[Sorry Jakub, mid-air collision. I just posted this comment to the PR.]
Jakub Jelinek <ja...@redhat.com> writes:
> Optimizing (truncate:A (subreg:B (truncate:C X) 0)) into
> (truncate:A X) is wrong if C mode is narrower than A mode.
> The following patch fixes that and simplifies it into
> (subreg:A (truncate:C X 0)) instead.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
Thanks for the fix. Looks good to me FWIW. However...
> --- gcc/simplify-rtx.c.jj 2013-02-12 11:23:37.000000000 +0100
> +++ gcc/simplify-rtx.c 2013-03-04 11:32:44.111721542 +0100
> @@ -757,8 +757,18 @@ simplify_truncation (enum machine_mode m
> && SCALAR_INT_MODE_P (GET_MODE (SUBREG_REG (op)))
> && GET_CODE (SUBREG_REG (op)) == TRUNCATE
> && subreg_lowpart_p (op))
> - return simplify_gen_unary (TRUNCATE, mode, XEXP (SUBREG_REG (op), 0),
> - GET_MODE (XEXP (SUBREG_REG (op), 0)));
> + {
> + rtx inner = XEXP (SUBREG_REG (op), 0);
> + if (GET_MODE_PRECISION (mode)
> + <= GET_MODE_PRECISION (GET_MODE (SUBREG_REG (op))))
> + return simplify_gen_unary (TRUNCATE, mode, inner, GET_MODE (inner));
> + /* If subreg above is paradoxical and C is narrower
> + than A, return (subreg:A (truncate:C X) 0). */
> + if (GET_MODE_PRECISION (op_mode)
> + > GET_MODE_PRECISION (GET_MODE (SUBREG_REG (op))))
> + return simplify_gen_subreg (mode, SUBREG_REG (op),
> + GET_MODE (SUBREG_REG (op)), 0);
> + }
...truncations can't be to a narrower mode (already asserted at the top
of the function) so in some ways I think it would be clearer to have an
"if ... else" rather than two "if"s.
Richard
