MIR Shl/Shr: the offset can be computed with rem_euclid

compiler/rustc_const_eval/src/interpret/operator.rs

@@ -112,25 +112,20 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
 
  // Shift ops can have an RHS with a different numeric type.
  if matches!(bin_op, Shl | ShlUnchecked | Shr | ShrUnchecked) {
- let size = left.layout.size.bits();
+ let l_bits = left.layout.size.bits();
  // Compute the equivalent shift modulo `size` that is in the range `0..size`. (This is
  // the one MIR operator that does *not* directly map to a single LLVM operation.)
  let (shift_amount, overflow) = if right.layout.abi.is_signed() {
  let shift_amount = r_signed();
- let overflow = shift_amount < 0 || shift_amount >= i128::from(size);
- // Deliberately wrapping `as` casts: shift_amount *can* be negative, but the result
- // of the `as` will be equal modulo `size` (since it is a power of two).
- let masked_amount = (shift_amount as u128) % u128::from(size);
- assert_eq!(overflow, shift_amount != i128::try_from(masked_amount).unwrap());
- (masked_amount, overflow)
+ let rem = shift_amount.rem_euclid(l_bits.into());
+ // `rem` is guaranteed positive, so the `unwrap` cannot fail
+ (u128::try_from(rem).unwrap(), rem != shift_amount)
  } else {
  let shift_amount = r_unsigned();
- let overflow = shift_amount >= u128::from(size);
- let masked_amount = shift_amount % u128::from(size);
- assert_eq!(overflow, shift_amount != masked_amount);
- (masked_amount, overflow)
+ let rem = shift_amount.rem_euclid(l_bits.into());
+ (rem, rem != shift_amount)
  };
- let shift_amount = u32::try_from(shift_amount).unwrap(); // we masked so this will always fit
+ let shift_amount = u32::try_from(shift_amount).unwrap(); // we brought this in the range `0..size` so this will always fit
  // Compute the shifted result.
  let result = if left.layout.abi.is_signed() {
  let l = l_signed();

compiler/rustc_middle/src/mir/syntax.rs

@@ -1490,15 +1490,17 @@ pub enum BinOp {
  BitOr,
  /// The `<<` operator (shift left)
  ///
- /// The offset is (uniquely) determined as follows:
+ /// The offset is given by `RHS.rem_euclid(LHS::BITS)`.
+ /// In other words, it is (uniquely) determined as follows:
  /// - it is "equal modulo LHS::BITS" to the RHS
  /// - it is in the range `0..LHS::BITS`
  Shl,
  /// Like `Shl`, but is UB if the RHS >= LHS::BITS or RHS < 0
  ShlUnchecked,
  /// The `>>` operator (shift right)
  ///
- /// The offset is (uniquely) determined as follows:
+ /// The offset is given by `RHS.rem_euclid(LHS::BITS)`.
+ /// In other words, it is (uniquely) determined as follows:
  /// - it is "equal modulo LHS::BITS" to the RHS
  /// - it is in the range `0..LHS::BITS`
  ///