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From: Jason Merrill <jason@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org Subject: [gcc r9-10177] c++: NRV in lambda in template [PR91217] Date: Fri, 13 May 2022 17:42:09 +0000 (GMT) [thread overview] Message-ID: <20220513174209.8B5A9395BC3E@sourceware.org> (raw) https://gcc.gnu.org/g:da999d938e516ff12b39a0aa7e8f6d97fbee60df commit r9-10177-gda999d938e516ff12b39a0aa7e8f6d97fbee60df Author: Jason Merrill <jason@redhat.com> Date: Sat Apr 3 01:07:36 2021 -0400 c++: NRV in lambda in template [PR91217] tsubst_lambda_expr was producing a function with two blocks that claimed to be the outermost block in the function body, one from the call to start_lambda_function in tsubst_lambda_expr, and one from tsubsting the block added by start_lambda_function when we first parsed the lambda. This messed with the named return value optimization, which only works for variables in the outermost block. gcc/cp/ChangeLog: PR c++/91217 * pt.c (tsubst_lambda_expr): Skip the body block from DECL_SAVED_TREE. gcc/testsuite/ChangeLog: PR c++/91217 * g++.dg/opt/nrv20.C: New test. Diff: --- gcc/cp/pt.c | 9 +++++++-- gcc/testsuite/g++.dg/opt/nrv20.C | 20 ++++++++++++++++++++ 2 files changed, 27 insertions(+), 2 deletions(-) diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c index eeee23181fc..4bae467fa71 100644 --- a/gcc/cp/pt.c +++ b/gcc/cp/pt.c @@ -18472,8 +18472,13 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl) the purposes of template argument deduction. */ complain = tf_warning_or_error; - tsubst_expr (DECL_SAVED_TREE (oldfn), args, complain, r, - /*constexpr*/false); + tree saved = DECL_SAVED_TREE (oldfn); + if (TREE_CODE (saved) == BIND_EXPR && BIND_EXPR_BODY_BLOCK (saved)) + /* We already have a body block from start_lambda_function, we don't + need another to confuse NRV (91217). */ + saved = BIND_EXPR_BODY (saved); + + tsubst_expr (saved, args, complain, r, /*constexpr*/false); finish_lambda_function (body); diff --git a/gcc/testsuite/g++.dg/opt/nrv20.C b/gcc/testsuite/g++.dg/opt/nrv20.C new file mode 100644 index 00000000000..ade0c28824a --- /dev/null +++ b/gcc/testsuite/g++.dg/opt/nrv20.C @@ -0,0 +1,20 @@ +// PR c++/91217 +// { dg-do compile { target c++11 } } +// { dg-additional-options -fdump-tree-gimple } +// { dg-final { scan-tree-dump-not "<retval> = a" "gimple" } } + +struct A +{ + int ar[42]; +}; + +template <class T> +A f() +{ + return [] { A a; return a; }(); +} + +int main() +{ + f<int>(); +}
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