ARM assembly puzzle

First of all, I'm not sure if solution even exists. I spent more than a couple of hours trying to come up with one, so beware.

The problem:

r1 contains an arbitrary integer, flags are not set according to its value. Set r0 to 1 if r1 is 0x80000000, to 0 otherwise, using only two instructions.

It's easy to do that in 3 instruc开发者_开发知识库tions (there are many ways), however doing it in 2 seems very hard, and may very well be impossible.

---

something like

SMMUL r0,r1,r1
MOV r0,r0,lsr #30

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Here's a partial solution that gives the correct answer in the top bit of r0, so it is available as a shifter operand (r0 lsr #31).

; r0 = r1 & -r1
rsb r0, r1, #0
and r0, r0, r1

This works because 0 and 0x80000000 are the only numbers that retain their sign bits when negated. I'm fairly sure an exact solution is impossible.

EDIT: no, it's not impossible. See Martin's answer.

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adds  r0, r1, #0x80000000 ; if r1 is 0x80000000, r0 will now hold 0
movne r0, #1              ; otherwise, set r0 to 1

This is equivalent to:

unsigned int r0, r1;
r0 = r1 + 0x80000000; // 32 bits, so top bit will be lost on overflow
if (r0 != 0)
{
    r0 = 1;
}

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Something like:

mov r0,r1,lsr #31

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Tough puzzle if one wants to use "fast" instructions. I can't quite come up with a solution, but can offer a couple more 'notions':

; If goal were to have value of zero if $80000000 and something else otherwise:
  adds r0,r1,r1 ; Overflow only if $80000000
  movvc r0,#whatever

; If goal were to have value of $80000000 if $80000000 and zero otherwise
  subs r0,r1,#0  ; Overflow only if $80000000
  movvc r0,#0 ; Or whatever

; If the goal were to have value of $7FFFFFFF if $80000000 and zero otherwise
  adds r0,r1,r1,asr #31 ; Overflow only if $80000000
  movvc r0,#0

; If carry were known to be set beforehand
  addcs r0,r1,r1 ; Overflow only if $80000000 (value is 1)
  movvc r0,#0

; If register r2 were known to hold #1
  adds r0,r1,r1,asr #31
  ; If $80000000, MSB and carry set
  sbc r0,r2,r0,lsr #31

None of those is a perfect solution, but they're interesting.