On 08/09/2019 16:46, Andrew Haley wrote:
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.
Having its address taken is not enough (AFAIK) - the address must be
used in a way that forces the data into memory. You can do useful
things with an address of a local variable without the variable being
put in memory:
#include <string.h>
unsigned int raw_float(const float f) {
unsigned int x;
memcpy(&x, &f, sizeof (x));
return x;
}
Generated x86 code (gcc -O2):
raw_float(float):
movd eax, xmm0
ret
Taking the address is not enough to make the local part of memory.
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.
