On January 29, 2017 4:56:46 PM GMT+01:00, parmenides via gcc <gcc@gcc.gnu.org>
wrote:
>Hi,
>
> I want to know how to use inline assembler instruction, and wonder
>what is the meaning of "memory" in clobbered register list. According
>to
>the manual of GCC, the "memory" will cause GCC to not keep memory
>values
>cached in registers across the assembler instruction and not optimize
>stores or loads to that memory. IMO, that means the "memory" notifies
>GCC the assembler instruction meant to modify memory locations; If
>variables are cached into registers before the assembler instruction,
>then these varialbes should be written back to relevant memory
>locations
>before the assembler instruction, and read when being used again after
>the assembler instruction. On the other hand, the "memory" affected
>inputs and outputs of the assembler instruction only. Then the volatile
>
>keyword play its role, if the memory affected is not listed in the
>inputs and outputs of the assembler instruction. So, I think the
>"memory" and the volatile keyword should be used to implement a
>barried.
>I write the following C code to test the effect of the twos:
>
> /* asm.c */
> int foobar() {
> int i, t, sum = 0;
>
> for (i = 0; i <= 10; i++) {
> t = 1 << i;
> sum += t;
> }
> asm volatile ("nop" : : : "memory");
> return sum;
> }
> ... ...
>
>I compiled the asm.c using gcc 4.4.7:
> gcc -S -O1 asm.c
>
>Then, the asm.s is generated as follows:
>
> foobar:
> pushl %ebp
> movl %esp, %ebp
> pushl %ebx
> movl $0, %eax
> movl $0, %ecx
> movl $1, %edx
> .L2:
> movl %edx, %ebx
> sall %cl, %ebx
> addl %ebx, %eax
> addl $1, %ecx
> cmpl $11, %ecx
> jne .L2
> #APP
> # 10 "hello.c" 1
> nop
> # 0 "" 2
> #NO_APP
> negl %eax
> popl %ebx
> popl %ebp
> ret
> ... ...
>
>It is obvious that every variables are cached into register, and even
>though the 'sum' is used after the barrier, it does not written back to
>
>its memory location and read again. I think my idea about the "memory"
>and the volatile keyword must be misunderstanding. Any suggestion?
Local variables are not memory unless they have their address taken and it
escapes.
Richard.
