[all-commits] [llvm/llvm-project] ddff97: [BPF] Prevent disassembly segfault for NOP insn

[yonghong-song via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: ddff9799d2d0052653aa0385969b418a87bf5d7c
      https://github.com/llvm/llvm-project/commit/ddff9799d2d0052653aa0385969b418a87bf5d7c
  Author: Yonghong Song <yhs at fb.com>
  Date:   2020-05-18 (Mon, 18 May 2020)

  Changed paths:
    M llvm/lib/Target/BPF/BPFInstrInfo.td
    A llvm/test/CodeGen/BPF/objdump_nop.ll

  Log Message:
  -----------
  [BPF] Prevent disassembly segfault for NOP insn

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 NOP insn for disassembling.

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

Differential Revision: https://reviews.llvm.org/D80156