GCC Assembly "+t"

Question

I'm currently testing some inline assembly in C++ on an old compiler (GCC circa 2004) and I wanted to perform the square root function on a floating point number. After trying and searching for a successful method, I came across the following code

float r3(float n){
    __asm__("fsqrt" : "+t" (n));
    return n;
};

which worked. The issue is, even though I understand the assembly instructions used, I'm unable to find any particular documentation as to what the "+t" flag means on the n variable. I'm under the genuine idea that it seems to be a manner by which to treat the variable n as both the input and output variable but I was unable to find any information on it. So, what exactly is the "t" flag and how does it work here?

Answer 1

+

Means that this operand is both read and written by the instruction. (From here)

t

Top of 80387 floating-point stack (%st(0)). (From here)