On Tue, Jun 17, 2014 at 4:11 AM, Zhenqiang Chen
<zhenqiang.c...@linaro.org> wrote:
> Hi,
>
> For some large constant, ports like ARM, need one more instructions to
> operate it. e.g
>
> #define MASK 0xfe00ff
> void maskdata (int * data, int len)
> {
> int i = len;
> for (; i > 0; i -= 2)
> {
> data[i] &= MASK;
> data[i + 1] &= MASK;
> }
> }
>
> Need two instructions for each AND operation:
>
> and r3, r3, #16711935
> bic r3, r3, #65536
>
> If we keep the MASK in a register, loop2_invariant pass can hoist it
> out the loop. And it can be shared by different references.
>
> So the patch skips constant propagation if it makes INSN's cost higher.
So cprop undos invariant motions work here?
Should we make sure we add a REG_EQUAL note when not propagating?
> Bootstrap and no make check regression on X86-64 and ARM Chrome book.
>
> OK for trunk?
>
> Thanks!
> -Zhenqiang
>
> ChangeLog:
> 2014-06-17 Zhenqiang Chen <zhenqiang.c...@linaro.org>
>
> * cprop.c (try_replace_reg): Check cost for constants.
>
> diff --git a/gcc/cprop.c b/gcc/cprop.c
> index aef3ee8..c9cf02a 100644
> --- a/gcc/cprop.c
> +++ b/gcc/cprop.c
> @@ -733,6 +733,14 @@ try_replace_reg (rtx from, rtx to, rtx insn)
> rtx src = 0;
> int success = 0;
> rtx set = single_set (insn);
> + int old_cost = 0;
> + bool copy_p = false;
> + bool speed = optimize_bb_for_speed_p (BLOCK_FOR_INSN (insn));
> +
> + if (set && SET_SRC (set) && REG_P (SET_SRC (set)))
> + copy_p = true;
> + else
> + old_cost = set_rtx_cost (set, speed);
Looks bogus for set == NULL?
Also what about register pressure?
I think this kind of change needs wider testing as RTX costs are
usually not fully implemented and you introduce a new use kind
(or is it already used elsewhere in this way to compute cost
difference of a set with s/reg/const?).
What kind of performance difference do you see?
Thanks,
Richard.
> /* Usually we substitute easy stuff, so we won't copy everything.
> We however need to take care to not duplicate non-trivial CONST
> @@ -740,6 +748,20 @@ try_replace_reg (rtx from, rtx to, rtx insn)
> to = copy_rtx (to);
>
> validate_replace_src_group (from, to, insn);
> +
> + /* For CONSTANT_P (TO), loop2_invariant pass might hoist it out the loop.
> + And it can be shared by different references. So skip propagation if
> + it makes INSN's rtx cost higher. */
> + if (set && !copy_p && CONSTANT_P (to))
> + {
> + int new_cost = set_rtx_cost (set, speed);
> + if (new_cost > old_cost)
> + {
> + cancel_changes (0);
> + return false;
> + }
> + }
> +
> if (num_changes_pending () && apply_change_group ())
> success = 1;
