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c++: cxx_eval_vec_init after zero-initialization [PR96282]

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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
CONSTRUCTOR in cxx_eval_vec_init_1 before we begin appending array
elements to it.  We propagate its zeroed out state during evaluation by
clearing CONSTRUCTOR_NO_CLEARING on each new appended aggregate 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 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.

Co-authored-by: Jason Merrill <jason@redhat.com>
This commit is contained in:
Patrick Palka
2020-08-05 15:05:30 -04:00
parent dd30d93f1a
commit d21252de6c
4 changed files with 59 additions and 1 deletions
Download Patch File Download Diff File Expand all files Collapse all files

18
gcc/cp/constexpr.c
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@@ -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 (!CONSTRUCTOR_NO_CLEARING (ctx->ctor))
{
/* We're initializing an array object that had been zero-initialized
earlier. Truncate ctx->ctor, and propagate its zeroed state by
clearing CONSTRUCTOR_NO_CLEARING on each of the aggregate element
initializers we append to it. */
gcc_checking_assert (initializer_zerop (ctx->ctor));
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. */

13
gcc/testsuite/g++.dg/cpp0x/constexpr-array26.C Normal file
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@@ -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;

13
gcc/testsuite/g++.dg/cpp0x/constexpr-array27.C Normal file
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@@ -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;

16
gcc/testsuite/g++.dg/cpp2a/constexpr-init18.C Normal file
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@@ -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);