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

Patrick Palka ppalka@redhat.com
Mon Aug 3 21:06:15 GMT 2020


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.

> 
> 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



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