Hi!
On Sun, Jul 19, 2020 at 10:42:16AM +0100, Roger Sayle wrote:
> This patch to simplify-rtx.c
> simplifies (sign_extend:HI (truncate:QI (?shiftrt:HI x 8))) to just
> (ashiftrt:HI x 8), as the inner shift already sets the high bits
> appropriately.
> The one oddity of the patch is that it tests for
> LSHIFTRT as inner shift, as simplify/combine has already canonicalized
> this to a logical shift, assuming that the distinction is unimportant
> following the truncation.
If simplify-rtx does that, that is a bug. But combine will do this, I
think that is what you are seeing? Can you verify it already works if
the ASHIFTRT is not changed to an LSHIFTRT?
> + if (is_a <scalar_int_mode> (mode, &m_mode)
> + && is_a <scalar_int_mode> (GET_MODE (op), &n_mode)
> + && is_a <scalar_int_mode> (GET_MODE (old_shift), &o_mode)
> + && GET_MODE_PRECISION (o_mode) - GET_MODE_PRECISION (n_mode)
> + == INTVAL (XEXP (old_shift, 1)))
> + {
> + rtx new_shift = simplify_gen_binary (ASHIFTRT,
> + GET_MODE (old_shift),
> + XEXP (old_shift, 0),
> + XEXP (old_shift, 1));
> + if (GET_MODE_PRECISION (m_mode) > GET_MODE_PRECISION (o_mode))
> + return simplify_gen_unary (SIGN_EXTEND, mode, new_shift,
> + GET_MODE (new_shift));
> + if (mode != GET_MODE (new_shift))
> + return simplify_gen_unary (TRUNCATE, mode, new_shift,
> + GET_MODE (new_shift));
> + return new_shift;
> + }
Yeah looks like it :-)
You could say combine should be smarter about this, but this is a valid
simplification in itself. So, okay for trunk. Thank you!
Segher
