Great question—let’s dive into your three core technical questions, since you’re exploring those make-or-break 0.1% scenarios where every CPU cycle matters for performance-critical systems.

1. How to completely remove bounds checks in Rust loops? #

Your previous attempts (foo1 and foo2) still had implicit bounds checks because even in an unsafe block, using buffer[buffer_index] or ptr::replace with a slice index doesn’t automatically skip bounds checking—you need to explicitly opt into unchecked memory access. Here are two reliable ways to eliminate all bounds checks for your example:

Option 1: Use get_unchecked_mut for direct unchecked slice access #

This lets you bypass bounds checks while still working with slice abstractions:

pub unsafe fn foo_checked_removed(elements: &Vec<i32>, mut buffer: [i32; 64], pivot: i32) {
    let mut buffer_index: usize = 0;
    for i in 0..64 {
        // Explicitly access buffer without bounds checks
        *buffer.get_unchecked_mut(buffer_index) = i as i32;
        // Unchecked access to elements (since we know i < 64 and elements has at least 64 items)
        buffer_index += (elements.get_unchecked(i) < &pivot) as usize;
    }
}

Option 2: Use raw pointers for maximum control #

For even closer alignment with C-style memory operations, convert slices to raw pointers and manipulate them directly:

pub unsafe fn foo_raw_ptr(elements: &Vec<i32>, buffer: &mut [i32; 64], pivot: i32) {
    let mut buffer_ptr = buffer.as_mut_ptr();
    let elements_ptr = elements.as_ptr();
    
    for i in 0..64 {
        *buffer_ptr = i as i32;
        // Increment buffer pointer only if elements[i] < pivot (no bounds checks)
        if *elements_ptr.add(i) < pivot {
            buffer_ptr = buffer_ptr.add(1);
        }
    }
}

Both approaches will generate assembly identical to optimized C code (assuming LLVM is set to equivalent optimization levels like -O3).

2. Is analyzing these edge cases meaningful? Can Rust optimize full algorithms automatically? #

Are edge cases worth it? #

Absolutely. These 0.1% scenarios are often the core of performance-sensitive systems: high-frequency trading engines, real-time embedded firmware, or low-latency network stacks. Even small performance gaps here can have outsized business or functional impacts. Rust’s safety guarantees are great, but if it can’t match C’s performance in these critical paths, it’s a non-starter for those use cases.

Will full algorithms optimize better than fragments? #

In many cases, yes. LLVM (Rust’s backend) excels at cross-function optimization, loop unrolling, and dead code elimination when it has full context. For your example:

• If the entire algorithm enforces constraints like fixed-length arrays (64 elements) at compile time, LLVM can unroll the loop completely and eliminate conditional branches (matching GCC’s output for the C code).
• Using Rust’s iterator abstractions (instead of manual index loops) can sometimes lead to better optimization, since iterators are designed as zero-cost abstractions that LLVM can easily inline and optimize.
• Adding hints like #[inline(always)] or #[must_use] can help the compiler make better decisions about code layout and optimization.

That said, LLVM and GCC have different optimization heuristics—so you might still need to tweak code (like using raw pointers) for specific edge cases where GCC’s optimizations outperform LLVM.

3. Can Rust be made as "straightforward" and low-level as C? #

Yes, but it’s an opt-in choice. Rust’s design balances safety and control:

• Unsafe Rust gives you all the low-level capabilities of C: raw pointer arithmetic, manual memory allocation/deallocation, direct hardware access, and unchecked memory operations. You can write code that’s just as "bare metal" as C—you just have to explicitly mark those sections with unsafe to acknowledge you’re taking responsibility for memory safety.
• Unlike C, Rust still enforces some guardrails even in unsafe code (e.g., preventing double-frees via ownership rules unless you use raw pointers directly), but these are designed to reduce undefined behavior, not restrict low-level access.
• Rust also provides direct access to inline assembly, memory ordering primitives, and low-level allocation APIs (via std::alloc), making it just as capable as C for system programming.

The key difference is that Rust doesn’t force you to write unsafe code by default—you get memory safety for free in most cases, but you can drop down to C-style low-level code when you need it.

内容的提问来源于stack exchange，提问作者mcmayer