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mrustc (master)
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pub struct C (*const (), usize, u32, u16, u8, u8); const LENGTH_MASK: u8 = 192; const HEAP_MASK: u8 = 216; const HEAP_MASK_AFTER_SUB: usize = (HEAP_MASK as usize) .wrapping_sub(LENGTH_MASK as usize); pub fn is_empty(c: &C) -> bool { let len_heap = ensure_read(c.1); let last_byte = c.5 as usize; let mut len = last_byte.wrapping_sub(LENGTH_MASK as usize); if last_byte >= HEAP_MASK as usize { len = len_heap; } len == 0 } pub fn is_empty_new(c: &C) -> bool { let len_heap = ensure_read(c.1); let last_byte = c.5 as usize; let mut len = last_byte.wrapping_sub(LENGTH_MASK as usize); // NB `==` not `>=` if len == HEAP_MASK_AFTER_SUB { len = len_heap; } len == 0 } pub fn is_empty_new2(c: &C) -> bool { let len_heap = c.1; let last_byte = c.5 as usize; let len = last_byte.wrapping_sub(LENGTH_MASK as usize); let is_empty_inline = len == 0; // NB `==` not `>=` let is_empty_heap = len == HEAP_MASK_AFTER_SUB && len_heap == 0; is_empty_inline || is_empty_heap } /// Returns the supplied value, and ensures that the value is eagerly loaded into a register. #[inline(always)] fn ensure_read(value: usize) -> usize { // SAFETY: This assembly instruction is a noop that only affects the instruction ordering. #[cfg(all( not(miri), any( target_arch = "x86", target_arch = "x86_64", target_arch = "arm", target_arch = "aarch64", target_arch = "loongarch", target_arch = "riscv", ) ))] unsafe { core::arch::asm!( "/* {value} */", value = in(reg) value, options(nomem, nostack), ); }; value }
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