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fix saturating_sub() underflow for unsigned ints #58030

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This commit is contained in:
Patrick McCarter 2019-02-06 18:10:08 -05:00
parent c5586ebe26
commit f857199073
2 changed files with 15 additions and 11 deletions
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22
src/librustc_mir/interpret/intrinsics.rs
View file

@ -139,12 +139,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
Scalar::from_uint(1u128 << (num_bits - 1), Size::from_bits(num_bits))
}
} else {
if num_bits == 128 {
Scalar::from_uint(u128::max_value(), Size::from_bits(128))
} else {
Scalar::from_uint(u128::max_value() & ((1 << num_bits) - 1),
Size::from_bits(num_bits))
}
Scalar::from_uint(u128::max_value() >> (128 - num_bits), Size::from_bits(num_bits))
};
self.write_scalar(val, dest)?;
} else {
@ -158,12 +153,17 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
if overflowed {
let first_term: u128 = l.to_scalar()?.to_bits(l.layout.size)?;
let num_bits = l.layout.size.bits();
let val = if first_term & (1 << (num_bits-1)) == 0 { // first term is positive
// so overflow is positive
Scalar::from_uint((1u128 << (num_bits - 1)) - 1, Size::from_bits(num_bits))
let val = if l.layout.abi.is_signed() {
if first_term & (1 << (num_bits-1)) == 0 { // first term is positive
// so overflow is positive
Scalar::from_uint((1u128 << (num_bits - 1)) - 1, Size::from_bits(num_bits))
} else {
// if first term negative, overflow must be negative
Scalar::from_uint(1u128 << (num_bits - 1), Size::from_bits(num_bits))
}
} else {
// if first term negative, overflow must be negative
Scalar::from_uint(1u128 << (num_bits - 1), Size::from_bits(num_bits))
// unsigned underflow saturates to 0
Scalar::from_uint(0u128, Size::from_bits(num_bits))
};
self.write_scalar(val, dest)?;
} else {

4
src/test/run-pass/const-int-saturating-arith.rs
View file

@ -5,9 +5,11 @@ const INT_I128: i128 = i128::max_value().saturating_add(1);
const INT_I128_NEG: i128 = i128::min_value().saturating_add(-1);
const INT_U32_NO_SUB: u32 = (42 as u32).saturating_sub(2);
const INT_U32_SUB: u32 = (1 as u32).saturating_sub(2);
const INT_I32_NO_SUB: i32 = (-42 as i32).saturating_sub(2);
const INT_I32_NEG_SUB: i32 = i32::min_value().saturating_sub(1);
const INT_I32_POS_SUB: i32 = i32::max_value().saturating_sub(-1);
const INT_U128_SUB: u128 = (0 as u128).saturating_sub(1);
const INT_I128_NEG_SUB: i128 = i128::min_value().saturating_sub(1);
const INT_I128_POS_SUB: i128 = i128::max_value().saturating_sub(-1);
@ -19,9 +21,11 @@ fn main() {
assert_eq!(INT_I128_NEG, i128::min_value());
assert_eq!(INT_U32_NO_SUB, 40);
assert_eq!(INT_U32_SUB, 0);
assert_eq!(INT_I32_NO_SUB, -44);
assert_eq!(INT_I32_NEG_SUB, i32::min_value());
assert_eq!(INT_I32_POS_SUB, i32::max_value());
assert_eq!(INT_U128_SUB, 0);
assert_eq!(INT_I128_NEG_SUB, i128::min_value());
assert_eq!(INT_I128_POS_SUB, i128::max_value());
}