Am 24.09.23 um 03:48 schrieb Ricardo Cosme:
Hi,
I have a question regarding the reordering of statements involving a
builtin function and extended asm volatile expressions in avr-gcc.
Consider the following code snippet:
//C++
{
asm volatile(/* template plus operands */);
__builtin_avr_delay_cycles(cycles);
asm volatile(/* template plus operands */);
}
I understand that the compiler is allowed to reorder statements as long
as it doesn't affect the observable behavior of the program. However, in
this specific case, the __builtin_avr_delay_cycles function is a
compiler intrinsic.
Assuming there are no other expressions on the above block, is it
possible for the compiler to reorder the three statements?
Cheers,
Ricardo Cosme
No. __builtin_avr_delay_cycles is implicitly volatile, so it may not
be reordered against other volatile statements.
This also applies to built-ins like
__builtin_avr_sei/cli/nop/nops/wdr/sleep,
whereas arithmetic built-ins like __builtin_avr_swap/fmul/insert_bits
etc. are purely arithmetic any may be reordered or optimized out.
The volatile built-ins also have a memory clobber similar to
__asm volatile ("" ::: "memory");
so they may not be reordered against memory accesses. They may be
reordered against arithmetic operations like divisions though, cf.
https://stackoverflow.com/q/37897848/1556746
Johann
