Hi!
On Wed, Dec 16, 2020 at 04:49:49PM +0800, Kewen.Lin wrote:
> Currently we have the check:
>
> if (!insn
> || (value && rsp->last_set_table_tick >= label_tick_ebb_start))
> rsp->last_set_invalid = 1;
>
> which means if we want to record some value for some reg and
> this reg got refered before in a valid scope,
If we already know it is *set* in this same extended basic block.
Possibly by the same instruction btw.
> we invalidate the
> set of reg (last_set_invalid to 1). It avoids to find the wrong
> set for one reg reference, such as the case like:
>
> ... op regX // this regX could find wrong last_set below
> regX = ... // if we think this set is valid
> ... op regX
Yup, exactly.
> But because of retry's existence, the last_set_table_tick could
> be set by some later reference insns, but we see it's set due
> to retry on the set (for that reg) insn again, such as:
>
> insn 1
> insn 2
>
> regX = ... --> (a)
> ... op regX --> (b)
>
> insn 3
>
> // assume all in the same BB.
>
> Assuming we combine 1, 2 -> 3 sucessfully and replace them as two
> (3 insns -> 2 insns),
This will delete insn 1 and write the combined result to insns 2 and 3.
> retrying from insn1 or insn2 again:
Always 2, but your point remains valid.
> it will scan insn (a) again, the below condition holds for regX:
>
> (value && rsp->last_set_table_tick >= label_tick_ebb_start)
>
> it will mark this set as invalid set. But actually the
> last_set_table_tick here is set by insn (b) before retrying, so it
> should be safe to be taken as valid set.
Yup.
> This proposal is to check whether the last_set_table safely happens
> after the current set, make the set still valid if so.
> Full SPEC2017 building shows this patch gets more sucessful combines
> from 1902208 to 1902243 (trivial though).
Do you have some example, or maybe even a testcase? :-)
> + /* Record the luid of the insn whose expression involving register n. */
> +
> + int last_set_table_luid;
"Record the luid of the insn for which last_set_table_tick was set",
right?
> -static void update_table_tick (rtx);
> +static void update_table_tick (rtx, int);
Please remove this declaration instead, the function is not used until
after its actual definition :-)
> @@ -13243,7 +13247,21 @@ update_table_tick (rtx x)
> for (r = regno; r < endregno; r++)
> {
> reg_stat_type *rsp = ®_stat[r];
> - rsp->last_set_table_tick = label_tick;
> + if (rsp->last_set_table_tick >= label_tick_ebb_start)
> + {
> + /* Later references should not have lower ticks. */
> + gcc_assert (label_tick >= rsp->last_set_table_tick);
This should be obvious, but checking it won't hurt, okay.
> + /* Should pick up the lowest luid if the references
> + are in the same block. */
> + if (label_tick == rsp->last_set_table_tick
> + && rsp->last_set_table_luid > insn_luid)
> + rsp->last_set_table_luid = insn_luid;
Why? Is it conservative for the check you will do later? Please spell
this out, it is crucial!
> @@ -13359,7 +13378,10 @@ record_value_for_reg (rtx reg, rtx_insn *insn, rtx
> value)
>
> /* Mark registers that are being referenced in this value. */
> if (value)
> - update_table_tick (value);
> + {
> + gcc_assert (insn);
> + update_table_tick (value, DF_INSN_LUID (insn));
> + }
Don't add that assert please. If you really want one it should come
right at the start of the function, not 60 lines later :-)
Looks good if I understood this correctly :-)
Segher
