I'm having difficulty understanding the role constraints play in GCC inline assembly (x86). I've read the manual, which explains exactly what each constraint does. The problem is that even though I understand what each constraint does, I have very little understanding of why you would use one constraint over another, or what the implications might be.
I realize this is a very broad topic, so a small example should help narrow the focus. The following is a simple asm routine which just adds two numbers. If an integer overflow occurs, it writes a value of 1 to an output C variable.
int32_t a = 10, b = 5;
int32_t c = 0; // overflow flag
__asm__
(
"addl %2,%3;" // Do a + b (the result goes into b)
"jno 0f;" // Jump ahead if an overflow occurred
"movl $1, %1;" // Copy 1 into c
"0:" // We're done.
:"=r"(b), "=m"(c) // Output list
:"r"(a), "0"(b) // Input list
);
Now this works fine, except I had to arbitrarily fiddle with the constraints until I got it to work correctly. Originally, I used the following constraints:
:"=r"(b), "=m"(c) // Output list
:"r"(a), "m"(b) // Input list
Note that instead of a "0", I use an "m" constraint for b. This had a weird side effect where if I compiled with optimization flags and called the function twice, for some reason the result of the addition operation would also get stored in c. I eventually read about "matching constraints", which allows you to specify that a variable is to be used as both an input and output operand. When I changed "m"(b) to "0"(b) it worked.
But I don't really understand why you would use one constraint over another. I mean yeah, I understand that "r" means the variable should be in a register and "m" means it should be in memory - but I don't really understand what the implications of choosing one over another are, or why the addition operation doesn't work correctly if I choose a certain combination of constraints.
Questions: 1) In the above example code, why did the "m" constraint on b cause c to get written to? 2) Is there any tutorial or online resource which goes into more detail about constraints?