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builder: fix checked_binop for signed addition/subtraction.

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Eduard-Mihai Burtescu
2026-03-11 06:28:19 +02:00
parent 39eec00a8f
commit 451d966d7b
1 changed files with 14 additions and 8 deletions
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22
crates/rustc_codegen_spirv/src/builder/builder_methods.rs
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@@ -1748,28 +1748,34 @@ impl<'a, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'tcx> {
};
// adopted partially from https://github.com/ziglang/zig/blob/master/src/codegen/spirv.zig
// FIXME(eddyb) ^^ find a working a permalink, and also inform them
// of the difference between these two approaches:
// - broken: https://alive2.llvm.org/ce/z/Q3Pchi
// - correct: https://alive2.llvm.org/ce/z/aWvThi
// when adding, overflow could happen if
// - rhs is positive and result < lhs; or
// - rhs is negative and result > lhs
// this is equivalent to (rhs < 0) == (result > lhs)
// - rhs is negative and result >= lhs
// (`result == lhs` impossible, `>=` used as it's `!(result < lhs)`)
// this is equivalent to (rhs < 0) == (result >= lhs)
//
// when subtracting, overflow happens if
// - rhs is positive and result > lhs; or
// - rhs is negative and result < lhs
// this is equivalent to (rhs < 0) == (result < lhs)
// - rhs is negative and result <= lhs
// (`result == lhs` impossible, `<=` used as it's `!(result > lhs)`)
// this is equivalent to (rhs < 0) == (result <= lhs)
let rhs_lt_zero = self.icmp(IntPredicate::IntSLT, rhs, self.constant_int(rhs.ty, 0));
let result_gt_lhs = self.icmp(
let result_ge_lhs = self.icmp(
match oop {
OverflowOp::Add => IntPredicate::IntSGT,
OverflowOp::Sub => IntPredicate::IntSLT,
OverflowOp::Add => IntPredicate::IntSGE,
OverflowOp::Sub => IntPredicate::IntSLE,
OverflowOp::Mul => unreachable!(),
},
result,
lhs,
);
let overflowed = self.icmp(IntPredicate::IntEQ, rhs_lt_zero, result_gt_lhs);
let overflowed = self.icmp(IntPredicate::IntEQ, rhs_lt_zero, result_ge_lhs);
return (result, overflowed);
}