How to tell GCC that a pointer argument is always double-word-aligned?

by

If the attributes don’t work, or aren’t an option ….

I’m not sure, but try this:

void vecadd (float * restrict a, float * restrict b, float * restrict c)
{
   a = __builtin_assume_aligned (a, 8);
   b = __builtin_assume_aligned (b, 8);
   c = __builtin_assume_aligned (c, 8);

   for ....

That should tell GCC that the pointers are aligned. From that whether it does what you want depends on whether the compiler can use that information effectively; it might not be smart enough: these optimizations aren’t easy.

Another option might be to wrap the float inside a union that must be 8-byte aligned:

typedef union {
  float f;
  long long dummy;
} aligned_float;

void vedadd (aligned_float * a, ......

I think that should enforce 8-byte alignment, but again, I don’t know if the compiler is smart enough to use it.

More Related Contents:

  1. What is the difference between C, C99, ANSI C and GNU C?
  2. Realistic usage of the C99 ‘restrict’ keyword?
  3. Create statically-linked binary that uses getaddrinfo?
  4. c99 goto past initialization
  5. What is the default C -std standard version for the current GCC (especially on Ubuntu)?
  6. Anonymous union within struct not in c99?
  7. Is there a GCC keyword to allow structure-reordering?
  8. Printf long long int in C with GCC?
  9. Declaring an array of negative length
  10. Is there a 128 bit integer in gcc?
  11. Return value range of the main function
  12. Is an empty initializer list valid C code?
  13. Working of fork() in linux gcc [duplicate]
  14. How does GCC implement variable-length arrays?
  15. Where is PATH_MAX defined in Linux?
  16. How to verify if a void pointer (void *) is one of two data types?
  17. typeof operator in C
  18. Recommended gcc warning options for C [closed]
  19. Build .so file from .c file using gcc command line
  20. How to place a variable at a given absolute address in memory (with GCC)
  21. What is the official status of C99 support in VS2013?
  22. Why do I get different results when I dereference a pointer after freeing it?
  23. How to implement memmove in standard C without an intermediate copy?
  24. Bitshift and integer promotion?
  25. dyld: Library not loaded: ….. How to correctly tell GCC Compiler where to find another static library?
  26. How to set up a cron job to run an executable every hour?
  27. Returning struct containing array
  28. Compiling a C program that uses OpenGl in Mac OS X
  29. Why does malloc allocate a different number of bytes than requested?
  30. & operator optional in function pointer assignment

Leave a Comment