什么叫非显式的整型立即数

zylthinking

folklore 发表于 2012-10-15 11:56
看了楼主的英文，大意是说：
如果指令
ins n

insn 是英文指令的意思， 是英文单词， 没有说 ins 是指令，  n 是操作数

folklore

回复 6# zylthinking


    是，但是事实上是常数

folklore

回复 8# zylthinking


    asm ("mov %1 %0" : "=r"(result) : "s" (????????))

mov result,????
????要填常数

所以，你给出的 ???????? 必须是常数表达式，也就是 1+1
或
const int n=10;
    asm ("mov %1 %0" : "=r"(result) : "s" (n+1))

这类东西。

zylthinking

folklore 发表于 2012-10-15 12:17
回复 8# zylthinking

1.     const int nn=10;
   
2.     int result;
   
3.     asm ("mov %1 %0" : "=r"(result) : "s" (nn+1));
   
4.    

复制代码

fatal error: error in backend: Invalid operand for inline asm constraint 's'!

zylthinking

goldenfort 发表于 2012-10-15 11:48
回复 3# zylthinking

1.     const int nn=10;
   
2.     int result;
   
3.     asm ("mov %1 %0" : "=r"(result) : "s" (&nn));

复制代码

fatal error: error in backend: Invalid operand for inline asm constraint 's'!

folklore

回复 11# zylthinking


没见过，学习了。   

folklore

回复 14# zylthinking


    也就是编译器不认为const int n=0;中的n是常量（这是正确的）

1+1可以吧。

folklore

回复 15# zylthinking


    int n;
  &n <<不是常量，要Run-time计算的。

这里要求常量表达式。

zylthinking

folklore 发表于 2012-10-15 12:24
回复 15# zylthinking

你说怎么写就行了

sonicling

楼主真是的，也不贴全，后面不是说明原因了吗。

‘s’
An immediate integer operand whose value is not an explicit integer is allowed.
This might appear strange; if an insn allows a constant operand with a value
not known at compile time, it certainly must allow any known value. So why
use ‘s’ instead of ‘i’? Sometimes it allows better code to be generated.

For example, on the 68000 in a fullword instruction it is possible to use an
immediate operand; but if the immediate value is between −128 and 127, better
code results from loading the value into a register and using the register. This
is because the load into the register can be done with a ‘moveq’ instruction. We
arrange for this to happen by defining the letter ‘K’ to mean “any integer outside
the range −128 to 127”, and then specifying ‘Ks’ in the operand constraints.

‘I’, ‘J’, ‘K’, . . . ‘P’
Other letters in the range ‘I’ through ‘P’ may be defined in a machine-dependent
fashion to permit immediate integer operands with explicit integer values in
specified ranges. For example, on the 68000, ‘I’ is defined to stand for the
range of values 1 to 8. This is the range permitted as a shift count in the shift
instructions.