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Rollup merge of #91162 - RalfJung:miri-shift-truncation, r=oli-obk

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explain why CTFE/Miri perform truncation on shift offset

Closes https://github.com/rust-lang/miri/issues/1920
This commit is contained in:
Matthias Krüger 2021-11-25 15:05:39 +01:00 committed by GitHub
commit 5fff1d77e7
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2 changed files with 14 additions and 1 deletions
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7
compiler/rustc_const_eval/src/interpret/operator.rs
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@ -130,7 +130,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
let signed = left_layout.abi.is_signed();
let size = u128::from(left_layout.size.bits());
let overflow = r >= size;
let r = r % size; // mask to type size
// The shift offset is implicitly masked to the type size, to make sure this operation
// is always defined. This is the one MIR operator that does *not* directly map to a
// single LLVM operation. See
// <https://github.com/rust-lang/rust/blob/a3b9405ae7bb6ab4e8103b414e75c44598a10fd2/compiler/rustc_codegen_ssa/src/common.rs#L131-L158>
// for the corresponding truncation in our codegen backends.
let r = r % size;
let r = u32::try_from(r).unwrap(); // we masked so this will always fit
let result = if signed {
let l = self.sign_extend(l, left_layout) as i128;

8
compiler/rustc_middle/src/mir/mod.rs
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@ -2246,8 +2246,12 @@ pub enum BinOp {
/// The `*` operator (multiplication)
Mul,
/// The `/` operator (division)
///
/// Division by zero is UB.
Div,
/// The `%` operator (modulus)
///
/// Using zero as the modulus (second operand) is UB.
Rem,
/// The `^` operator (bitwise xor)
BitXor,
@ -2256,8 +2260,12 @@ pub enum BinOp {
/// The `|` operator (bitwise or)
BitOr,
/// The `<<` operator (shift left)
///
/// The offset is truncated to the size of the first operand before shifting.
Shl,
/// The `>>` operator (shift right)
///
/// The offset is truncated to the size of the first operand before shifting.
Shr,
/// The `==` operator (equality)
Eq,