On-stack memory alignment

Jay Gowdy81418

Other Parts Discussed in Thread: OMAP-L138

I am using the TI C/C++ C6000 compiler, v7.3.1 on a C6748 in an OMAP-L138.

I would like to know if it is possible to specify alignment for a local array declared on the stack. Using the DATA_ALIGN pragma on a local variable is not allowed. Is there some other way?

Can I get the effect I want by declaring a double precision floating point variable, followed by my array, i.e.,

void foo()
{
double foo;
unsigned char double_aligned_array[256];
.
.

}

If this is effective, will I have to play games to make sure foo doesn't disappear when optimized, i.e., declare it volatile or some such thing?

(And yes, I know that declaring it global and using a DATA_ALIGN there will do it, but for various reasons I really want to ensure a local array's alignment, and I want to know if it is possible).

Jay

over 13 years ago

0 Archaeologist over 13 years ago

TI__Guru* 84285 points

No, there is no pragma which will affect the alignment of local objects, and the alignment of the array is not guaranteed to be affected by any other local variable. You can, however, do it the old-fashioned way, with a union:

void foo()
{
  union
  {
    double foo;
    unsigned char double_aligned_array[256];
  } u;
  u.double_aligned_array[0] = 0;
}

0 Jay Gowdy81418 over 13 years ago in reply to Archaeologist

Intellectual 810 points

Yup, I can see how that would do it, if somewhat messily. Thanks!

(I wouldn't mind a future extension of the DATA_ALIGN pragma to work on local variables, but I understand if that is difficult).

0 Chris Thomas over 13 years ago in reply to Jay Gowdy81418

Expert 1310 points

Have a look in spru187u, 7.5.11.1 gives information on the alignment you get by default.

Chris

0 Jay Gowdy81418 over 13 years ago in reply to Chris Thomas

Intellectual 810 points

Chris,

That was exactly the pointer I needed. I read that section to say that if I declare an array of bytes on the stack, it will be 8-byte aligned (on my C6748) and ready to "match up" to a _nassert statement, which is exactly what I wanted to ensure.

Thanks,

Jay

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On-stack memory alignment