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Rollup merge of #94659 - RalfJung:signed-shift, r=oli-obk

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explain why shift with signed offset works the way it does

I was worried for a bit here that Miri/CTFE would be inconsistent with codegen, but I *think* everything is all right, actually.

Cc `@oli-obk` `@eddyb`
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Matthias Krüger 2022-03-06 19:08:37 +01:00 committed by GitHub
commit c2d2f6fd14
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1 changed files with 15 additions and 3 deletions
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18
compiler/rustc_const_eval/src/interpret/operator.rs
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@ -127,17 +127,29 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
// Shift ops can have an RHS with a different numeric type.
if bin_op == Shl || bin_op == Shr {
let signed = left_layout.abi.is_signed();
let size = u128::from(left_layout.size.bits());
// Even if `r` is signed, we treat it as if it was unsigned (i.e., we use its
// zero-extended form). This matches the codegen backend:
// <https://github.com/rust-lang/rust/blob/c274e4969f058b1c644243181ece9f829efa7594/compiler/rustc_codegen_ssa/src/base.rs#L315-L317>.
// The overflow check is also ignorant to the sign:
// <https://github.com/rust-lang/rust/blob/c274e4969f058b1c644243181ece9f829efa7594/compiler/rustc_codegen_ssa/src/mir/rvalue.rs#L728>.
// This would behave rather strangely if we had integer types of size 256: a shift by
// -1i8 would actually shift by 255, but that would *not* be considered overflowing. A
// shift by -1i16 though would be considered overflowing. If we had integers of size
// 512, then a shift by -1i8 would even produce a different result than one by -1i16:
// the first shifts by 255, the latter by u16::MAX % 512 = 511. Lucky enough, our
// integers are maximally 128bits wide, so negative shifts *always* overflow and we have
// consistent results for the same value represented at different bit widths.
assert!(size <= 128);
let overflow = r >= 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>
// <https://github.com/rust-lang/rust/blob/c274e4969f058b1c644243181ece9f829efa7594/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 result = if left_layout.abi.is_signed() {
let l = self.sign_extend(l, left_layout) as i128;
let result = match bin_op {
Shl => l.checked_shl(r).unwrap(),