From 7854067044466de2dd123ca4a3fb6251c0683ead Mon Sep 17 00:00:00 2001 From: Patrick McCarter Date: Fri, 8 Feb 2019 13:04:11 -0500 Subject: [PATCH] Saturating add/sub intrinsic emulation refactor/comments #58030 --- src/librustc_mir/interpret/intrinsics.rs | 17 ++++++++++++----- 1 file changed, 12 insertions(+), 5 deletions(-) diff --git a/src/librustc_mir/interpret/intrinsics.rs b/src/librustc_mir/interpret/intrinsics.rs index e8dc22b8a596..48fa85683960 100644 --- a/src/librustc_mir/interpret/intrinsics.rs +++ b/src/librustc_mir/interpret/intrinsics.rs @@ -132,15 +132,22 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M> BinOp::Sub }, l, r)?; let val = if overflowed { - // For signed ints the saturated value depends on the - // sign of the first term - let first_term: u128 = l.to_scalar()?.to_bits(l.layout.size)?; let num_bits = l.layout.size.bits(); if l.layout.abi.is_signed() { - if first_term & (1 << (num_bits-1)) == 0 { // first term is positive + // 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_pos = first_term & (1 << (num_bits-1)) == 0; + if first_term_pos { + // 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 { // first term is negative + } else { + // Positive overflow not possible for similar reason // max negative Scalar::from_uint(1u128 << (num_bits - 1), Size::from_bits(num_bits)) }