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

User · Answer

Instead of using the new pragmas  you can also use   attribute    optimize  O0     for your needs  This has the advantage of just applying to a single function and not all functions defined in the same file   Usage example   void   attribute    optimize  O0     foo unsigned char data           unmodifiable compiler code

User · Answer

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

User · Answer

Turning off optimization fixes the problem  but it is unnecessary   A safer alternative is to make it illegal for the compiler to optimize out the store by using the volatile type qualifier      Assuming pageptr is unsigned char   already    unsigned char  pageptr          unsigned char volatile   pageptr  0    pageptr 0     The volatile type qualifier instructs the compiler to be strict about memory stores and loads   One purpose of volatile is to let the compiler know that the memory access has side effects  and therefore must be preserved   In this case  the store has the side effect of causing a page fault  and you want the compiler to preserve the page fault   This way  the surrounding code can still be optimized  and your code is portable to other compilers which don t understand GCC s  pragma or   attribute   syntax

[c] How to prevent gcc optimizing some statements in C?

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