On 9/8/19 1:03 PM, Ayrat Gaskarov wrote:
> From gcc docs: 'The "memory" clobber tells the compiler that the assembly
> code performs memory reads or writes to items other than those listed in
> the input and output operands (for example, accessing the memory pointed to
> by one of the input parameters). To ensure memory contains correct values,
> GCC may need to flush specific register values to memory before executing
> the asm. Further, the compiler does not assume that any values read from
> memory before an asm remain unchanged after that asm; it reloads them as
> needed. Using the "memory" clobber effectively forms a read/write memory
> barrier for the compiler.'
> Does this mean that all data could be accessed including global variables,
> data in heap and local variables? Or is it not true for local variables?
Yes, but only if the local variable has had its address taken. If it has not,
then it is not reachable.
> For example:
> int local = 5;
> asm volatile("" ::: "memory");
> local += 6;
> Could it be reordered in the following way (because 'local' is local
> variable and could not be accessed):
> int local = 5;
> local += 6;
> asm volatile("" ::: "memory");
Yes.
> Could 'local' be optimized out in this case?
Yes.
--
Andrew Haley (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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