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interpret: move saturating_add/sub into (pub) helper method #94685

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merged 2 commits into from
Mar 7, 2022
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interpret: move saturating_add/sub into (pub) helper method #94685

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956659e
move saturating_add/sub into (pub) helper method
RalfJung Mar 7, 2022
ac84498
use singed_int_max/min helper methods
RalfJung Mar 7, 2022
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84 changes: 45 additions & 39 deletions compiler/rustc_const_eval/src/interpret/intrinsics.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -219,48 +219,11 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
sym::saturating_add | sym::saturating_sub => {
let l = self.read_immediate(&args[0])?;
let r = self.read_immediate(&args[1])?;
let is_add = intrinsic_name == sym::saturating_add;
let (val, overflowed, _ty) = self.overflowing_binary_op(
if is_add { BinOp::Add } else { BinOp::Sub },
let val = self.saturating_arith(
if intrinsic_name == sym::saturating_add { BinOp::Add } else { BinOp::Sub },
&l,
&r,
)?;
let val = if overflowed {
let size = l.layout.size;
let num_bits = size.bits();
if l.layout.abi.is_signed() {
// For signed ints the saturated value depends on the sign of the first
// term since the sign of the second term can be inferred from this and
// the fact that the operation has overflowed (if either is 0 no
// overflow can occur)
let first_term: u128 = l.to_scalar()?.to_bits(l.layout.size)?;
let first_term_positive = first_term & (1 << (num_bits - 1)) == 0;
if first_term_positive {
// Negative overflow not possible since the positive first term
// can only increase an (in range) negative term for addition
// or corresponding negated positive term for subtraction
Scalar::from_uint(
(1u128 << (num_bits - 1)) - 1, // max positive
Size::from_bits(num_bits),
)
} else {
// Positive overflow not possible for similar reason
// max negative
Scalar::from_uint(1u128 << (num_bits - 1), Size::from_bits(num_bits))
}
} else {
// unsigned
if is_add {
// max unsigned
Scalar::from_uint(size.unsigned_int_max(), Size::from_bits(num_bits))
} else {
// underflow to 0
Scalar::from_uint(0u128, Size::from_bits(num_bits))
}
}
} else {
val
};
self.write_scalar(val, dest)?;
}
sym::discriminant_value => {
Expand Down Expand Up @@ -508,6 +471,49 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
self.binop_ignore_overflow(BinOp::Div, &a, &b, dest)
}

pub fn saturating_arith(
&self,
mir_op: BinOp,
l: &ImmTy<'tcx, M::PointerTag>,
r: &ImmTy<'tcx, M::PointerTag>,
) -> InterpResult<'tcx, Scalar<M::PointerTag>> {
assert!(matches!(mir_op, BinOp::Add | BinOp::Sub));
let (val, overflowed, _ty) = self.overflowing_binary_op(mir_op, l, r)?;
Ok(if overflowed {
let size = l.layout.size;
let num_bits = size.bits();
if l.layout.abi.is_signed() {
// For signed ints the saturated value depends on the sign of the first
// term since the sign of the second term can be inferred from this and
// the fact that the operation has overflowed (if either is 0 no
// overflow can occur)
let first_term: u128 = l.to_scalar()?.to_bits(l.layout.size)?;
let first_term_positive = first_term & (1 << (num_bits - 1)) == 0;
if first_term_positive {
// Negative overflow not possible since the positive first term
// can only increase an (in range) negative term for addition
// or corresponding negated positive term for subtraction
Scalar::from_int(size.signed_int_max(), size)
} else {
// Positive overflow not possible for similar reason
// max negative
Scalar::from_int(size.signed_int_min(), size)
}
} else {
// unsigned
if matches!(mir_op, BinOp::Add) {
// max unsigned
Scalar::from_uint(size.unsigned_int_max(), size)
} else {
// underflow to 0
Scalar::from_uint(0u128, size)
}
}
} else {
val
})
}

/// Offsets a pointer by some multiple of its type, returning an error if the pointer leaves its
/// allocation. For integer pointers, we consider each of them their own tiny allocation of size
/// 0, so offset-by-0 (and only 0) is okay -- except that null cannot be offset by _any_ value.
Expand Down