golang asm代码中未知的“ ptr”变量

Question

Recently I just begin to read the source code of atomic.LoadUint64, then I got an unknown variable "ptr" in the following asm code:

TEXT runtime∕internal∕atomic·Load64(SB), NOSPLIT, $0-12                                                                                                                                                                                       
    MOVL    ptr+0(FP), AX                                                                                                                                                                                                                     
    TESTL   $7, AX                                                                                                                                                                                                                            
    JZ  2(PC)                                                                                                                                                                                                                                 
    MOVL    0, AX // crash with nil ptr deref                                                                                                                                                                                                 
    MOVQ    (AX), M0                                                                                                                                                                                                                          
    MOVQ    M0, ret+4(FP)                                                                                                                                                                                                                     
    EMMS                                                                                                                                                                                                                                      
    RET

I couldn't find the declaration of this variable, and any documents about this variable, can anyone tell me about it?

Answer 1

A Quick Guide to Go's Assembler

Symbols

The FP pseudo-register is a virtual frame pointer used to refer to function arguments. The compilers maintain a virtual frame pointer and refer to the arguments on the stack as offsets from that pseudo-register. Thus 0(FP) is the first argument to the function, 8(FP) is the second (on a 64-bit machine), and so on. However, when referring to a function argument this way, it is necessary to place a name at the beginning, as in first_arg+0(FP) and second_arg+8(FP). (The meaning of the offset—offset from the frame pointer—distinct from its use with SB, where it is an offset from the symbol.) The assembler enforces this convention, rejecting plain 0(FP) and 8(FP). The actual name is semantically irrelevant but should be used to document the argument's name. It is worth stressing that FP is always a pseudo-register, not a hardware register, even on architectures with a hardware frame pointer.

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ptr, in ptr+0(FP), is a name for the first argument to the function. The argument is probably a pointer.