On Tue, 28 Nov 2023, Reinette Chatre wrote:
On 11/20/2023 3:13 AM, Ilpo Järvinen wrote:
The fill_buf code prevents compiler optimizating the entire read loop away by writing the final value of the variable into a file. While it achieves the goal, writing into a file requires significant amount of work within the innermost test loop and also error handling.
A simpler approach is to take advantage of volatile. Writing to a variable through a volatile pointer is enough to prevent compiler from optimizing the write away, and therefore compiler cannot remove the read loop either.
Add a volatile 'value_sink' into resctrl_tests.c and make fill_buf to write into it. As a result, the error handling in fill_buf.c can be simplified.
The subject and changelog describes the need for a volatile variable. The patch introduces two volatile variables. Could you please elaborate why two volatile variables are needed?
Well, the other "volatile variable" is a pointer to a volatile variable.
I've seen gcc to kill a static volatile int so I prefer to not take change with its optimizer. Thus, I placed the sink into different compilation unit and just present a pointer to the actual "volatile" variable.
I guess the sink could be marked as plain int instead but this being trickery to begin with I don't see much value either way. It's still a trick.
I'll alter the changelog's wording though, "a volatile variable" is not accurate as it's "a pointer to a volatile variable".