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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); } }
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