On Mon, 3 Aug 2020, Patrick Palka wrote:
> On Mon, 3 Aug 2020, Jason Merrill wrote:
>
> > On 8/3/20 2:45 PM, Patrick Palka wrote:
> > > On Mon, 3 Aug 2020, Jason Merrill wrote:
> > >
> > > > On 8/3/20 8:53 AM, Patrick Palka wrote:
> > > > > On Mon, 3 Aug 2020, Patrick Palka wrote:
> > > > >
> > > > > > In the first testcase below, expand_aggr_init_1 sets up t's default
> > > > > > constructor such that the ctor first zero-initializes the entire
> > > > > > base
> > > > > > b,
> > > > > > followed by calling b's default constructor, the latter of which
> > > > > > just
> > > > > > default-initializes the array member b::m via a VEC_INIT_EXPR.
> > > > > >
> > > > > > So upon constexpr evaluation of this latter VEC_INIT_EXPR, ctx->ctor
> > > > > > is
> > > > > > nonempty due to the prior zero-initialization, and we proceed in
> > > > > > cxx_eval_vec_init to append new constructor_elts to the end of
> > > > > > ctx->ctor
> > > > > > without first checking if a matching constructor_elt already exists.
> > > > > > This leads to ctx->ctor having two matching constructor_elts for
> > > > > > each
> > > > > > index.
> > > > > >
> > > > > > This patch partially fixes this issue by making the RANGE_EXPR
> > > > > > optimization in cxx_eval_vec_init truncate ctx->ctor before adding
> > > > > > the
> > > > > > single RANGE_EXPR constructor_elt. This isn't a complete fix
> > > > > > because
> > > > > > the RANGE_EXPR optimization applies only when the constant
> > > > > > initializer
> > > > > > is relocatable, so whenever it's not relocatable we can still build
> > > > > > up
> > > > > > an invalid CONSTRUCTOR, e.g. if in the first testcase we add an
> > > > > > NSDMI
> > > > > > such as 'e *p = this;' to struct e, then the ICE still occurs even
> > > > > > with
> > > > > > this patch.
> > > > >
> > > > > A complete but more risky one-line fix would be to always truncate
> > > > > ctx->ctor beforehand, not just when the RANGE_EXPR optimization
> > > > > applies.
> > > > > If it's true that the initializer of a VEC_INIT_EXPR can't observe the
> > > > > previous elements of the target array, then it should be safe to
> > > > > always
> > > > > truncate I think?
> > > >
> > > > What if default-initialization of the array element type doesn't fully
> > > > initialize the elements, e.g. if 'e' had another member without a
> > > > default
> > > > initializer? Does truncation first mean we lose the zero-initialization
> > > > of
> > > > such a member?
> > >
> > > Hmm, it looks like we would lose the zero-initialization of such a
> > > member with or without truncation first (so with any one of the three
> > > proposed fixes). I think it's because the evaluation loop in
> > > cxx_eval_vec_init disregards each element's prior (zero-initialized)
> > > state.
> > >
> > > >
> > > > We could probably still do the truncation, but clear the
> > > > CONSTRUCTOR_NO_CLEARING flag on the element initializer.
> > >
> > > Ah, this seems to work well. Like this?
> > >
> > > -- >8 --
> > >
> > > Subject: [PATCH] c++: cxx_eval_vec_init after zero initialization
> > > [PR96282]
> > >
> > > In the first testcase below, expand_aggr_init_1 sets up t's default
> > > constructor such that the ctor first zero-initializes the entire base b,
> > > followed by calling b's default constructor, the latter of which just
> > > default-initializes the array member b::m via a VEC_INIT_EXPR.
> > >
> > > So upon constexpr evaluation of this latter VEC_INIT_EXPR, ctx->ctor is
> > > nonempty due to the prior zero-initialization, and we proceed in
> > > cxx_eval_vec_init to append new constructor_elts to the end of ctx->ctor
> > > without first checking if a matching constructor_elt already exists.
> > > This leads to ctx->ctor having two matching constructor_elts for each
> > > index.
> > >
> > > This patch fixes this issue by truncating a zero-initialized array
> > > object in cxx_eval_vec_init_1 before we begin appending
> > > default-initialized
> > > array elements to it. Since default-initialization may leave parts of
> > > the element type unitialized, we also preserve the array's prior
> > > zero-initialized state by clearing CONSTRUCTOR_NO_CLEARING on each
> > > appended element initializers.
> > >
> > > gcc/cp/ChangeLog:
> > >
> > > PR c++/96282
> > > * constexpr.c (cxx_eval_vec_init_1): Truncate ctx->ctor and
> > > then clear CONSTRUCTOR_NO_CLEARING on each appended element
> > > initializer if we're default-initializing a previously
> > > zero-initialized array object.
> > >
> > > gcc/testsuite/ChangeLog:
> > >
> > > PR c++/96282
> > > * g++.dg/cpp0x/constexpr-array26.C: New test.
> > > * g++.dg/cpp0x/constexpr-array27.C: New test.
> > > * g++.dg/cpp2a/constexpr-init18.C: New test.
> > > ---
> > > gcc/cp/constexpr.c | 17 ++++++++++++++++-
> > > gcc/testsuite/g++.dg/cpp0x/constexpr-array26.C | 13 +++++++++++++
> > > gcc/testsuite/g++.dg/cpp0x/constexpr-array27.C | 13 +++++++++++++
> > > gcc/testsuite/g++.dg/cpp2a/constexpr-init18.C | 16 ++++++++++++++++
> > > 4 files changed, 58 insertions(+), 1 deletion(-)
> > > create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-array26.C
> > > create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-array27.C
> > > create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-init18.C
> > >
> > > diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
> > > index b1c1d249c6e..706bef323b2 100644
> > > --- a/gcc/cp/constexpr.c
> > > +++ b/gcc/cp/constexpr.c
> > > @@ -4171,6 +4171,17 @@ cxx_eval_vec_init_1 (const constexpr_ctx *ctx, tree
> > > atype, tree init,
> > > pre_init = true;
> > > }
> > > + bool zero_initialized_p = false;
> > > + if ((pre_init || value_init || !init) && initializer_zerop (ctx->ctor))
> >
> > Does initializer_zerop capture the difference between a default-initialized
> > or
> > value-initialized containing object? I would expect it to be true for both.
> > Maybe check CONSTRUCTOR_NO_CLEARING (ctx->ctor) instead?
>
> D'oh, indeed it looks like initializer_zerop is not what we want here.
Hmm, might we need to check both? !CONSTRUCTOR_NO_CLEARING tells us
about the omitted initializers, but it seems we'd still need to test if
the explicit initializers are zero.
>
> >
> > This all seems parallel to the no_zero_init code in
> > cxx_eval_store_expression.
>
> I am testing the following, which inspects CONSTRUCTOR_NO_CLEARING and
> also beefs up the tests to verify that we handle the multidimensional
> array case correctly.
>
> -- >8 --
>
> Subject: [PATCH] c++: cxx_eval_vec_init after zero initialization [PR96282]
>
> In the first testcase below, expand_aggr_init_1 sets up t's default
> constructor such that the ctor first zero-initializes the entire base b,
> followed by calling b's default constructor, the latter of which just
> default-initializes the array member b::m via a VEC_INIT_EXPR.
>
> So upon constexpr evaluation of this latter VEC_INIT_EXPR, ctx->ctor is
> nonempty due to the prior zero-initialization, and we proceed in
> cxx_eval_vec_init to append new constructor_elts to the end of ctx->ctor
> without first checking if a matching constructor_elt already exists.
> This leads to ctx->ctor having two matching constructor_elts for each
> index.
>
> This patch fixes this issue by truncating a zeroed out array CONSTRUCTOR
> in cxx_eval_vec_init_1 before we begin appending array elements to it.
> We preserve its zeroed out state during evaluation by clearing
> CONSTRUCTOR_NO_CLEARING on each new appended element.
>
> gcc/cp/ChangeLog:
>
> PR c++/96282
> * constexpr.c (cxx_eval_vec_init_1): Truncate ctx->ctor and
> then clear CONSTRUCTOR_NO_CLEARING on each appended element
> initializer if we're initializing a previously zeroed out
> array object.
>
> gcc/testsuite/ChangeLog:
>
> PR c++/96282
> * g++.dg/cpp0x/constexpr-array26.C: New test.
> * g++.dg/cpp0x/constexpr-array27.C: New test.
> * g++.dg/cpp2a/constexpr-init18.C: New test.
> ---
> gcc/cp/constexpr.c | 18 +++++++++++++++++-
> gcc/testsuite/g++.dg/cpp0x/constexpr-array26.C | 13 +++++++++++++
> gcc/testsuite/g++.dg/cpp0x/constexpr-array27.C | 13 +++++++++++++
> gcc/testsuite/g++.dg/cpp2a/constexpr-init18.C | 16 ++++++++++++++++
> 4 files changed, 59 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-array26.C
> create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-array27.C
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-init18.C
>
> diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
> index b1c1d249c6e..364630f6333 100644
> --- a/gcc/cp/constexpr.c
> +++ b/gcc/cp/constexpr.c
> @@ -4171,6 +4171,18 @@ cxx_eval_vec_init_1 (const constexpr_ctx *ctx, tree
> atype, tree init,
> pre_init = true;
> }
>
> + bool zeroed_out = false;
> + if ((pre_init || value_init || !init)
> + && !CONSTRUCTOR_NO_CLEARING (ctx->ctor))
> + {
> + /* We're initializing an array object that had been zeroed out
> + earlier. Truncate ctx->ctor and preserve its prior state by
> + propagating CONSTRUCTOR_NO_CLEARING to each of the element
> + initializers we append to it. */
> + zeroed_out = true;
> + vec_safe_truncate (*p, 0);
> + }
> +
> tree nelts = get_array_or_vector_nelts (ctx, atype, non_constant_p,
> overflow_p);
> unsigned HOST_WIDE_INT max = tree_to_uhwi (nelts);
> @@ -4182,7 +4194,11 @@ cxx_eval_vec_init_1 (const constexpr_ctx *ctx, tree
> atype, tree init,
> constexpr_ctx new_ctx;
> init_subob_ctx (ctx, new_ctx, idx, pre_init ? init : elttype);
> if (new_ctx.ctor != ctx->ctor)
> - CONSTRUCTOR_APPEND_ELT (*p, idx, new_ctx.ctor);
> + {
> + if (zeroed_out)
> + CONSTRUCTOR_NO_CLEARING (new_ctx.ctor) = false;
> + CONSTRUCTOR_APPEND_ELT (*p, idx, new_ctx.ctor);
> + }
> if (TREE_CODE (elttype) == ARRAY_TYPE)
> {
> /* A multidimensional array; recurse. */
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-array26.C
> b/gcc/testsuite/g++.dg/cpp0x/constexpr-array26.C
> new file mode 100644
> index 00000000000..274f55a88bf
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-array26.C
> @@ -0,0 +1,13 @@
> +// PR c++/96282
> +// { dg-do compile { target c++11 } }
> +
> +struct e { bool v = true; };
> +
> +template<int N>
> +struct b { e m[N]; };
> +
> +template<int N>
> +struct t : b<N> { constexpr t() : b<N>() {} };
> +
> +constexpr t<1> h1;
> +constexpr t<42> h2;
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-array27.C
> b/gcc/testsuite/g++.dg/cpp0x/constexpr-array27.C
> new file mode 100644
> index 00000000000..1234caef31d
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-array27.C
> @@ -0,0 +1,13 @@
> +// PR c++/96282
> +// { dg-do compile { target c++11 } }
> +
> +struct e { bool v = true; e *p = this; };
> +
> +template<int N>
> +struct b { e m[N][N]; };
> +
> +template<int N>
> +struct t : b<N> { constexpr t() : b<N>() {} };
> +
> +constexpr t<1> h1;
> +constexpr t<42> h2;
> diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-init18.C
> b/gcc/testsuite/g++.dg/cpp2a/constexpr-init18.C
> new file mode 100644
> index 00000000000..47ad11f2290
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-init18.C
> @@ -0,0 +1,16 @@
> +// PR c++/96282
> +// { dg-do compile { target c++20 } }
> +
> +struct e { bool v = true; bool w; };
> +
> +template<int N>
> +struct b { e m[N][N]; };
> +
> +template<int N>
> +struct t : b<N> { constexpr t() : b<N>() {} };
> +
> +constexpr t<1> h1;
> +static_assert(h1.m[0][0].w == false);
> +
> +constexpr t<42> h2;
> +static_assert(h2.m[17][17].w == false);
> --
> 2.28.0.89.g85b4e0a6dc
>
>
