DMD compiler(seems from 1.029) produces bad optimized opcode which access a popped register.
$cat test.d
class foo {
uint a, b;
}
void main () {
foo f = new foo;
uint c;
with (f) {
c = 0;
b = 0;
a = 0;
if (a == b && (a + (c = 66)) <= 66)
assert(c == 66);
}
}
$dmd -O test.d
$./test
Error: AssertError Failure test.d(13)
$objdump -d test.o
...
...
25: 53 push %ebx
26: bb 42 00 00 00 mov $0x42,%ebx
2b: 89 d9 mov %ebx,%ecx
2d: 5b pop %ebx <----------- here
2e: 8d 14 0b lea (%ebx,%ecx,1),%edx
31: 3b d6 cmp %esi,%edx
33: 77 0f ja 44 <_Dmain+0x44>
35: 83 fb 42 cmp $0x42,%ebx <---------- and here
38: 74 0a je 44 <_Dmain+0x44>
...
...
Comment #1 by clugdbug — 2009-11-23T01:57:30Z
Reduced test case shows that this is very low level. It's very specific, changing the expression order slightly will make the problem go away.
The assignment to c gets optimised away, so that it remains as 0.
-------------------------
void bug3521(int *a)
{
int c = 0;
*a = 0;
if (*a == 0 && (*a + (c = 2)) == 2)
assert(c == 2);
}
void main ()
{
int x;
bug3521(&x);
}
Comment #2 by clugdbug — 2009-11-27T00:15:10Z
I investigated this a bit, so far without success. I'm writing notes here for when I come back to it.
It's something to do with the register allocation. The compiler is somehow forgetting that it already assigned a register for c when doing the assignment. I removed the assert to make disassembly even easier: this hits a breakpoint only when compiled with -O. It only happens if c is zero. You can change the != into any kind of comparison, and the c=200 into c+=200, without affecting the bug. However, changing it into ((c = 200)!=*a) avoids the bug.
void crash(int x)
{
if (x==200) return;
asm { int 3; }
}
void bug3521(int *a){
int c = 0;
*a = 0;
if ( *a || (*a != (c = 200)) )
crash(c);
}
void main (){
int x;
bug3521(&x);
}
Comment #3 by bugzilla — 2009-11-30T14:16:29Z
I've found the cause of the problem; it's when a registered variable is on both sides of an operator and the right side tries to modify the variable. The fix isn't easy, I'll work on it.