[BUG] struct with multiple SIMD fields rounds sizeof up to nearest power of 2

[mikowals]

Bug description

I expected a struct's sizeof to be the total of the sizeof each of its fields individually. This usually is true but does not seem to be the case for SIMD fields when the sizes don't total to a power of 2. I am not sure if this is really a bug or just something that needs better documentation. But it was unexpected and can be a lot of extra memory use.

Steps to reproduce

Gist from playground or code below:

from sys import sizeof
struct Foo:
  var a: SIMD[DType.uint8, 8]
  var b: SIMD[DType.uint8, 4]
 
def main():
    print("sizeof[Foo](): ", sizeof[Foo]())
    print("sizeof[SIMD[DType.uint8, 8]]():", sizeof[SIMD[DType.uint8, 8]]())
    print("sizeof[SIMD[DType.uint8, 4]]():", sizeof[SIMD[DType.uint8, 4]]())

output:

sizeof[Foo]():  16
sizeof[SIMD[DType.uint8, 8]](): 8
sizeof[SIMD[DType.uint8, 4]](): 4

System information

In playground on 17-May-2024 and locally on:
- macOS Sonoma 14.4.1 (23E224)
- mojo 2024.5.1607 (bd1f55d0)
- modular 0.7.2 (d0adc668)

[bgreni]

This doesn't seem to be exclusive to SIMD so there could be a reason for this behaviour?

struct Bar:
    var d: Int
    var e: Bool
    

fn main():
    print(sizeof[Int]()) # 8
    print(sizeof[Bool]()) # 1
    print(sizeof[Bar]()) # 16
``