Updates comments on Int to Float conversion
Int to float for unsigned ints has merged, but there were
some comments on a different PR for the same pull request that
are addressed in this PR
@@ -2298,13 +2298,24 @@ fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
} else {
// Converting packed unsigned integers to packed floats requires a few steps.
// There is no single instruction lowering for converting unsigned floats but there
- // is for converted packed signed integers to float (cvtdq2ps). In the steps below
+ // is for converting packed signed integers to float (cvtdq2ps). In the steps below
// we isolate the upper half (16 bits) and lower half (16 bits) of each lane and
// then we convert each half separately using cvtdq2ps meant for signed integers.
// In order for this to work for the upper half bits we must shift right by 1
// (divide by 2) these bits in order to ensure the most significant bit is 0 not
// signed, and then after the conversion we double the value. Finally we add the
// converted values where addition will correctly round.
+ //
+ // Sequence:
+ // -> A = 0xffffffff
+ // -> Ah = 0xffff0000
+ // -> Al = 0x0000ffff
+ // -> Convert(Al) // Convert int to float
+ // -> Ah = Ah >> 1 // Shift right 1 to assure Ah conversion isn't treated as signed
+ // -> Convert(Ah) // Convert .. with no loss of significant digits from previous shift
+ // -> Ah = Ah + Ah // Double Ah to account for shift right before the conversion.
+ // -> dst = Ah + Al // Add the two floats together
+
assert_eq!(ctx.input_ty(insn, 0), types::I32X4);
let src = put_input_in_reg(ctx, inputs[0]);
let dst = get_output_reg(ctx, outputs[0]);
