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Question

I'm trying to write an asm statement (inline assembly in GCC ), that just calls some function, that returns one value in floating-point register and has no operands, but potentially clobbers all the floating-point registers.

asm("call *%1"
    : "=t"(result_)
    : "d"(code_.data())
    : "memory", "cc", "ax", "%st(1)", "%st(2)", "%st(3)", "%st(4)", "%st(5)", "%st(6)", "%st(7)"
    );

My problem is that I cannot tell the assembler, that is clobbering the top floating-point register %st(0) too, because I cannot specify "%st(0)" (or "%st" ) in clobber list (it results in an compilation error).

Answer 1

You are returning a result in %st(0) ; that is what the t constraint means. The compiler therefore knows it is modified.

I am not sure why your GCC is not recognizing %st(0) or %st as a name in the clobber list, but this should not cause a problem in this situation.

Apple clang version 4.0 (tags/Apple/clang-418.0.60) accepts %st in the clobber list, even with =t as an output constraint.