Vector sizes are different in x86_64 backend

[IntegratedQuantum]

Zig Version

0.15.0-dev.828+3ce8d19f7

Steps to Reproduce and Observed Behavior

pub fn main() !void {
	@compileLog(@sizeOf(@Vector(2, f32)));
}

Observe that this prints different values depending on the backend

$ zig-debug run test.zig
...
Compile Log Output:
@as(comptime_int, 16)
$ zig-debug run test.zig -fllvm
...
Compile Log Output:
@as(comptime_int, 8)

Expected Behavior

The size of vector types, which are usable in extern structs, must be independent of the backend.
Current behavior violates the promise that extern structs have a well-defined in-memory layout.

[alexrp]

Labeling for Jacob to triage; not sure if this is in fact an x86_64 backend bug.

[mlugg]

Oops, I meant to reply to this issue (I'm not Jacob but I know about this :P).

Because the optimal vector representation is highly target-dependent, it's intended that vectors have ill-defined layout, meaning nothing about their in-memory representation, including their @sizeOf/@alignOf, is guaranteed. We're currently in a bit of a transitional period, where the compiler claims the layout is well-defined, but it actually differs per backend. Once #19755 is implemented (that should come first so we don't temporarily break everyones' bitcasts), vectors can be marked in the compiler as having ill-defined layout, so that making assumptions about their representation actually invokes Illegal Behavior.

Passing vectors over ABI boundaries is currently an unsolved problem.

In short: working as intended. cc @jacobly0 to confirm this is all accurate