Re: Odd error with the "X" inline assembly constraint

[LIU Hao <lh_mouse@126.com>]

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在 2024/1/6 0:30, David Brown via Gcc-help 写道:
> I'd imagine that the "X" operand doesn't see much use in real inline assembly - on x86 and ARM the 
> assembly instruction template would usually depend on where the data is put.  But if anyone can 
> explain this behaviour to me, I am very curious to know what is going on.

I haven't used it at all and know nothing about its original intention. From GCC documentation, it 
looks like `X` specifies that the argument is allowed to be passed via either a register or memory, 
so it should be eligible for overloaded mnemonics, such as the MOV, FLD and ADDPD instructions on x86.

For example, given
(https://gcc.godbolt.org/z/vxb1aTe9c)
```
static int value;

int
test_X(void)
   {
     int a;
     __asm__("mov %0, %1" : "=&r"(a), "+X"(value));
     return a;
   }
```

GCC passes `value` directly via memory
```
test_X:
         mov eax, DWORD PTR value[rip]
         ret
```

and Clang passes it via the ECX register
```
test_X:
         mov     ecx, dword ptr [rip + value]
         mov     eax, ecx
         mov     dword ptr [rip + value], ecx
         ret
```

If `static int value;` is changed to `extern int value;` then GCC will reject it, as the operand no 
longer fits into a single operand; but the clang approach still works, with `value` replaced with 
`value@GOTPCREL`.

It doesn't seem very useful for RISC machines.


-- 
Best regards,
LIU Hao


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