speice.io/_posts/2019-02-08-compiler-optimizations.md

layout	title	description	category	tags
post	Compiler Optimizations: What It's Done Lately	A lot. The answer is a lot.		rust understanding-allocations

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.

But 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 main() {
    // Turn on panicking if we allocate on the heap
    DO_PANIC.store(true, Ordering::SeqCst);
    
    // This code will only run with the mode set to "Release".
    // If you try running in "Debug", you'll get a panic.
    let x = Box::new(0);
    drop(x);
    
    // Turn off panicking, as there are some deallocations
    // when we exit main.
    DO_PANIC.store(false, Ordering::SeqCst);
}

#[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);
    }
}

-- Rust Playground

Vectors of Usual Size

With some collections, LLVM can predict how large they will become and allocate the entire size on the stack instead of the heap. This works with both the pre-allocation (Vec::with_capacity) and re-allocation (Vec::push) methods for collection types. Not only can LLVM predict sizing if you reserve everything up front, it can see through the resizing operations and find the total size. While this specific optimization is unlikely to come up in production usage, it's cool to note that LLVM does a considerable amount of work to understand what the code will do:

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

fn main() {
    // Turn on panicking if we allocate on the heap
    DO_PANIC.store(true, Ordering::SeqCst);
    
    // If the compiler can predict how large a vector will be,
    // it can optimize out the heap storage needed.
    let mut x: Vec<u64> = Vec::new();
    x.push(12);
    
    let mut y: Vec<u64> = Vec::with_capacity(1);
    y.push(12);
    
    assert_eq!(x[0], y[0]);
    drop(x);
    drop(y);
    
    // Turn off panicking, as there are some deallocations
    // when we exit main.
    DO_PANIC.store(false, Ordering::SeqCst);
}

#[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);
    }
}

-- 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();
}

-- Compiler Explorer
-- Rust Playground