I’m seeing what looks like a nasty bug in DC 9.50. I have the following code:
printf("q = ++p(%p)
", p);
if (!p)
q = ++p;
else
{
++p;
q=p;
}
printf("[done]q(%p)=++p(%p)
", q, p);
This generates the output:
q = ++p(8453)
[done]q(8711)=++p(8454)
Yikes!
If I invert that if condition (whose sole purpose is probing this bug), I get the following output:
q = ++p(8453)
[done]q(8454)=++p(8454)
Before we were using DC 9.25, but upgraded because we need to do commandline compiles, and 9.25’s commandline compiler can’t compile the tcp library. 
The assembly listing generated by DC9.50 for this is:
[MOTION.C(985:1)]: printf("q = ++p(%p)
", p);
00:e527 E4F0 ld hl, (ix + -16) 9
00:e529 E5 push hl 10
00:e52a 21082B ld hl, 0x2B08 6
00:e52d E5 push hl 10
[MOTION.C(985:23)]: , p);
00:e52e CFEDE919 lcall printf 19
00:e532 2704 add sp, 4 4
[MOTION.C(986:1)]: if (p)
00:e534 E4F0 ld hl, (ix + -16) 9
00:e536 CC bool hl 2
00:e537 CA67E5 jp z, 0xE567 7
[MOTION.C(987:3)]: q = ++p;
00:e53a 211801 ld hl, 0x0118 6
00:e53d D5 push de 10
00:e53e DD5E00 ld e, (ix + 0) 9
00:e541 DD5601 ld d, (ix + 1) 9
00:e544 19 add hl, de 2
00:e545 D1 pop de 7
00:e546 E5 push hl 10
00:e547 211A01 ld hl, 0x011A 6
00:e54a D5 push de 10
00:e54b DD5E00 ld e, (ix + 0) 9
00:e54e DD5601 ld d, (ix + 1) 9
00:e551 19 add hl, de 2
00:e552 D1 pop de 7
00:e553 E5 push hl 10
00:e554 DDE400 ld hl, (hl + 0) 11
00:e557 23 inc hl 2
00:e558 44 ld b, h 2
00:e559 4D ld c, l 2
00:e55a E1 pop hl 7
00:e55b 71 ld (hl), c 6
00:e55c 23 inc hl 2
00:e55d 70 ld (hl), b 6
00:e55e 44 ld b, h 2
00:e55f 4D ld c, l 2
00:e560 E1 pop hl 7
00:e561 71 ld (hl), c 6
00:e562 23 inc hl 2
00:e563 70 ld (hl), b 6
00:e564 C393E5 jp 0xE593 7
[MOTION.C(988:1)]: else
[MOTION.C(990:1)]: ++p;
00:e567 211A01 ld hl, 0x011A 6
00:e56a D5 push de 10
00:e56b DD5E00 ld e, (ix + 0) 9
00:e56e DD5601 ld d, (ix + 1) 9
00:e571 19 add hl, de 2
00:e572 D1 pop de 7
00:e573 E5 push hl 10
00:e574 DDE400 ld hl, (hl + 0) 11
00:e577 23 inc hl 2
00:e578 44 ld b, h 2
00:e579 4D ld c, l 2
00:e57a E1 pop hl 7
00:e57b 71 ld (hl), c 6
00:e57c 23 inc hl 2
00:e57d 70 ld (hl), b 6
[MOTION.C(991:1)]: q=p;
00:e57e 211801 ld hl, 0x0118 6
00:e581 D5 push de 10
00:e582 DD5E00 ld e, (ix + 0) 9
00:e585 DD5601 ld d, (ix + 1) 9
00:e588 19 add hl, de 2
00:e589 D1 pop de 7
00:e58a E5 push hl 10
00:e58b E4F0 ld hl, (ix + -16) 9
00:e58d 44 ld b, h 2
00:e58e 4D ld c, l 2
00:e58f E1 pop hl 7
00:e590 71 ld (hl), c 6
00:e591 23 inc hl 2
00:e592 70 ld (hl), b 6
[MOTION.C(992:1)]: }
[MOTION.C(993:1)]: printf("[done]q(%p)=++p(%p)
", q, p);
00:e593 E4F0 ld hl, (ix + -16) 9
00:e595 E5 push hl 10
00:e596 E4EE ld hl, (ix + -18) 9
00:e598 E5 push hl 10
00:e599 213C2A ld hl, 0x2A3C 6
00:e59c E5 push hl 10
[MOTION.C(993:31)]: , q, p);
00:e59d CFEDE919 lcall printf 19
00:e5a1 2706 add sp, 6 4
I’m far from an expert at rabbit assembly, but the two sides of the if ought to look similar, and there are some marked differences between the two.