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MIR Shl/Shr: the offset can be computed with rem_euclid
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@ -112,25 +112,20 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
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// Shift ops can have an RHS with a different numeric type.
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if matches!(bin_op, Shl | ShlUnchecked | Shr | ShrUnchecked) {
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let size = left.layout.size.bits();
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let l_bits = left.layout.size.bits();
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// Compute the equivalent shift modulo `size` that is in the range `0..size`. (This is
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// the one MIR operator that does *not* directly map to a single LLVM operation.)
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let (shift_amount, overflow) = if right.layout.abi.is_signed() {
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let shift_amount = r_signed();
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let overflow = shift_amount < 0 || shift_amount >= i128::from(size);
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// Deliberately wrapping `as` casts: shift_amount *can* be negative, but the result
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// of the `as` will be equal modulo `size` (since it is a power of two).
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let masked_amount = (shift_amount as u128) % u128::from(size);
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assert_eq!(overflow, shift_amount != i128::try_from(masked_amount).unwrap());
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(masked_amount, overflow)
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let rem = shift_amount.rem_euclid(l_bits.into());
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// `rem` is guaranteed positive, so the `unwrap` cannot fail
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(u128::try_from(rem).unwrap(), rem != shift_amount)
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} else {
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let shift_amount = r_unsigned();
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let overflow = shift_amount >= u128::from(size);
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let masked_amount = shift_amount % u128::from(size);
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assert_eq!(overflow, shift_amount != masked_amount);
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(masked_amount, overflow)
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let rem = shift_amount.rem_euclid(l_bits.into());
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(rem, rem != shift_amount)
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};
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let shift_amount = u32::try_from(shift_amount).unwrap(); // we masked so this will always fit
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let shift_amount = u32::try_from(shift_amount).unwrap(); // we brought this in the range `0..size` so this will always fit
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// Compute the shifted result.
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let result = if left.layout.abi.is_signed() {
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let l = l_signed();
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@ -1490,7 +1490,8 @@ pub enum BinOp {
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BitOr,
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/// The `<<` operator (shift left)
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///
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/// The offset is (uniquely) determined as follows:
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/// The offset is given by `RHS.rem_euclid(LHS::BITS)`.
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/// In other words, it is (uniquely) determined as follows:
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/// - it is "equal modulo LHS::BITS" to the RHS
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/// - it is in the range `0..LHS::BITS`
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Shl,
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@ -1498,7 +1499,8 @@ pub enum BinOp {
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ShlUnchecked,
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/// The `>>` operator (shift right)
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///
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/// The offset is (uniquely) determined as follows:
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/// The offset is given by `RHS.rem_euclid(LHS::BITS)`.
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/// In other words, it is (uniquely) determined as follows:
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/// - it is "equal modulo LHS::BITS" to the RHS
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/// - it is in the range `0..LHS::BITS`
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///
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