Method fptoint_sat

crates/rustc_codegen_spirv/src/builder/builder_methods.rs:457–633

(
        &mut self,
        signed: bool,
        val: SpirvValue,
        dest_ty: <Self as BackendTypes>::Type,
    )

    }

    fn fptoint_sat(
        &mut self,
        signed: bool,
        val: SpirvValue,
        dest_ty: <Self as BackendTypes>::Type,
    ) -> SpirvValue {
        // This uses the old llvm emulation to implement saturation

        let src_ty = self.cx.val_ty(val);
        let (float_ty, int_ty) = if self.cx.type_kind(src_ty) == TypeKind::Vector {
            assert_eq!(
                self.cx.vector_length(src_ty),
                self.cx.vector_length(dest_ty)
            );
            (self.cx.element_type(src_ty), self.cx.element_type(dest_ty))
        } else {
            (src_ty, dest_ty)
        };
        let int_width = self.cx().int_width(int_ty);
        let float_width = self.cx().float_width(float_ty);
        // LLVM's fpto[su]i returns undef when the input x is infinite, NaN, or does not fit into the
        // destination integer type after rounding towards zero. This `undef` value can cause UB in
        // safe code (see issue #10184), so we implement a saturating conversion on top of it:
        // Semantically, the mathematical value of the input is rounded towards zero to the next
        // mathematical integer, and then the result is clamped into the range of the destination
        // integer type. Positive and negative infinity are mapped to the maximum and minimum value of
        // the destination integer type. NaN is mapped to 0.
        //
        // Define f_min and f_max as the largest and smallest (finite) floats that are exactly equal to
        // a value representable in int_ty.
        // They are exactly equal to int_ty::{MIN,MAX} if float_ty has enough significand bits.
        // Otherwise, int_ty::MAX must be rounded towards zero, as it is one less than a power of two.
        // int_ty::MIN, however, is either zero or a negative power of two and is thus exactly
        // representable. Note that this only works if float_ty's exponent range is sufficiently large.
        // f16 or 256 bit integers would break this property. Right now the smallest float type is f32
        // with exponents ranging up to 127, which is barely enough for i128::MIN = -2^127.
        // On the other hand, f_max works even if int_ty::MAX is greater than float_ty::MAX. Because
        // we're rounding towards zero, we just get float_ty::MAX (which is always an integer).
        // This already happens today with u128::MAX = 2^128 - 1 > f32::MAX.
        let int_max = |signed: bool, int_width: u64| -> u128 {
            let shift_amount = 128 - int_width;
            if signed {
                i128::MAX as u128 >> shift_amount
            } else {
                u128::MAX >> shift_amount
            }
        };
        let int_min = |signed: bool, int_width: u64| -> i128 {
            if signed {
                i128::MIN >> (128 - int_width)
            } else {
                0
            }
        };

        let compute_clamp_bounds_single = |signed: bool, int_width: u64| -> (u128, u128) {
            let rounded_min =
                ieee::Single::from_i128_r(int_min(signed, int_width), Round::TowardZero);