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explain why CTFE/Miri perform truncation on shift offset

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This commit is contained in:
Ralf Jung 2021-11-23 14:12:14 -05:00
parent 311fa1f14d
commit 94c9c22b39
1 changed files with 6 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 @@ fn binary_int_op(
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;