`#[thread_local]` alignment broken on Windows, GNU ABI

I tried this code:

#![feature(thread_local)]

#[repr(align(256))]
struct HighAlignment(u8);

#[thread_local]
static EXAMPLE1: HighAlignment = const { HighAlignment(0) };
#[thread_local]
static EXAMPLE2: HighAlignment = const { HighAlignment(0) };
#[thread_local]
static EXAMPLE3: HighAlignment = const { HighAlignment(0) };

fn main() {
    println!(
        "{:?}, {:?}, {:?}",
        &EXAMPLE1 as *const HighAlignment,
        &EXAMPLE2 as *const HighAlignment,
        &EXAMPLE3 as *const HighAlignment
    );
}

I expected to see this happen: three 0x100-aligned pointers.

Instead, this happened: metal pipe falling

• x86_64, GNU ABI, real Windows: 0x1f39e953850, 0x1f39e953950, 0x1f39e953a50 (unaligned)
• x86_64, MSVC ABI, real Windows: 0x1c287893900, 0x1c287893a00, 0x1c287893b00 (aligned)
• i686, GNU ABI, real Windows: 0xf9a5f8, 0xf9a6f8, 0xf9a7f8 (unaligned)
• i686, MSVC ABI, real Windows: 0x6d2f00, 0x6d3000, 0x6d3100 (aligned)
• x86_64, GNU ABI, Wine: 0x232c80, 0x232d80, 0x232e80 (unaligned)
• x86_64, MSVC ABI, Wine: 0x1320e0, 0x1321e0, 0x1322e0 (unaligned)
• i686, GNU ABI, Wine: 0x341f28, 0x342028, 0x342128 (unaligned)
• i686, MSVC ABI, Wine: 0x34f010, 0x34f110, 0x34f210 (unaligned)

So either this affects all of Windows and I just got lucky twice, or this is two bugs in a trenchcoat, one affecting GNU ABI and another one in Wine. Not sure which.

Notably, checking mixed-alignment thread locals, like u8, u8, u128, u128, u128, shows that the variables are correctly aligned with respect to each other within TLS, but the TLS base itself seems to be unaligned.

Tentatively marking as I-unsound because of unaligned references bad, feel free to retag.

Meta

rustc --version --verbose:

rustc 1.86.0-nightly (419b3e2d3 2025-01-15)
binary: rustc
commit-hash: 419b3e2d3e350822550eee0e82eeded4d324d584
commit-date: 2025-01-15
host: x86_64-unknown-linux-gnu
release: 1.86.0-nightly
LLVM version: 19.1.6

wine --version: wine-9.22 (Staging)

Tracking issue for visibility: #29594

Start with the supplied #[thread_local] reproducer and compare its pointer alignment across the listed GNU and MSVC ABI, Windows, and Wine configurations. Trace the Windows TLS alignment behavior implicated by the results; done means the three thread-local pointers satisfy the requested 0x100 alignment in the affected configurations, with regression coverage for the observed cases.