From: Patrick Palka Date: Fri, 31 Jul 2020 02:21:41 +0000 (-0400) Subject: c++: decl_constant_value and unsharing [PR96197] X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=8c00059ce058ea2aec2933319e270f5443b8b909;p=gcc.git c++: decl_constant_value and unsharing [PR96197] In the testcase from the PR we're 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, and from there decl_constant_value makes an unshared copy of the VAR_DECL's initializer. But unsharing here is unnecessary because callers of cxx_eval_constant_expression already unshare its result when necessary. To fix this excessive unsharing, this patch adds a new defaulted parameter unshare_p to decl_really_constant_value and decl_constant_value so that callers can control whether to unshare. As a simplification, we can also move the call to unshare_expr in constant_value_1 outside of the loop, since doing unshare_expr on a DECL_P is a no-op. Now that we no longer unshare the result of decl_constant_value and decl_really_constant_value from cxx_eval_constant_expression, 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) : 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. --- 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 ef4b3c4dc3c..cb9bd2dbfba 100644 --- a/gcc/cp/init.c +++ b/gcc/cp/init.c @@ -2272,10 +2272,12 @@ 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, return an unshared + copy of the initializer. */ 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) @@ -2343,9 +2345,9 @@ 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 @@ -2357,26 +2359,36 @@ 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, return an unshared copy of the initializer. */ 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();