Function doNotOptimizeAway [src]

Force an evaluation of the expression; this tries to prevent the compiler from optimizing the computation away even if the result eventually gets discarded.

Prototype

pub fn doNotOptimizeAway(val: anytype) void

Example

 test doNotOptimizeAway {
    comptime doNotOptimizeAway("test");

    doNotOptimizeAway(null);
    doNotOptimizeAway(true);
    doNotOptimizeAway(0);
    doNotOptimizeAway(0.0);
    doNotOptimizeAway(@as(u1, 0));
    doNotOptimizeAway(@as(u3, 0));
    doNotOptimizeAway(@as(u8, 0));
    doNotOptimizeAway(@as(u16, 0));
    doNotOptimizeAway(@as(u32, 0));
    doNotOptimizeAway(@as(u64, 0));
    doNotOptimizeAway(@as(u128, 0));
    doNotOptimizeAway(@as(u13, 0));
    doNotOptimizeAway(@as(u37, 0));
    doNotOptimizeAway(@as(u96, 0));
    doNotOptimizeAway(@as(u200, 0));
    doNotOptimizeAway(@as(f32, 0.0));
    doNotOptimizeAway(@as(f64, 0.0));
    doNotOptimizeAway([_]u8{0} ** 4);
    doNotOptimizeAway([_]u8{0} ** 100);
    doNotOptimizeAway(@as(std.builtin.Endian, .little));
}

Source

  pub fn doNotOptimizeAway(val: anytype) void {
    if (@inComptime()) return;

    const max_gp_register_bits = @bitSizeOf(c_long);
    const t = @typeInfo(@TypeOf(val));
    switch (t) {
        .void, .null, .comptime_int, .comptime_float => return,
        .@"enum" => doNotOptimizeAway(@intFromEnum(val)),
        .bool => doNotOptimizeAway(@intFromBool(val)),
        .int => {
            const bits = t.int.bits;
            if (bits <= max_gp_register_bits and builtin.zig_backend != .stage2_c) {
                const val2 = @as(
                    std.meta.Int(t.int.signedness, @max(8, std.math.ceilPowerOfTwoAssert(u16, bits))),
                    val,
                );
                asm volatile (""
                    :
                    : [val2] "r" (val2),
                );
            } else doNotOptimizeAway(&val);
        },
        .float => {
            if ((t.float.bits == 32 or t.float.bits == 64) and builtin.zig_backend != .stage2_c) {
                asm volatile (""
                    :
                    : [val] "rm" (val),
                );
            } else doNotOptimizeAway(&val);
        },
        .pointer => {
            if (builtin.zig_backend == .stage2_c) {
                doNotOptimizeAwayC(val);
            } else {
                asm volatile (""
                    :
                    : [val] "m" (val),
                    : "memory"
                );
            }
        },
        .array => {
            if (t.array.len * @sizeOf(t.array.child) <= 64) {
                for (val) |v| doNotOptimizeAway(v);
            } else doNotOptimizeAway(&val);
        },
        else => doNotOptimizeAway(&val),
    }
}  