Miscompliation due to LLVM IR alignment issue (?)

[cbeuw]

Reproduction:

#![feature(custom_mir, core_intrinsics)]
#![allow(unused_parens, unused_assignments, overflowing_literals)]
extern crate core;
use core::intrinsics::mir::*;

pub fn enter() {
    fn13(core::hint::black_box((0, [false; 6], [0; 4], 0, 0)));
}

#[custom_mir(dialect = "runtime", phase = "initial")]
fn fn13(mut _6: (i8, [bool; 6], [i64; 4], i128, u8)) {
    mir! {
    let _1: [bool; 6];
    let _14: [u16; 6];
    let _17: *const (i8, [bool; 6], [i64; 4], i128, u8);
    let _19: Adt59;
    let _27: (i8, [bool; 6], [i64; 4], i128, u8);
    let _34: Adt54;
    let _49: ();
    {
    _1 = [false,true,true,true,true,true];
    _19.fld4 = [25264_u16,54664_u16,47032_u16,55453_u16,33511_u16,43043_u16];
    _17 = core::ptr::addr_of!(_6);
    _1 = (*_17).1;
    (*_17).1 = [false,true,true,false,false,true];
    Call(_49 = fn14(_6.1), ReturnTo(bb3), UnwindUnreachable())
    }
    bb3 = {
    (*_17).1 = _1;
    _14 = _19.fld4;
    _17 = core::ptr::addr_of!(_27);
    _34.fld5 = (-1228700359_i32);
    _34.fld1 = _6;
    match _34.fld5 {
    0 => bb3,
    340282366920938463463374607430539511097 => bb12,
    _ => bb18
    }
    }
    bb12 = {
    (*_17) = (_6.0, _6.1, _6.2, _34.fld1.3, _6.4);
    _1 = _27.1;
    Call(_49 = p1(Move(_1)), ReturnTo(bb17), UnwindUnreachable())
    }
    bb17 = {
    Call(_49 = hide(13_usize, 25_usize, 0_usize, 11_usize, 0_usize, 14_usize, Move(_14), 6_usize, Move(_6)), ReturnTo(bb18), UnwindUnreachable())
    }
    bb18 = {
    Return()
    }

    }
}

#[custom_mir(dialect = "runtime", phase = "initial")]
pub fn fn14(mut _16: [bool; 6]) {
    mir! {
    let _20: (i8, [bool; 6], [i64; 4], i128, u8);
    let unit: ();
    let _44: ();
    {
    Call(_44 = core::hint::black_box(unit), ReturnTo(bb17), UnwindUnreachable())
    }
    bb17 = {
    Call(_20.1 = core::hint::black_box(Move(_16)), ReturnTo(bb18), UnwindUnreachable())
    }
    bb18 = {
    Return()
    }

    }
}
pub fn main() {
    enter();
}

#[derive(Debug)]
pub struct Adt54 {
    fld1: (i8, [bool; 6], [i64; 4], i128, u8),
    fld5: i32,
}
#[derive(Debug)]
pub struct Adt59 {
    fld4: [u16; 6],
}

extern "C" {
    fn printf(fmt: *const core::ffi::c_char, ...) -> core::ffi::c_int;
}
#[cfg(not(miri))]
fn print_bool(x: bool) {
    unsafe {
        printf(b"%d\n\0".as_ptr().cast(), x as core::ffi::c_int);
    }
}

#[cfg(miri)]
fn print_bool(x: bool) {
    println!("{}", x as i32);
}

#[inline(never)]
pub fn p1(x: [bool; 6]) {
    for b in x {
        print_bool(b);
    }
}

#[inline(never)]
fn hide(
    f: usize,
    var0: usize,
    val0: usize,
    var1: usize,
    val1: usize,
    var2: usize,
    val2: [u16; 6],
    var3: usize,
    val3: (i8, [bool; 6], [i64; 4], i128, u8),
) {
    core::hint::black_box(val2);
    core::hint::black_box(val3);
}

Miri and -Copt-level <= 2 prints

0
0
0
0
0
0

But -Zmir-opt-level=0 -Copt-level=3 prints

0
0
1
0
0
0

I reported this to LLVM llvm/llvm-project#80911 but was told that the IR generated from MIR has alignment issues.

I really don't know much about LLVM IR but I suspect the problem is

%Adt54 = type { { [4 x i64], i128, [6 x i8], i8, i8, [8 x i8] }, i32, [3 x i32] }
[snip]
  %_7 = alloca %Adt54, align 16
[snip]
  %30 = getelementptr inbounds %Adt54, ptr %_7, i32 0, i32 1
  store i32 -1228700359, ptr %30, align 16
[snip]
  %31 = getelementptr inbounds %Adt54, ptr %_7, i32 0, i32 1
  %32 = load i32, ptr %31, align 16, !noundef !5
[snip]

The second element of %Adt54 (i32) is not guaranteed to be 16-byte aligned

cc @nikic

[saethlin]

Can you reproduce this without using u128/i128? And/or does this reproduce with LLVM 18? #120055

[cbeuw]

It's still reproducible on nikic's llvm-18 branch. If i replace all i128 with i64 then it no longer reproduces, but of course I don't know if there are other examples out there that triggers the same bug without i128

[saethlin]

Ah; what I meant is can you reproduce this issue using a type which in Rust has the same layout as i128, but which is not i128? Perhaps #[repr(align(16))] struct I128([u64; 2]);?

[cbeuw]

Yes, still reproducible after replacing it with a repr(align(16)) struct (both zero-sized and with members)

#![feature(custom_mir, core_intrinsics)]
#![allow(unused_parens, unused_assignments, overflowing_literals)]
extern crate core;
use core::intrinsics::mir::*;

#[repr(align(16))]
struct I128();

pub fn enter() {
    fn13(core::hint::black_box((0, [false; 6], [0; 4], I128(), 0)));
}

#[custom_mir(dialect = "runtime", phase = "initial")]
fn fn13(mut _6: (i8, [bool; 6], [i64; 4], I128, u8)) {
    mir! {
    let _1: [bool; 6];
    let _14: [u16; 6];
    let _17: *const (i8, [bool; 6], [i64; 4], I128, u8);
    let _19: Adt59;
    let _27: (i8, [bool; 6], [i64; 4], I128, u8);
    let _34: Adt54;
    let _49: ();
    {
    _1 = [false,true,true,true,true,true];
    _19.fld4 = [25264_u16,54664_u16,47032_u16,55453_u16,33511_u16,43043_u16];
    _17 = core::ptr::addr_of!(_6);
    _1 = (*_17).1;
    (*_17).1 = [false,true,true,false,false,true];
    Call(_49 = fn14(_6.1), ReturnTo(bb3), UnwindUnreachable())
    }
    bb3 = {
    (*_17).1 = _1;
    _14 = _19.fld4;
    _17 = core::ptr::addr_of!(_27);
    _34.fld5 = (-1228700359_i32);
    _34.fld1 = _6;
    match _34.fld5 {
    0 => bb3,
    340282366920938463463374607430539511097 => bb12,
    _ => bb18
    }
    }
    bb12 = {
    (*_17) = (_6.0, _6.1, _6.2, _34.fld1.3, _6.4);
    _1 = _27.1;
    Call(_49 = p1(Move(_1)), ReturnTo(bb17), UnwindUnreachable())
    }
    bb17 = {
    Call(_49 = hide(13_usize, 25_usize, 0_usize, 11_usize, 0_usize, 14_usize, Move(_14), 6_usize, Move(_6)), ReturnTo(bb18), UnwindUnreachable())
    }
    bb18 = {
    Return()
    }

    }
}

#[custom_mir(dialect = "runtime", phase = "initial")]
pub fn fn14(mut _16: [bool; 6]) {
    mir! {
    let _20: (i8, [bool; 6], [i64; 4], I128, u8);
    let unit: ();
    let _44: ();
    {
    Call(_44 = core::hint::black_box(unit), ReturnTo(bb17), UnwindUnreachable())
    }
    bb17 = {
    Call(_20.1 = core::hint::black_box(Move(_16)), ReturnTo(bb18), UnwindUnreachable())
    }
    bb18 = {
    Return()
    }

    }
}
pub fn main() {
    enter();
}

pub struct Adt54 {
    fld1: (i8, [bool; 6], [i64; 4], I128, u8),
    fld5: i32,
}
#[derive(Debug)]
pub struct Adt59 {
    fld4: [u16; 6],
}

extern "C" {
    fn printf(fmt: *const core::ffi::c_char, ...) -> core::ffi::c_int;
}
#[cfg(not(miri))]
fn print_bool(x: bool) {
    unsafe {
        printf(b"%d\n\0".as_ptr().cast(), x as core::ffi::c_int);
    }
}

#[cfg(miri)]
fn print_bool(x: bool) {
    println!("{}", x as i32);
}

#[inline(never)]
pub fn p1(x: [bool; 6]) {
    for b in x {
        print_bool(b);
    }
}

#[inline(never)]
fn hide(
    f: usize,
    var0: usize,
    val0: usize,
    var1: usize,
    val1: usize,
    var2: usize,
    val2: [u16; 6],
    var3: usize,
    val3: (i8, [bool; 6], [i64; 4], I128, u8),
) {
    core::hint::black_box(val2);
    core::hint::black_box(val3);
}

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high