[BPF] Prevent disassembly segfault for NOP insn

[yonghong-song]

Summary

For a simple program like below:

-bash-4.4$ cat t.c 
int test() {
  asm volatile("r0 = r0" ::);
  return 0;
}

compiled with

clang -target bpf -O2 -c t.c

the following llvm-objdump command will segfault.

llvm-objdump -d t.o 

0:       bf 00 00 00 00 00 00 00 nop 
llvm-objdump: ../include/llvm/ADT/SmallVector.h:180
... 
Assertion `idx < size()' failed
... 
abort
... 
llvm::BPFInstPrinter::printOperand
llvm::BPFInstPrinter::printInstruction
...

The reason is both NOP and MOV_rr (r0 = r0) having the same encoding.
The disassembly getInstruction() decodes to be a NOP instruciton but
during printInstruction() the same encoding is interpreted as
a MOV_rr instruction. Such a mismatcch caused the segfault.

The fix is to make NOP instruction as CodeGen only so disassembler
will skip it.

Note that instruction "r0 = r0" should not appear in non inline
asm codes, we BPF Machine Instruction Peephole optimization will
remove it.

llvm/lib/Target/BPF/BPFInstrInfo.td

: TYPE_ALU_JMP<BPF_MOV.Value, BPF_X.Value,
                    !strconcat(OpcodeStr, "\t$imm"),
                    []> {
   // mov r0, r0 == nop
   let Inst{55-52} = 0;
   let Inst{51-48} = 0;
   let BPFClass = BPF_ALU64;
 }
 
-let hasSideEffects = 0 in
+let hasSideEffects = 0, isCodeGenOnly = 1 in
   def NOP : NOP_I<"nop">;
 
 class RET<string OpcodeStr>
     : TYPE_ALU_JMP<BPF_EXIT.Value, BPF_K.Value,
                    (outs),
                    (ins),
                    !strconcat(OpcodeStr, ""),
                    [(BPFretflag)]> {

llvm/test/CodeGen/BPF/objdump_nop.ll

+; RUN: llc -march=bpfel -filetype=obj -o - %s | llvm-objdump -d - | FileCheck %s
+;
+; Source:
+;   int test() {
+;     asm volatile("r0 = r0" ::);
+;     return 0;
+;   }
+; Compilation flag:
+;   clang -target bpf -O2 -S -emit-llvm t.c
+
+; Function Attrs: nounwind
+define dso_local i32 @test() local_unnamed_addr {
+entry:
+  tail call void asm sideeffect "r0 = r0", ""()
+  ret i32 0
+}
+; CHECK-LABEL: test
+; CHECK:       r0 = r0
+; CHECK:       r0 = 0