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

Patrick Palka ppalka@redhat.com
Mon Aug 3 18:45:32 GMT 2020


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))
+    {
+      /* We're default-initializing an array object that had been
+	 zero-initialized earlier.  We'll preserve this prior state
+	 by clearing CONSTRUCTOR_NO_CLEARING on each of its element
+	 initializers.  */
+      zero_initialized_p = 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 +4193,11 @@ 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 (zero_initialized_p)
+	    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..bd5dd58d1ab
--- /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]; };
+
+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..47c15edfc73
--- /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]; };
+
+template<int N>
+struct t : b<N> { constexpr t() : b<N>() {} };
+
+constexpr t<1> h1;
+static_assert(h1.m[0].w == false);
+
+constexpr t<42> h2;
+static_assert(h2.m[17].w == false);
-- 
2.28.0.89.g85b4e0a6dc



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