
 // CHECK:  movq $12, foo(%rip)
 // CHECK: encoding: [0x48,0xc7,0x05,A,A,A,A,0x0c,0x00,0x00,0x00]
 // CHECK:    fixup A - offset: 3, value: foo-8, kind: reloc_riprel_4byte
 
 movl  foo(%eip), %eax
 // CHECK: movl  foo(%eip), %eax
 // CHECK: encoding: [0x67,0x8b,0x05,A,A,A,A]