Debug mode - Referencing field copies the object (self hosted backend) #25184
Description
Zig Version
0.15.1
Steps to Reproduce and Observed Behavior
I'm not sure if this is a duplicate of #25111 but I didn't think it was, based on the first comment...
The following code (in debug mode, not using llvm) has fn slow running significantly slower than fn fast:
const std = @import("std");
pub fn main() !void {
var foo: Foo = .init();
var slow_data: [100]i128 = undefined;
for (0..100) |i| slow_data[i] = foo.slow();
var fast_data: [100]i128 = undefined;
for (0..100) |i| fast_data[i] = foo.fast();
std.mem.sort(i128, &slow_data, {}, std.sort.asc(i128));
std.mem.sort(i128, &fast_data, {}, std.sort.asc(i128));
std.log.err("min: {} - max: {} - median: {}", .{ slow_data[0], slow_data[99], slow_data[49] });
std.log.err("min: {} - max: {} - median: {}", .{ fast_data[0], fast_data[99], fast_data[49] });
}
const Map = struct {
pub const Self = @This();
tile_size: f32,
data: [144 * 144 * 16]u8 = undefined,
pub fn init(tile_size: f32) Self {
return .{ .tile_size = tile_size };
}
};
const Foo = struct {
pub const Self = @This();
map: Map,
pub fn init() Self {
return .{ .map = .init(16.0) };
}
pub fn slow(self: *Self) i128 {
const start_time = std.time.nanoTimestamp();
const map_size = self.map.tile_size;
_ = map_size;
return std.time.nanoTimestamp() - start_time;
}
pub fn fast(self: *Self) i128 {
const start_time = std.time.nanoTimestamp();
const map_size = &(self.map).tile_size;
_ = map_size;
return std.time.nanoTimestamp() - start_time;
}
};zig run foo.zig -freference-trace
error: min: 5811 - max: 150233 - median: 6301
error: min: 60 - max: 81 - median: 70
zig run -OReleaseSafe foo.zig
error: min: 10 - max: 40 - median: 20
error: min: 10 - max: 30 - median: 20
zig run -fllvm foo.zig
error: min: 20 - max: 140 - median: 30
error: min: 20 - max: 31 - median: 30
I haven't delved into the assembly, but I'm assuming this is causing a copy of Map (used Map.data to emphasize the slowdown)
Expected Behavior
Self hosted backend debug mode should have speeds somewhat similar to debug + llvm, or ReleaseSafe