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Compiler Optimizations: What It's Done Lately


A lot. The answer is a lot.

  • rust
  • understanding-allocations

Update 2019-02-10: When debugging a related issue, it was discovered that the original code worked because LLVM optimized out the entire function, rather than just the allocation segments. The code has been updated with proper use of read_volatile, and a previous section on vector capacity has been removed.


Up to this point, we’ve been discussing memory usage in the Rust language by focusing on simple rules that are mostly right for small chunks of code. We’ve spent time showing how those rules work themselves out in practice, and become familiar with reading the assembly code needed to see each memory type (global, stack, heap) in action.

Throughout the series so far, we’ve put a handicap on the code. In the name of consistent and understandable results, we’ve asked the compiler to pretty please leave the training wheels on. Now is the time where we throw out all the rules and take off the kid gloves. As it turns out, both the Rust compiler and the LLVM optimizers are incredibly sophisticated, and we’ll step back and let them do their job.

Similar to “What Has My Compiler Done For Me Lately?”, we’re focusing on interesting things the Rust language (and LLVM!) can do with memory management. We’ll still be looking at assembly code to understand what’s going on, but it’s important to mention again: please use automated tools like alloc-counter to double-check memory behavior if it’s something you care about. It’s far too easy to mis-read assembly in large code sections, you should always verify behavior if you care about memory usage.

The guiding principal as we move forward is this: optimizing compilers won’t produce worse programs than we started with. There won’t be any situations where stack allocations get moved to heap allocations. There will, however, be an opera of optimization.

The Case of the Disappearing Box

Our first optimization comes when LLVM can reason that the lifetime of an object is sufficiently short that heap allocations aren’t necessary. In these cases, LLVM will move the allocation to the stack instead! The way this interacts with #[inline] attributes is a bit opaque, but the important part is that LLVM can sometimes do better than the baseline Rust language:

use std::alloc::{GlobalAlloc, Layout, System};
use std::sync::atomic::{AtomicBool, Ordering};

pub fn cmp(x: u32) {
    // Turn on panicking if we allocate on the heap
    DO_PANIC.store(true, Ordering::SeqCst);
    
    // The compiler is able to see through the constant `Box`
    // and directly compare `x` to 24 - assembly line 73
    let y = Box::new(24);
    let equals = x == *y;

    // This call to drop is eliminated
    drop(y);

    // Need to mark the comparison result as volatile so that
    // LLVM doesn't strip out all the code. If `y` is marked
    // volatile instead, allocation will be forced.
    unsafe { std::ptr::read_volatile(&equals) };
    
    // Turn off panicking, as there are some deallocations
    // when we exit main.
    DO_PANIC.store(false, Ordering::SeqCst);
}

fn main() {
    cmp(12)
}

#[global_allocator]
static A: PanicAllocator = PanicAllocator;
static DO_PANIC: AtomicBool = AtomicBool::new(false);
struct PanicAllocator;

unsafe impl GlobalAlloc for PanicAllocator {
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        if DO_PANIC.load(Ordering::SeqCst) {
            panic!("Unexpected allocation.");
        }
        System.alloc(layout)
    }
    
    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
        if DO_PANIC.load(Ordering::SeqCst) {
            panic!("Unexpected deallocation.");
        }
        System.dealloc(ptr, layout);
    }
}

Compiler Explorer
Rust Playground

Dr. Array or: How I Learned to Love the Optimizer

Finally, this isn’t so much about LLVM figuring out different memory behavior, but LLVM stripping out code that doesn’t do anything. Optimizations of this type have a lot of nuance to them; if you’re not careful, they can make your benchmarks look impossibly good. In Rust, the black_box function (implemented in both libtest and criterion) will tell the compiler to disable this kind of optimization. But if you let LLVM remove unnecessary code, you can end up running programs that previously caused errors:

#[derive(Default)]
struct TwoFiftySix {
    _a: [u64; 32]
}

#[derive(Default)]
struct EightK {
    _a: [TwoFiftySix; 32]
}

#[derive(Default)]
struct TwoFiftySixK {
    _a: [EightK; 32]
}

#[derive(Default)]
struct EightM {
    _a: [TwoFiftySixK; 32]
}

pub fn main() {
    // Normally this blows up because we can't reserve size on stack
    // for the `EightM` struct. But because the compiler notices we
    // never do anything with `_x`, it optimizes out the stack storage
    // and the program completes successfully.
    let _x = EightM::default();
}

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