Problem disassembling fldt instruction

[lukemaurer]

(This came up in Refurbish, but it's almost certainly in Flexdis.)

The musl implementation of floating-point formatting has the instructions

  40103e:       db ac 24 90 1d 00 00    fldt   0x1d90(%rsp)
  401045:       48 8d 05 f8 33 00 00    lea    0x33f8(%rip),%rax        # 404444 <_fini+0xa0>

Refurbish rewrites them as

  40103e:       db 2c 24                fldt   (%rsp)
  401041:       90                      nop
  401042:       1d 00 00 48 8d          sbb    $0x8d480000,%eax
  401047:       05 f8 33 00 00          add    $0x33f8,%eax

What seems to be happening is that the ac in fldt gets misread as 2c, which shortens the instruction, causing the next few instructions to get mangled.

You can test this by adding a line to test-fp.c in the Refurbish tests:

int main() {
  entry();
  printf("%.9f", -0.169075164); /* new line */
  return 0;
}

This will cause a segfault when someone tries to access the nonsense pointer.

[travitch]

Confirmed as a problem in either the disassembler or re-assembler. I have a test case in the roundtrip tests that exhibit the problem (not checked in yet since I don't have a fix yet)