[Bug 1925512] Re: UNDEFINED case for instruction BLX

[JIANG Muhui]

Hi

I still feel QEMU's implementation is not right. Could you please check
it again.

According to https://developer.arm.com/documentation/ddi0406/c
/Application-Level-Architecture/Instruction-Details/Alphabetical-list-
of-instructions/BL--BLX--immediate-?lang=en

The encoding T2 for BLX is below:

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 
 1  1  1  1  0  S |       imm10H     | 1  1 J1  0 J2 |       imm10L      |H

In the ASL of ARM,  we have  H == '1' then UNDEFINED;

Symbol *H* represents the last bit of this instruction. I am not sure
whether a->imm includes the symbol *H*. I double checked the file
`t32.decode` and it seems so (It would be great if you can tell me what
a->imm indeed represents in BLX).

However, UNDEFINED means unallocated encoding in ARM manual. The right
behavior might be something like below:

    if (s->thumb && (a->imm & 2)) {
        unallocated_encoding(s);
        return true;
    }

Correct me if I am wrong. I can also provide test case if you need. Many
Thanks

Regards
Muhui

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https://bugs.launchpad.net/bugs/1925512

Title:
  UNDEFINED case for instruction BLX

Status in QEMU:
  Invalid

Bug description:
  Hi

  I refer to the instruction BLX imm (T2 encoding) in ARMv7 (Thumb
  mode).

  11110 S imm10H  11 J1 0 J2 imm10L H

  
  if H == '1' then UNDEFINED;
  I1 = NOT(J1 EOR S);  I2 = NOT(J2 EOR S);  imm32 = 
SignExtend(S:I1:I2:imm10H:imm10L:'00', 32);
  targetInstrSet = InstrSet_A32;
  if InITBlock() && !LastInITBlock() then UNPREDICTABLE;

  According to the manual, if H equals to 1, this instruction should be
  an UNDEFINED instruction. However, it seems QEMU does not check this
  constraint in function trans_BLX_i. Thanks

  Regards
  Muhui

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