Is placing data with align(32) on the stack with 16-byte alignment an error?

[Johan Engelen via Digitalmars-d]

On Sunday, 29 May 2016 at 12:07:02 UTC, Marco Leise wrote:
> 
> void main() {
>     import core.simd;
>     Matrix4x4 matrix;  // No warning
>     float8 vector;     // No warning
> }

Did you do some LDC IR/asm testing?

With LDC, the type `float8` has 32-byte alignment and so will be 
placed with that alignment on the stack. For your Matrix4x4 user 
type (I'll assume you meant to write `align(64)`), that alignment 
becomes part of the type and will be put on the stack with 
64-byte alignment. (aliasing does not work: `alias Byte8 = 
align(8) byte; Byte8 willBeUnaligned;`)
I believe LDC respects the type's alignment when selecting 
instructions, so when you specified align(32) byte for your type 
it can use the aligned load instructions. If you did not specify 
that alignment, or a lower alignment, it will use unaligned loads.

A problem arises when you cast a (pointer of a) type with lower 
alignment to a type with higher alignment; in that case, 
currently LDC assumes that cast was valid in terms of alignment 
and <boom>!

-Johan