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Make intrinsics::write_bytes const #90081

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Dec 12, 2021
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24 changes: 24 additions & 0 deletions compiler/rustc_const_eval/src/interpret/intrinsics.rs
Original file line number Diff line number Diff line change
Expand Up @@ -322,6 +322,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
sym::copy => {
self.copy_intrinsic(&args[0], &args[1], &args[2], /*nonoverlapping*/ false)?;
}
sym::write_bytes => {
self.write_bytes_intrinsic(&args[0], &args[1], &args[2])?;
}
sym::offset => {
let ptr = self.read_pointer(&args[0])?;
let offset_count = self.read_scalar(&args[1])?.to_machine_isize(self)?;
Expand Down Expand Up @@ -543,6 +546,27 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
self.memory.copy(src, align, dst, align, size, nonoverlapping)
}

pub(crate) fn write_bytes_intrinsic(
&mut self,
dst: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::PointerTag>,
byte: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::PointerTag>,
count: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::PointerTag>,
) -> InterpResult<'tcx> {
let layout = self.layout_of(dst.layout.ty.builtin_deref(true).unwrap().ty)?;

let dst = self.read_pointer(&dst)?;
let byte = self.read_scalar(&byte)?.to_u8()?;
let count = self.read_scalar(&count)?.to_machine_usize(self)?;

let len = layout
.size
.checked_mul(count, self)
.ok_or_else(|| err_ub_format!("overflow computing total size of `write_bytes`"))?;

let bytes = std::iter::repeat(byte).take(len.bytes_usize());
self.memory.write_bytes(dst, bytes)
}

pub(crate) fn raw_eq_intrinsic(
&mut self,
lhs: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::PointerTag>,
Expand Down
20 changes: 18 additions & 2 deletions library/core/src/intrinsics.rs
Original file line number Diff line number Diff line change
Expand Up @@ -2242,13 +2242,29 @@ pub const unsafe fn copy<T>(src: *const T, dst: *mut T, count: usize) {
/// assert_eq!(*v, 42);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
#[inline]
pub unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize) {
pub const unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize) {
extern "rust-intrinsic" {
#[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
fn write_bytes<T>(dst: *mut T, val: u8, count: usize);
}

debug_assert!(is_aligned_and_not_null(dst), "attempt to write to unaligned or null pointer");
#[cfg(debug_assertions)]
fn runtime_check<T>(ptr: *mut T) {
debug_assert!(
is_aligned_and_not_null(ptr),
"attempt to write to unaligned or null pointer"
);
}
#[cfg(debug_assertions)]
const fn compiletime_check<T>(_ptr: *mut T) {}
#[cfg(debug_assertions)]
// SAFETY: runtime debug-assertions are a best-effort basis; it's fine to
// not do them during compile time
unsafe {
const_eval_select((dst,), compiletime_check, runtime_check);
}

// SAFETY: the safety contract for `write_bytes` must be upheld by the caller.
unsafe { write_bytes(dst, val, count) }
Expand Down
30 changes: 30 additions & 0 deletions library/core/tests/intrinsics.rs
Original file line number Diff line number Diff line change
Expand Up @@ -35,3 +35,33 @@ fn test_assume_can_be_in_const_contexts() {
let rs = unsafe { foo(42, 97) };
assert_eq!(rs, 0);
}

#[test]
#[cfg(not(bootstrap))]
const fn test_write_bytes_in_const_contexts() {
use core::intrinsics::write_bytes;

const TEST: [u32; 3] = {
let mut arr = [1u32, 2, 3];
unsafe {
write_bytes(arr.as_mut_ptr(), 0, 2);
}
arr
};

assert!(TEST[0] == 0);
assert!(TEST[1] == 0);
assert!(TEST[2] == 3);

const TEST2: [u32; 3] = {
let mut arr = [1u32, 2, 3];
unsafe {
write_bytes(arr.as_mut_ptr(), 1, 2);
}
arr
};

assert!(TEST2[0] == 16843009);
assert!(TEST2[1] == 16843009);
assert!(TEST2[2] == 3);
}