How to prevent gcc optimizing some statements in C?

Question

In order to make a page dirty (switching on the dirty bit in the page table entry), I touch the first bytes of the page like this:

pageptr[0] = pageptr[0];

But in practice gcc will ignore the statement by dead store elimination. In order to prevent gcc optimizing it, I re-write the statement as follows:

volatile int tmp;
tmp = pageptr[0];
pageptr[0] = tmp;

It seems the trick works, but somewhat ugly. I would like to know is there any directives or syntax which has the same effect? And I don't want to use a -O0 flag, since it will bring great performance penalty as well.

Answer 1

You can use

#pragma GCC push_options
#pragma GCC optimize ("O0")

your code

#pragma GCC pop_options

to disable optimizations since GCC 4.4.

See the GCC documentation if you need more details.