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Re: [C++ PATCH] Reuse certain cxx_eval_constant_expression results in cxx_eval_vec_init_1 (PR c++/70001)
- From: Patrick Palka <patrick at parcs dot ath dot cx>
- To: Jakub Jelinek <jakub at redhat dot com>
- Cc: Jason Merrill <jason at redhat dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Thu, 10 Mar 2016 12:34:40 -0500
- Subject: Re: [C++ PATCH] Reuse certain cxx_eval_constant_expression results in cxx_eval_vec_init_1 (PR c++/70001)
- Authentication-results: sourceware.org; auth=none
- References: <20160310170351 dot GX3017 at tucnak dot redhat dot com>
On Thu, Mar 10, 2016 at 12:03 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> Hi!
>
> As mentioned in the PR, the compile time and compile memory are wasted
> if a large array is is using value or default initialization, and
> if the resulting initializer value is simple enough, we can just share
> it by all the elements.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> 2016-03-10 Patrick Palka <ppalka@gcc.gnu.org>
> Jakub Jelinek <jakub@redhat.com>
>
> PR c++/70001
> * constexpr.c (cxx_eval_vec_init_1): For pre_init case, reuse
> return value from cxx_eval_constant_expression from earlier
> elements if it is valid constant initializer requiring no
> relocations.
>
> * g++.dg/cpp0x/constexpr-70001-1.C: New test.
> * g++.dg/cpp0x/constexpr-70001-2.C: New test.
> * g++.dg/cpp0x/constexpr-70001-3.C: New test.
>
> --- gcc/cp/constexpr.c.jj 2016-03-08 21:04:43.050564671 +0100
> +++ gcc/cp/constexpr.c 2016-03-10 12:52:04.016852313 +0100
> @@ -2340,6 +2340,7 @@ cxx_eval_vec_init_1 (const constexpr_ctx
> vec<constructor_elt, va_gc> **p = &CONSTRUCTOR_ELTS (ctx->ctor);
> vec_alloc (*p, max + 1);
> bool pre_init = false;
> + tree pre_init_elt = NULL_TREE;
> unsigned HOST_WIDE_INT i;
>
> /* For the default constructor, build up a call to the default
> @@ -2389,9 +2390,18 @@ cxx_eval_vec_init_1 (const constexpr_ctx
> {
> /* Initializing an element using value or default initialization
> we just pre-built above. */
> - eltinit = (cxx_eval_constant_expression
> - (&new_ctx, init,
> - lval, non_constant_p, overflow_p));
> + if (pre_init_elt == NULL_TREE)
> + pre_init_elt
> + = cxx_eval_constant_expression (&new_ctx, init, lval,
> + non_constant_p, overflow_p);
> + eltinit = pre_init_elt;
> + /* Don't reuse the result of cxx_eval_constant_expression
> + call if it isn't a constant initializer or if it requires
> + relocations. */
> + if (initializer_constant_valid_p (pre_init_elt,
> + TREE_TYPE (pre_init_elt))
> + != null_pointer_node)
> + pre_init_elt = NULL_TREE;
Doesn't this mean that we call initializer_constant_valid_p at each
iteration? This would slow down the non-constant case even further.
So I wonder if the return value of initializer_constant_valid_p could
be cached or something, since it seems like a potentially expensive
predicate.