How to use ".ifnb" in Gnu Assembly? And how to accept a blank value as an argument for a ".macro" function in GAS?

Question

I am trying to pass a blank value to a macro funtion in GNU Assembly on x64, this function is supposed to add three value togather or just two according to if the first argument is blank, e.g.,

.macro foo1 t0, t1, t2
  .ifnb t0
    add \t0, \t1
  .endif
    add \t1, \t2
.endm

foo2:
    foo1 , %rax, %rbx
    foo1 %rax, %rbx, %rcx

But when I complie the above code, the gcc said expecting operand before ','; got nothing because I call foo1 use the folloing format foo1 , %rax, %rbx. So I am confused about how to use .ifnb pseudo-op, and how to pass a blank argument to a macro like foo1.

I tried to search in google, but I got few things about gnu assembly blank value. https://sourceware.org/binutils/docs/as/If.html I also read GAS official docs but it also not mention how to use .ifnb and no blank argument example was given for .macro. https://sourceware.org/binutils/docs/as/Macro.html

Answer 1

The correct syntax for .ifnb is as follows:

.macro foo1 t0, t1, t2
  .ifnb \t0             ; <------ Note the backslash preceding the `t0`
    add \t0, \t1
  .endif
    add \t1, \t2
.endm

You are getting a confusing error message pointing to the line of macro invocation, because it seem to be generated after the macro expansion to add , %rax but still refer to the line in the original code.