Stack Alignment for Numerics

[Don]

dsimcha wrote:
> Does the freezing of the D2 language spec and the publication of TDPL preclude
> fixing low level ABI issues like stack alignment?  I have some numerics code
> that is taking a massive performance hit because the stack keeps ending up
> aligned such that none of my doubles are aligned on 8-byte boundaries,
> resulting in something like a 2x performance hit.
> 
> If not, this is a pretty serious performance problem.  Is there a "standard"
> solution to the stack alignment problem that will allow consistently good
> performance on numerics code that uses double-precision floats?

I agree. See bugzilla 2278. Something that's changed since this issue 
was last raised, is that the DMD backend now has 8-byte stack alignment 
for the Mac compiler. So the hard work has already been done. All that 
would be required to support it on Windows and Linux as well, is to 
enable it, and to align the stack to 8 bytes around every extern(C) call.

As a workaround, I've been doing things like:

// Align the stack to a multiple of 64 bytes
void main()
{
     asm {
         naked;
         mov EBP, ESP;
         and ESP, 0xFFFF_FFC0;
         call alignedmain;
         mov ESP, EBP;
         ret;
     }
}