asm volatile statement reordering

[Xi Ruoyao]

On 2017-10-17 00:27 +0200, David Brown wrote:

> My testing suggests that gcc will re-order "asm volatile" statements 
> that have an output, such as the "save the PRIMASK into status"Â 
> statement, but it will /not/ re-order "asm volatile" statements that 
> have no outputs.
> 
> Is that correct?
> 
> Is that the intended behaviour of "asm volatile" ?

Yes.  It's said in the documentation
<https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gcc/Extended-Asm.html>:

Note that the compiler can move even volatile asm instructions relative
to other code, including across jump instructions. For example, on many
targets there is a system register that controls the rounding mode of
floating-point operations. Setting it with a volatile asm, as in the
following PowerPC example, does not work reliably.

asm volatile("mtfsf 255, %0" : : "f" (fpenv));
sum = x + y;

The compiler may move the addition back before the volatile asm. To
make it work as expected, add an artificial dependency to the asm by
referencing a variable in the subsequent code, for example:

asm volatile ("mtfsf 255,%1" : "=X" (sum) : "f" (fpenv));
sum = x + y;

> If so, is that a good design choice or should it be changed?

I don't know.  I think it is just a choice, not so good and not so
bad.

> Could the documentation in the gcc web page be improved?

The documentation is very clear.

> Or is this a bug in gcc, and the statements should not have been re-ordered?
> 
> 
> David
-- 
Xi Ruoyao <ryxi@stu.xidian.edu.cn>
School of Aerospace Science and Technology, Xidian University