[PATCH] c++: decl_constant_value and unsharing [PR96197]
Patrick Palka
ppalka@redhat.com
Thu Jul 30 18:54:12 GMT 2020
On Thu, 30 Jul 2020, Jason Merrill wrote:
> On 7/30/20 9:49 AM, Patrick Palka wrote:
> > On Thu, 30 Jul 2020, Jason Merrill wrote:
> >
> > > On 7/21/20 3:07 PM, Patrick Palka wrote:
> > > > In the testcase from the PR we are seeing excessive memory use (> 5GB)
> > > > during constexpr evaluation, almost all of which is due to the call to
> > > > decl_constant_value in the VAR_DECL/CONST_DECL branch of
> > > > cxx_eval_constant_expression. We reach here every time we evaluate an
> > > > ARRAY_REF of a constexpr VAR_DECL, which in this testcase is quite
> > > > often, and from there decl_constant_value makes an unshared copy of the
> > > > VAR_DECL's initializer, even though the unsharing is not needed at this
> > > > call site (because it is up to callers of cxx_eval_constant_expression
> > > > to unshare).
> > > >
> > > > To fix this, this patch moves the responsibility of unsharing the result
> > > > of decl_constant_value, decl_really_constant_value and
> > > > scalar_constant_value from the callee to the caller.
> > >
> > > How about creating another entry point that doesn't unshare, and using
> > > that in
> > > constexpr evaluation?
> >
> > Is something like this what you have in mind? This adds a defaulted
> > bool parameter to the three routines that controls unsharing (except for
> > decl_constant_value, which instead needs a new overload if we don't want
> > to touch its common declaration in c-common.h.) Bootstrap and regtest
> > in progress.
>
> That looks good, though I don't think we need the added parameter for
> scalar_constant_value.
Hmm, I guess it should always be cheap to unshare an scalar initializer.
So consider the parameter removed for scalar_constant_value.
>
> > -- >8 --
> >
> > Subject: [PATCH] c++: decl_constant_value and unsharing [PR96197]
> >
> > In the testcase from the PR we are seeing excessive memory use (> 5GB)
> > during constexpr evaluation, almost all of which is due to the call to
> > decl_constant_value in the VAR_DECL/CONST_DECL branch of
> > cxx_eval_constant_expression. We reach here every time we evaluate an
> > ARRAY_REF of a constexpr VAR_DECL, which in this testcase is quite
> > often, and from there decl_constant_value makes an unshared copy of the
> > VAR_DECL's initializer, even though the unsharing is not needed at this
> > call site (because callers of cxx_eval_constant_expression already
> > unshare its result when necessary).
> >
> > To fix this excessive unsharing, this patch introduces a new defaulted
> > parameter unshare_p to scalar_constant_value, decl_really_constant_value
> > and decl_constant_value to allow callers to decide if these routines
> > should unshare their result before returning. (Since decl_constant_value
> > is declared in c-common.h, it instead gets a new overload declared in
> > cp-tree.h.)
> >
> > As a simplification, this patch also moves the call to unshare_expr in
> > constant_value_1 outside of the loop, since calling unshare_expr on a
> > DECL_P should be a no-op.
> >
> > Additionally, in unify there is one call to scalar_constant_value that
> > seems to be dead code since we apparently don't see unlowered
> > enumerators anymore, so this patch replaces it with an appropriate
> > gcc_assert.
I'll also push this change as a separate patch, now that
scalar_constant_value is unrelated to the rest of the patch.
Here is the main patch that I guess I'll commit after a full bootstrap
and regtest:
-- >8 --
Subject: [PATCH] c++: decl_constant_value and unsharing [PR96197]
In the testcase from the PR we are seeing excessive memory use (> 5GB)
during constexpr evaluation, almost all of which is due to the call to
decl_constant_value in the VAR_DECL/CONST_DECL branch of
cxx_eval_constant_expression. We reach here every time we evaluate an
ARRAY_REF of a constexpr VAR_DECL, which in this testcase is quite
often, and from there decl_constant_value makes an unshared copy of the
VAR_DECL's initializer, even though the unsharing is not needed at this
call site (because callers of cxx_eval_constant_expression already
unshare its result when necessary).
To fix this excessive unsharing, this patch introduces a new defaulted
parameter unshare_p to decl_really_constant_value and
decl_constant_value to allow callers to choose whether these routines
should unshare the returned result. (Since decl_constant_value is
declared in c-common.h, we introduce a new overload declared in
cp-tree.h instead of changing its existing declaration.)
As a simplification, this patch also moves the call to unshare_expr in
constant_value_1 outside of the loop, since calling unshare_expr on a
DECL_P should be a no-op.
Now that the the calls to decl_constant_value and
decl_really_constant_value from cxx_eval_constant_expression no longer
unshare their result, memory use during constexpr evaluation for the
testcase from the PR falls from ~5GB to 15MB according to -ftime-report.
gcc/cp/ChangeLog:
PR c++/96197
* constexpr.c (cxx_eval_constant_expression) <case CONST_DECL>:
Pass false to decl_constant_value and decl_really_constant_value
so that they don't unshare their result.
* cp-tree.h (decl_constant_value): New declaration with an added
bool parameter.
(decl_really_constant_value): Add bool parameter defaulting to
true to existing declaration.
* init.c (constant_value_1): Add bool parameter which controls
whether to unshare the initializer before returning. Call
unshare_expr at most once.
(scalar_constant_value): Pass true to constant_value_1's new
bool parameter.
(decl_really_constant_value): Add bool parameter and forward it
to constant_value_1.
(decl_constant_value): Likewise, but instead define a new
overload with an added bool parameter.
gcc/testsuite/ChangeLog:
PR c++/96197
* g++.dg/cpp1y/constexpr-array8.C: New test.
---
gcc/cp/constexpr.c | 4 +--
gcc/cp/cp-tree.h | 3 +-
gcc/cp/init.c | 34 +++++++++++++------
gcc/testsuite/g++.dg/cpp1y/constexpr-array8.C | 18 ++++++++++
4 files changed, 45 insertions(+), 14 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-array8.C
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 97dcc1b1d10..b1c1d249c6e 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -5695,9 +5695,9 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
TREE_CONSTANT (r) = true;
}
else if (ctx->strict)
- r = decl_really_constant_value (t);
+ r = decl_really_constant_value (t, /*unshare_p=*/false);
else
- r = decl_constant_value (t);
+ r = decl_constant_value (t, /*unshare_p=*/false);
if (TREE_CODE (r) == TARGET_EXPR
&& TREE_CODE (TARGET_EXPR_INITIAL (r)) == CONSTRUCTOR)
r = TARGET_EXPR_INITIAL (r);
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index ea4871f836a..1e583efd61d 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -6773,7 +6773,8 @@ extern tree build_vec_delete (location_t, tree, tree,
extern tree create_temporary_var (tree);
extern void initialize_vtbl_ptrs (tree);
extern tree scalar_constant_value (tree);
-extern tree decl_really_constant_value (tree);
+extern tree decl_constant_value (tree, bool);
+extern tree decl_really_constant_value (tree, bool = true);
extern int diagnose_uninitialized_cst_or_ref_member (tree, bool, bool);
extern tree build_vtbl_address (tree);
extern bool maybe_reject_flexarray_init (tree, tree);
diff --git a/gcc/cp/init.c b/gcc/cp/init.c
index 913fa4a0080..04404a52167 100644
--- a/gcc/cp/init.c
+++ b/gcc/cp/init.c
@@ -2277,10 +2277,12 @@ build_offset_ref (tree type, tree member, bool address_p,
recursively); otherwise, return DECL. If STRICT_P, the
initializer is only returned if DECL is a
constant-expression. If RETURN_AGGREGATE_CST_OK_P, it is ok to
- return an aggregate constant. */
+ return an aggregate constant. If UNSHARE_P, we unshare the
+ intializer before returning it. */
static tree
-constant_value_1 (tree decl, bool strict_p, bool return_aggregate_cst_ok_p)
+constant_value_1 (tree decl, bool strict_p, bool return_aggregate_cst_ok_p,
+ bool unshare_p)
{
while (TREE_CODE (decl) == CONST_DECL
|| decl_constant_var_p (decl)
@@ -2348,9 +2350,9 @@ constant_value_1 (tree decl, bool strict_p, bool return_aggregate_cst_ok_p)
&& !DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P (decl)
&& DECL_NONTRIVIALLY_INITIALIZED_P (decl))
break;
- decl = unshare_expr (init);
+ decl = init;
}
- return decl;
+ return unshare_p ? unshare_expr (decl) : decl;
}
/* If DECL is a CONST_DECL, or a constant VAR_DECL initialized by constant
@@ -2362,26 +2364,36 @@ tree
scalar_constant_value (tree decl)
{
return constant_value_1 (decl, /*strict_p=*/true,
- /*return_aggregate_cst_ok_p=*/false);
+ /*return_aggregate_cst_ok_p=*/false,
+ /*unshare_p=*/true);
}
-/* Like scalar_constant_value, but can also return aggregate initializers. */
+/* Like scalar_constant_value, but can also return aggregate initializers.
+ If UNSHARE_P, we unshare the initializer before returning it. */
tree
-decl_really_constant_value (tree decl)
+decl_really_constant_value (tree decl, bool unshare_p /*= true*/)
{
return constant_value_1 (decl, /*strict_p=*/true,
- /*return_aggregate_cst_ok_p=*/true);
+ /*return_aggregate_cst_ok_p=*/true,
+ /*unshare_p=*/unshare_p);
}
-/* A more relaxed version of scalar_constant_value, used by the
+/* A more relaxed version of decl_really_constant_value, used by the
common C/C++ code. */
tree
-decl_constant_value (tree decl)
+decl_constant_value (tree decl, bool unshare_p)
{
return constant_value_1 (decl, /*strict_p=*/processing_template_decl,
- /*return_aggregate_cst_ok_p=*/true);
+ /*return_aggregate_cst_ok_p=*/true,
+ /*unshare_p=*/unshare_p);
+}
+
+tree
+decl_constant_value (tree decl)
+{
+ return decl_constant_value (decl, /*unshare_p=*/true);
}
/* Common subroutines of build_new and build_vec_delete. */
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-array8.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-array8.C
new file mode 100644
index 00000000000..339abb69019
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-array8.C
@@ -0,0 +1,18 @@
+// PR c++/96197
+// { dg-do compile { target c++14 } }
+
+struct S {
+ S* p = this;
+};
+
+constexpr S ary[5000] = {};
+
+constexpr int foo() {
+ int count = 0;
+ for (int i = 0; i < 5000; i++)
+ if (ary[i].p != nullptr)
+ count++;
+ return count;
+}
+
+constexpr int bar = foo();
--
2.28.0.rc1
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