How to prevent GCC from optimizing out a busy wait loop?

by

I developed this answer after following a link from dmckee’s answer, but it takes a different approach than his/her answer.

Function Attributes documentation from GCC mentions:

noinline
This function attribute prevents a function from being considered for inlining. If the function does not have side-effects, there are optimizations other than inlining that causes function calls to be optimized away, although the function call is live. To keep such calls from being optimized away, put asm ("");

This gave me an interesting idea… Instead of adding a nop instruction at the inner loop, I tried adding an empty assembly code in there, like this:

unsigned char i, j;
j = 0;
while(--j) {
    i = 0;
    while(--i)
        asm("");
}

And it worked! That loop has not been optimized-out, and no extra nop instructions were inserted.

What’s more, if you use volatile, gcc will store those variables in RAM and add a bunch of ldd and std to copy them to temporary registers. This approach, on the other hand, doesn’t use volatile and generates no such overhead.

Update: If you are compiling code using -ansi or -std, you must replace the asm keyword with __asm__, as described in GCC documentation.

In addition, you can also use __asm__ __volatile__("") if your assembly statement must execute where we put it, (i.e. must not be moved out of a loop as an optimization).

More Related Contents:

  1. Competitive programming: "Time exceeded error"
  2. Disable all optimization options in GCC
  3. How many GCC optimization levels are there?
  4. Why doesn’t GCC optimize structs?
  5. Produce loops without cmp instruction in GCC
  6. Query on -ffunction-section & -fdata-sections options of gcc
  7. How come when I try to compile my C program by making a file named for the program it creates an application for it?
  8. Optimize this C Code [closed]
  9. Can’t compile C program on a Mac after upgrade to Mojave
  10. C loop optimization help for final assignment (with compiler optimization disabled)
  11. Is there a performance difference between i++ and ++i in C?
  12. Scanf skips every other while loop in C
  13. Why does GCC allocate more space than necessary on the stack, beyond what’s needed for alignment?
  14. Working of fork() in linux gcc [duplicate]
  15. Syscall implementation of exit()
  16. Why does GCC pad functions with NOPs?
  17. Tell gcc to specifically unroll a loop
  18. Can I mix static and shared-object libraries when linking?
  19. Installing OpenMP on Mac OS X 10.11
  20. Fused multiply add and default rounding modes
  21. Why do I get a warning every time I use malloc?
  22. An alternative for the deprecated __malloc_hook functionality of glibc
  23. Implementation of sizeof operator
  24. Standard C library in mingW
  25. Count each bit-position separately over many 64-bit bitmasks, with AVX but not AVX2
  26. How to compile executable for Windows with GCC with Linux Subsystem?
  27. Why is execstack required to execute code on the heap?
  28. gcc detect duplicate symbols/functions in static libraries
  29. Order of fields when using a bit field in C
  30. How much overhead can the -fPIC flag add?

Leave a Comment