Operator * and + produces wrong result in digital mars - c

Question

I am trying to calculate a number which produce Longest Collatz sequence. But here is a strange problem. 3n+1 become 38654705674 when n is 3. I do not see an error. here is the full code:
/* 6.c -- calculates Longest Collatz sequence */
#include <stdio.h>
long long get_collatz_length(long long);
int main(void)
{
long long i;
long long current, current_count, count;
current_count = 1;
current = 1;
for(i=2;i<1000000;i++)
{
// works fine when i is 2 the next line take eternity when i is 3;
count = get_collatz_length(i);
if(current_count <= count)
{
current = i;
current_count = count;
}
}
printf("%lld %lld\n", current, current_count);
return 0;
}
long long get_collatz_length(long long num)
{
long long count;
count = 1;
while(num != 1)
{
printf("%lld\n", num);
if(num%2)
{
num = num*3+1; // here it is;
}
else
{
num/=2;
}
count++;
}
puts("");
return count;
}

Answer 1

It's seems to be bug in dmc compiler, that fails to handle long long type correctly. Here is narrowed test-case:
#include <stdio.h>
int main(void)
{
long long num = 3LL;
/*printf("%lld\n", num);*/
num = num * 3LL;
char *t = (char *) &num;
for (int i = 0; i < 8; i++)
printf("%x\t", t[i]);
putchar('\n');
/*printf("%lld\n", num);*/
return 0;
}
It produces (little endian, so 0x900000009 == 38 654 705 673):
9 0 0 0 9 0 0 0
From dissasembly it looks that it stores 64-bit integer as two 32-bit registers:
.data:0x000000be 6bd203 imul edx,edx,0x3
.data:0x000000c1 6bc803 imul ecx,eax,0x3
.data:0x000000c4 03ca add ecx,edx
.data:0x000000c6 ba03000000 mov edx,0x3
.data:0x000000cb f7e2 mul edx
.data:0x000000cd 03d1 add edx,ecx
.data:0x000000cf 31c0 xor eax,eax
I additionaly tested it with objconv tool, that just confirms my initial diagnose:
#include <stdio.h>
void mul(void)
{
long long a;
long long c;
a = 5LL;
c = a * 3LL;
printf("%llx\n", c);
}
int main(void)
{
mul();
return 0;
}
disassembly (single section):
>objconv.exe -fmasm ..\dm\bin\check.obj
_mul PROC NEAR
mov eax, 5 ; 0000 _ B8, 00000005
cdq ; 0005 _ 99
imul edx, edx, 3 ; 0006 _ 6B. D2, 03
imul ecx, eax, 3 ; 0009 _ 6B. C8, 03
add ecx, edx ; 000C _ 03. CA
mov edx, 3 ; 000E _ BA, 00000003
mul edx ; 0013 _ F7. E2
add edx, ecx ; 0015 _ 03. D1
push edx ; 0017 _ 52
push eax ; 0018 _ 50
push offset FLAT:?_001 ; 0019 _ 68, 00000000(segrel)
call _printf ; 001E _ E8, 00000000(rel)
add esp, 12 ; 0023 _ 83. C4, 0C
ret ; 0026 _ C3
_mul ENDP
Note that mul edx operates implicitely on eax. The result is stored in both registers, higher part (in this case 0) in stored in edx, while lower in eax.