gcc - GNU assembly Inline: what do %1 and %0 mean? -


i new gnu assembly inlining, have read multiple write ups still not understand going on. understanding:

movl %eax, %ebx\n\t move whatever in %eax ebx, not add contents each other

addl %eax, %ebx\n\t add contents of %eax ebx , keep @ right register

addl %1, %0\n\t confused, adding 1 , 0? why need have %0 there?

the whole asm inline block looks like:

 asm [volatile] ( assemblertemplate                       : outputoperands                       [ : inputoperands                       [ : clobbers ] ])

or

 asm [volatile] ( assemblertemplate                       : outputoperands)

in assemblertemplate assembly code, , in output/inputoperands, can pass variable between c , asm.

then in asm, %0 refers first variable passed outputoperand or inputoperand, %1 second, etc.

example:

 int32_t = 10;  int32_t b;  asm volatile ("movl %1, %0" : "=r"(b) : "r"(a) : );

this asm code equivalent "b = a;"

a more detailed explanation here: https://gcc.gnu.org/onlinedocs/gcc/extended-asm.html


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