Register variable declaration

What exactly the following declaration will do?

Code:

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register int *ptr asm ("r9");

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Quote:

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1)Memory is allocated for ptr pointer variable and the address of that memory will be saved in r9 register?

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(OR)

Quote:

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2)what value we assign to ptr varible will be stored in r9 register?

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ptr is allocated on r9 register and "register" keyword tells the compiler that the address-of ptr cannot be taken.

Code:

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int *p asm ( "r8" );
register int *q asm ( "r9" );
int **pp = &p;  // this will generate an warning
int **qq = &q; // this will generate an error!

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So r9 register will act as a pointer, what ever value we assign to ptr will be stored in r9 register.

Quote:

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Originally Posted by krkr

So r9 register will act as a pointer, what ever value we assign to ptr will be stored in r9 register.

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The point here is that "register" keyword isn't a modifier to garantee register allocation... The semantics is "you cannot get the address-of this identifier"...
And, using the GCC extension to allocate a local variable in a specific register is only a hint that can be ignored:

Code:

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unsigned long f( unsigned long *p, unsigned int count )
{
  unsigned long sum = 0;
  unsigned long *q asm ("r9");

  q = p;
  while ( count-- )
    sum += *q++;

  return sum;
}

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Compiing (SysV x86-64 ABI):

Code:

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$  cc -O2 -c test.c
test.c: In function ‘f’:
test.c:4:18: warning: ignoring asm-specifier for non-static local variable ‘q’
  unsigned long *q asm ( "r9" );
                  ^

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The created code:

Code:

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f:
  test esi,esi
  lea  eax,[rsi-1]
  je  .L4
  lea  rdx,[rdi+rax*8+8]
  xor  eax,eax
.L3:
  add  rax,[rdi]
  add  rdi,8
  cmp  rdi,rdx
  jne  .L3
  ret
.L4:
  xor  eax,eax
  ret

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Where is r9? If you don't use -O2 option, the compiler will, still, ignore the hint. Try it.