I think I've run into a compiler bug, but I'm not sure how to pin it
down.
I'm using the TinyOS RandomMlcg random generator:
/* Return the next 32 bit random number */
async command uint32_t Random.rand32() {
uint32_t mlcg,p,q;
uint64_t tmpseed;
atomic
{
tmpseed = (uint64_t)33614U * (uint64_t)seed;
q = tmpseed; /* low */
q = q >> 1;
p = tmpseed >> 32 ; /* hi */
mlcg = p + q;
if (mlcg & 0x80000000) {
mlcg = mlcg & 0x7FFFFFFF;
mlcg++;
}
seed = mlcg;
}
return mlcg;
}
Now, in my program, I have a function that takes one argument. After the
call to Random.rand16(), this argument is overwritten by the value
0x834e - which is 33614U. This happens with mspgcc4 (20110813) but not
with 3.2.3. My trivial test program failed to reproduce this; I'll see
if I can find a way to trigger it.
The context in which it's called (noinlined so I can find it in the
disassembly):
void backoff(uint16_t fixed) __attribute__((noinline)) {
uint16_t delay = 0;
printf("fixed=%u\n", fixed); // here, fixed is
correct
delay = call Random.rand16();
printf("fixed=%u\n", fixed); // here, fixed == 0x834e
...
For reference, the disassembly of the working version (3.2.3) followed
by the broken one, up to the second call to printf:
3.2.3:
00005204 <SendP__backoff>:
5204: 1b 15 .word 0x151b; ????
5206: 0a 4f mov r15, r10
5208: 0f 12 push r15
520a: 30 12 fa 51 push #20986 ;#0x51fa
520e: b0 13 .word 0x13b0; ????
5210: 6a 7d subc.b @r13, r10
5212: 21 52 add #4, r1 ;r2 As==10
5214: b0 13 .word 0x13b0; ????
5216: 5e 52 0b 4e add.b &0x4e0b,r14
521a: 0a 12 push r10
521c: 30 12 fa 51 push #20986 ;#0x51fa
5220: b0 13 .word 0x13b0; ????
5222: 6a 7d subc.b @r13, r10
5224: 21 52 add #4, r1 ;r2 As==10
5226: 0a 4b mov r11, r10
5228: 0b 43 clr r11
522a: b0 13 .word 0x13b0; ????
522c: f6 34 jge $+494 ;abs 0x541a
522e: 53 12 push.b #1 ;r3 As==01
5230: 0b 12 push r11
5232: 0a 12 push r10
5234: 0d 4e mov r14, r13
5236: 0e 4f mov r15, r14
5238: 7f 40 03 00 mov.b #3, r15 ;#0x0003
523c: b0 13 .word 0x13b0; ????
523e: 4a 42 mov.b r2, r10
5240: 31 50 06 00 add #6, r1 ;#0x0006
5244: 1f 42 70 14 mov &0x1470,r15
5248: 0f 5f rla r15
524a: 82 4f 70 14 mov r15, &0x1470
524e: 3f 90 11 00 cmp #17, r15 ;#0x0011
5252: 03 28 jnc $+8 ;abs 0x525a
5254: b2 40 10 00 mov #16, &0x1470 ;#0x0010
5258: 70 14
525a: 1a 17 .word 0x171a; ????
525c: 10 01 .word 0x0110; ????
0000525e <RandomMlcgC__Random__rand32>:
525e: 4b 15 .word 0x154b; ????
5260: b0 13 .word 0x13b0; ????
5262: 8a 31 jn $+790 ;abs 0x5578
5264: 47 4f mov.b r15, r7
5266: 1c 42 c8 13 mov &0x13c8,r12
526a: 1d 42 ca 13 mov &0x13ca,r13
526e: 0e 43 clr r14
5270: 0f 43 clr r15
5272: 03 12 push #0 ;r3 As==00
5274: 03 12 push #0 ;r3 As==00
5276: 03 12 push #0 ;r3 As==00
5278: 30 12 4e 83 push #-31922 ;#0x834e
527c: b0 13 .word 0x13b0; ????
527e: f6 86 31 52 sub.b @r6+, 21041(r6);0x5231(r6)
5282: 0a 4c mov r12, r10
5284: 0b 4d mov r13, r11
5286: 12 c3 clrc
5288: 0b 10 rrc r11
528a: 0a 10 rrc r10
528c: 0c 4e mov r14, r12
528e: 0d 4f mov r15, r13
5290: 0e 43 clr r14
5292: 0f 43 clr r15
5294: 08 4c mov r12, r8
5296: 09 4d mov r13, r9
5298: 08 5a add r10, r8
529a: 09 6b addc r11, r9
529c: 39 b0 00 80 bit #-32768,r9 ;#0x8000
52a0: 04 24 jz $+10 ;abs 0x52aa
52a2: 39 f0 ff 7f and #32767, r9 ;#0x7fff
52a6: 18 53 inc r8
52a8: 09 63 adc r9
52aa: 82 48 c8 13 mov r8, &0x13c8
52ae: 82 49 ca 13 mov r9, &0x13ca
52b2: 4f 47 mov.b r7, r15
52b4: b0 13 .word 0x13b0; ????
52b6: 9e 31 jn $+830 ;abs 0x55f4
52b8: 0e 48 mov r8, r14
52ba: 0f 49 mov r9, r15
52bc: 47 17 .word 0x1747; ????
52be: 10 01 .word 0x0110; ????
20110813 inlines the call:
00004c90 <SendP__backoff>:
4c90: 0b 12 push r11
4c92: 0a 12 push r10
4c94: 09 12 push r9
4c96: 08 12 push r8
4c98: 07 12 push r7
4c9a: 06 12 push r6
4c9c: 05 12 push r5
4c9e: 04 12 push r4
4ca0: 21 83 decd r1
4ca2: 0f 12 push r15
4ca4: 0f 12 push r15
4ca6: 30 12 80 4c push #19584 ;#0x4c80
4caa: b0 12 b8 76 call #0x76b8
4cae: 31 50 06 00 add #6, r1 ;#0x0006
4cb2: b0 12 06 3a call #0x3a06
4cb6: c1 4f 00 00 mov.b r15, 0(r1) ;0x0000(r1)
4cba: 18 42 cc 13 mov &0x13cc,r8
4cbe: 19 42 ce 13 mov &0x13ce,r9
4cc2: 34 40 4e 83 mov #-31922,r4 ;#0x834e
4cc6: 05 43 clr r5
4cc8: b0 12 36 7c call #0x7c36
4ccc: 0a 4c mov r12, r10
4cce: 0b 4d mov r13, r11
4cd0: 12 c3 clrc
4cd2: 0b 10 rrc r11
4cd4: 0a 10 rrc r10
4cd6: 0a 5e add r14, r10
4cd8: 0b 6f addc r15, r11
4cda: 0b 93 tst r11
4cdc: 05 34 jge $+12 ;abs 0x4ce8
4cde: 3a f3 and #-1, r10 ;r3 As==11
4ce0: 3b f0 ff 7f and #32767, r11 ;#0x7fff
4ce4: 1a 53 inc r10
4ce6: 0b 63 adc r11
4ce8: 82 4a cc 13 mov r10, &0x13cc
4cec: 82 4b ce 13 mov r11, &0x13ce
4cf0: 6f 41 mov.b @r1, r15
4cf2: b0 12 1a 3a call #0x3a1a
4cf6: 04 12 push r4
4cf8: 04 12 push r4
4cfa: 30 12 80 4c push #19584 ;#0x4c80
4cfe: b0 12 b8 76 call #0x76b8
I'm afraid my assembly-fu isn't very strong, but I hope someone can
glean... something from it.
Best,
Michiel
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