Miscompilation with array inside of anonymous struct

[LiterallyVoid]

This code:

const std = @import("std");

pub noinline fn outer() u32 {
    var a: u32 = 42;
    return inner(.{
        .unused = a,
        .value = [1]u32{0},
    });
}

pub noinline fn inner(args: anytype) u32 {
    return args.value[0];
}

test "foo" {
    std.debug.warn("value: {}\n", .{outer()});
}

prints this:

1/1 test "foo"... value: 32767
OK
All 1 tests passed.

Relevant LLVM IR:

define internal fastcc i32 @outer() unnamed_addr #7 !dbg !2866 {
Entry:
  %result = alloca i32, align 4
  %a = alloca i32, align 4
  %0 = alloca %"struct:5:19", align 4
  store i32 42, i32* %a, align 4, !dbg !2872
  call void @llvm.dbg.declare(metadata i32* %a, metadata !2870, metadata !DIExpression()), !dbg !2872
  %1 = load i32, i32* %a, align 4, !dbg !2873
  %2 = getelementptr inbounds %"struct:5:19", %"struct:5:19"* %0, i32 0, i32 0, !dbg !2873
  store i32 %1, i32* %2, align 4, !dbg !2873
  %3 = call fastcc i32 @inner(%"struct:5:19"* %0), !dbg !2874
  store i32 %3, i32* %result, align 4, !dbg !2874
  %4 = load i32, i32* %result, align 4, !dbg !2875
  ret i32 %4, !dbg !2875
}

define internal fastcc i32 @inner(%"struct:5:19"* nonnull readonly align 4 %0) unnamed_addr #7 !dbg !3325 {
Entry:
  %result = alloca i32, align 4
  call void @llvm.dbg.declare(metadata %"struct:5:19"* %0, metadata !3337, metadata !DIExpression()), !dbg !3338
  %1 = getelementptr inbounds %"struct:5:19", %"struct:5:19"* %0, i32 0, i32 1, !dbg !3339
  %2 = getelementptr inbounds [1 x i32], [1 x i32]* %1, i64 0, i64 0, !dbg !3341
  %3 = load i32, i32* %2, align 4, !dbg !3341
  store i32 %3, i32* %result, align 4, !dbg !3341
  %4 = load i32, i32* %result, align 4, !dbg !3342
  ret i32 %4, !dbg !3342
}

Note how in @outer, the only store is into element 0 of the structure (which is unused.)

Workarounds

Put the array value into a constant.

pub noinline fn outer() u32 {
    var a: u32 = 42;
    const value = [1]u32{0};
    return inner(.{
        .unused = a,
        .value = value,
    });
}

Don't use an anonymous struct.

const S = struct {
    unused: u32,
    value: [1]u32,
};

pub noinline fn outer() u32 {
    var a: u32 = 42;
    return inner(S{
        .unused = a,
        .value = [1]u32{0},
    });
}