On Fri, Feb 3, 2023 at 2:06 PM Richard Biener via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> When cse_insn prunes src{,_folded,_eqv_here,_related} with the
> equivalence set in the *_same_value chain it also searches for
> an equivalence to the destination of the instruction with
>
> /* This is the same as the destination of the insns, we want
> to prefer it. Copy it to src_related. The code below will
> then give it a negative cost. */
> if (GET_CODE (dest) == code && rtx_equal_p (p->exp, dest))
> src_related = p->exp;
>
> this picks up the last such equivalence and in particular any
> later duplicate will be pruned by the preceeding
>
> else if (src_related && GET_CODE (src_related) == code
> && rtx_equal_p (src_related, p->exp))
> src_related = 0;
>
> first. This wastes cycles doing extra rtx_equal_p checks. The
> following instead searches for the first destination equivalence
> separately in this loop and delays using src_related for it until
> we are about to process that, avoiding another redundant rtx_equal_p
> check.
>
> I've came here because of a testcase with very large equivalence
> lists and compile-time of cse_insn. The patch below doesn't speed
> it up significantly since there's no equivalence on the destination.
>
> In theory this opens the possibility to track dest_related
> separately, avoiding the implicit pruning of any previous
> value in src_related. As is the change should be a no-op for
> code generation.
>
> Bootstrapped and tested on x86_64-unknown-linux-gnu, queued for
> stage1.
I have pushed this now after re-bootstrapping and testing on
x86_64-unknown-linux-gnu.
Richard.
> * cse.cc (cse_insn): Track an equivalence to the destination
> separately and delay using src_related for it.
> ---
> gcc/cse.cc | 51 +++++++++++++++++++++++++++------------------------
> 1 file changed, 27 insertions(+), 24 deletions(-)
>
> diff --git a/gcc/cse.cc b/gcc/cse.cc
> index 8fbda4ecc86..543cb1fe36f 100644
> --- a/gcc/cse.cc
> +++ b/gcc/cse.cc
> @@ -4614,6 +4614,7 @@ cse_insn (rtx_insn *insn)
> rtx src_eqv_here;
> rtx src_const = 0;
> rtx src_related = 0;
> + rtx dest_related = 0;
> bool src_related_is_const_anchor = false;
> struct table_elt *src_const_elt = 0;
> int src_cost = MAX_COST;
> @@ -5085,10 +5086,11 @@ cse_insn (rtx_insn *insn)
> src_related = 0;
>
> /* This is the same as the destination of the insns, we want
> - to prefer it. Copy it to src_related. The code below will
> - then give it a negative cost. */
> - if (GET_CODE (dest) == code && rtx_equal_p (p->exp, dest))
> - src_related = p->exp;
> + to prefer it. The code below will then give it a negative
> + cost. */
> + if (!dest_related
> + && GET_CODE (dest) == code && rtx_equal_p (p->exp, dest))
> + dest_related = p->exp;
> }
>
> /* Find the cheapest valid equivalent, trying all the available
> @@ -5130,27 +5132,28 @@ cse_insn (rtx_insn *insn)
> }
> }
>
> - if (src_related)
> + if (dest_related)
> {
> - if (rtx_equal_p (src_related, dest))
> - src_related_cost = src_related_regcost = -1;
> - else
> - {
> - src_related_cost = COST (src_related, mode);
> - src_related_regcost = approx_reg_cost (src_related);
> -
> - /* If a const-anchor is used to synthesize a constant that
> - normally requires multiple instructions then slightly prefer
> - it over the original sequence. These instructions are likely
> - to become redundant now. We can't compare against the cost
> - of src_eqv_here because, on MIPS for example, multi-insn
> - constants have zero cost; they are assumed to be hoisted from
> - loops. */
> - if (src_related_is_const_anchor
> - && src_related_cost == src_cost
> - && src_eqv_here)
> - src_related_cost--;
> - }
> + src_related_cost = src_related_regcost = -1;
> + /* Handle it as src_related. */
> + src_related = dest_related;
> + }
> + else if (src_related)
> + {
> + src_related_cost = COST (src_related, mode);
> + src_related_regcost = approx_reg_cost (src_related);
> +
> + /* If a const-anchor is used to synthesize a constant that
> + normally requires multiple instructions then slightly prefer
> + it over the original sequence. These instructions are likely
> + to become redundant now. We can't compare against the cost
> + of src_eqv_here because, on MIPS for example, multi-insn
> + constants have zero cost; they are assumed to be hoisted from
> + loops. */
> + if (src_related_is_const_anchor
> + && src_related_cost == src_cost
> + && src_eqv_here)
> + src_related_cost--;
> }
>
> /* If this was an indirect jump insn, a known label will really be
> --
> 2.35.3
