Bug: local array clobbered by PRNG call?

[david-vanderson]

This reproduces for me on zig-macos-x86_64-0.8.0-dev.2641+55811d8da but seems quite sensitive to rearranging statements. I looked at the assembly for a while but it's my first time trying to debug something like this. Any help or pointers would be appreciated. Can anyone reproduce this?

const std = @import( "std" );

pub fn main( ) void {
    // Problem only happens with Xoroshira128, not other PRNGs
    // Problem happens regardless of PRNG seed
    var g = std.rand.Xoroshiro128.init( 0 ).random;

    // Problem happens with some array sizes but not others
    var array = [_]i32 { 0, 0 };

    // Prints "0x0, 0x0"
    std.debug.print( "before: 0x{x}, 0x{x}\n", .{ array[0], array[1] } );

    // Problem happens regardless of what PRNG call we make here
    var x = g.int( i32 );

    // Prints garbage
    std.debug.print( " after: 0x{x}, 0x{x}\n", .{ array[0], array[1] } );
}

[SpexGuy]

var g = std.rand.Xoroshiro128.init( 0 ).random;

This line "truncates" the random interface, creating a copy of the vtable but not the extra data needed for the implementation. When you call g.int() it tries to get the extended data relative to g, which accesses stack memory that in this case happens to overlap with array.

The recommended way to write this to avoid truncation is

var rand_impl = std.rand.Xoroshiro128.init( 0 );
const rand = &rand_impl.random;

Once implemented, pinned structs (#7769) will make this mistake a compile error.

[david-vanderson]

Ahhh - Thank you!