What does a double-percent sign (%%) do in gcc inline assembly?

Question

I came across a code that looks like this:

asm volatile (
    # [...]
    "movl $1200, %%ecx;"
    # [...]
);

I know what movl $1200, %ecx does in x86. but I was confused by why there are two percent signs.

Answer 1

GCC inline assembly uses %0, %1, %2, etc. to refer to input and output operands. That means you need to use two %% for real registers.

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