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Rollup merge of #126469 - RalfJung:mir-shifts, r=scottmcm

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

r? ````@scottmcm````
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Matthias Krüger 2024-06-15 10:56:41 +02:00 committed by GitHub
commit f24509aa23
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2 changed files with 11 additions and 14 deletions
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19
compiler/rustc_const_eval/src/interpret/operator.rs
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@ -112,25 +112,20 @@ fn binary_int_op(
// Shift ops can have an RHS with a different numeric type. // Shift ops can have an RHS with a different numeric type.
if matches!(bin_op, Shl | ShlUnchecked | Shr | ShrUnchecked) { 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 // 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.) // 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, overflow) = if right.layout.abi.is_signed() {
let shift_amount = r_signed(); let shift_amount = r_signed();
let overflow = shift_amount < 0 || shift_amount >= i128::from(size); let rem = shift_amount.rem_euclid(l_bits.into());
// Deliberately wrapping `as` casts: shift_amount *can* be negative, but the result // `rem` is guaranteed positive, so the `unwrap` cannot fail
// of the `as` will be equal modulo `size` (since it is a power of two). (u128::try_from(rem).unwrap(), rem != shift_amount)
let masked_amount = (shift_amount as u128) % u128::from(size);
assert_eq!(overflow, shift_amount != i128::try_from(masked_amount).unwrap());
(masked_amount, overflow)
} else { } else {
let shift_amount = r_unsigned(); let shift_amount = r_unsigned();
let overflow = shift_amount >= u128::from(size); let rem = shift_amount.rem_euclid(l_bits.into());
let masked_amount = shift_amount % u128::from(size); (rem, rem != shift_amount)
assert_eq!(overflow, shift_amount != masked_amount);
(masked_amount, overflow)
}; };
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. // Compute the shifted result.
let result = if left.layout.abi.is_signed() { let result = if left.layout.abi.is_signed() {
let l = l_signed(); let l = l_signed();

6
compiler/rustc_middle/src/mir/syntax.rs
View File

@ -1490,7 +1490,8 @@ pub enum BinOp {
BitOr, BitOr,
/// The `<<` operator (shift left) /// 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 "equal modulo LHS::BITS" to the RHS
/// - it is in the range `0..LHS::BITS` /// - it is in the range `0..LHS::BITS`
Shl, Shl,
@ -1498,7 +1499,8 @@ pub enum BinOp {
ShlUnchecked, ShlUnchecked,
/// The `>>` operator (shift right) /// 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 "equal modulo LHS::BITS" to the RHS
/// - it is in the range `0..LHS::BITS` /// - it is in the range `0..LHS::BITS`
/// ///