From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by sourceware.org (Postfix) with ESMTPS id 1C5C13858D28 for ; Tue, 15 Mar 2022 14:03:15 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 1C5C13858D28 Received: from mail-qt1-f200.google.com (mail-qt1-f200.google.com [209.85.160.200]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-517-S23LznuZMJuOklv2UxfV9A-1; Tue, 15 Mar 2022 10:03:12 -0400 X-MC-Unique: S23LznuZMJuOklv2UxfV9A-1 Received: by mail-qt1-f200.google.com with SMTP id ay12-20020a05622a228c00b002e0659131baso13944755qtb.11 for ; Tue, 15 Mar 2022 07:03:12 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:from:date:to:cc:subject:in-reply-to:message-id :references:mime-version; bh=1EbhLT0V3kWKoA29LrAG2apsroNtV5joAn8Xmb/m0EA=; b=r8ila1s+ZUyp6/avwWbhANgL1CYD3CEVxQ7HREopJdnyDPJxwZuRd3sL2oRvp3C5XE 9c3u0xzT94ph+N+OGCcJqcTuG7C0xCru7SE19WWjloBhdXUKCqcgqHUsyeFIqeJUiLZ/ Ek+miEM5gMwgNd74aD+x2cMmx0286ygF1wXQWJUg9DvmqG0ohB+cG5k+p7QYALZBJh03 DKpKgROz8BnnPHbcjN8NcMvQKeSvd7GWulUQ6FJQ+oCzHyfA+KhrpLdLHnL3ZvfI0pzm CiaTcgOrWYQqhUxm/nxg+1Yjm3QNsHiUoYNbe1KxrvtiQgjFZGnxI4JTY3tnKI++tXRS 7jSA== X-Gm-Message-State: AOAM532UgRDXJZq0+0GQGC8tGduMIT47rRI1MG9f2UJjgcJ0R+kP8pWk +nVdC24V8ZR6cKE5WOc+frE4oPa6ICGnUlipNo67KfdSXNl1cK0i3me9lkWOFapiAV5qbImpURZ iSCp+bSA+UCEc70OMHQ== X-Received: by 2002:a05:622a:242:b0:2e1:bb73:220a with SMTP id c2-20020a05622a024200b002e1bb73220amr16384023qtx.664.1647352991331; Tue, 15 Mar 2022 07:03:11 -0700 (PDT) X-Google-Smtp-Source: ABdhPJwT/9IwcS9RokxRVxhwQLJfMwnE0zQMRGetu5IS5bdOph7c6q/GnnWw/dlVAfr2X2dOI8ui4w== X-Received: by 2002:a05:622a:242:b0:2e1:bb73:220a with SMTP id c2-20020a05622a024200b002e1bb73220amr16383959qtx.664.1647352990737; Tue, 15 Mar 2022 07:03:10 -0700 (PDT) Received: from [192.168.1.130] (ool-18e40894.dyn.optonline.net. [24.228.8.148]) by smtp.gmail.com with ESMTPSA id w3-20020a05622a190300b002e19fd2d2bcsm12908529qtc.90.2022.03.15.07.03.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 15 Mar 2022 07:03:10 -0700 (PDT) From: Patrick Palka X-Google-Original-From: Patrick Palka Date: Tue, 15 Mar 2022 10:03:09 -0400 (EDT) To: Jason Merrill cc: Patrick Palka , gcc-patches@gcc.gnu.org, Jonathan Wakely Subject: Re: [PATCH] c++: fold calls to std::move/forward [PR96780] In-Reply-To: Message-ID: <9db703fb-4517-5ee2-fe29-b84cdd30ed9e@idea> References: <20220301220821.1732163-1-ppalka@redhat.com> <1de0b889-66e7-8202-5c5e-526b728a36e8@redhat.com> <6b2257e0-cd0f-4771-7c12-1be4dbf88423@idea> <7b1f33b7-3c59-bf1e-1fa4-7226504a5ae2@redhat.com> MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII X-Spam-Status: No, score=-14.9 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, GIT_PATCH_0, RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H4, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_NONE, TXREP, T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on server2.sourceware.org X-BeenThere: gcc-patches@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 15 Mar 2022 14:03:17 -0000 On Mon, 14 Mar 2022, Jason Merrill wrote: > On 3/14/22 13:13, Patrick Palka wrote: > > On Fri, 11 Mar 2022, Jason Merrill wrote: > > > > > On 3/10/22 11:27, Patrick Palka wrote: > > > > On Wed, 9 Mar 2022, Jason Merrill wrote: > > > > > > > > > On 3/1/22 18:08, Patrick Palka wrote: > > > > > > A well-formed call to std::move/forward is equivalent to a cast, but > > > > > > the > > > > > > former being a function call means it comes with bloated debug info, > > > > > > which > > > > > > persists even after the call has been inlined away, for an operation > > > > > > that > > > > > > is never interesting to debug. > > > > > > > > > > > > This patch addresses this problem in a relatively ad-hoc way by > > > > > > folding > > > > > > calls to std::move/forward into casts as part of the frontend's > > > > > > general > > > > > > expression folding routine. After this patch with -O2 and a > > > > > > non-checking > > > > > > compiler, debug info size for some testcases decreases by about ~10% > > > > > > and > > > > > > overall compile time and memory usage decreases by ~2%. > > > > > > > > > > Impressive. Which testcases? > > > > > > > > I saw the largest percent reductions in debug file object size in > > > > various tests from cmcstl2 and range-v3, e.g. > > > > test/algorithm/set_symmetric_difference4.cpp and .../rotate_copy.cpp > > > > (which are among their biggest tests). > > > > > > > > Significant reductions in debug object file size can be observed in > > > > some libstdc++ testcases too, such as a 5.5% reduction in > > > > std/ranges/adaptor/join.cc > > > > > > > > > > > > > > Do you also want to handle addressof and as_const in this patch, as > > > > > Jonathan > > > > > suggested? > > > > > > > > Yes, good idea. Since each of their argument and return types are > > > > indirect types, I think we can use the same NOP_EXPR-based folding for > > > > them. > > > > > > > > > > > > > > I think we can do this now, and think about generalizing more in stage > > > > > 1. > > > > > > > > > > > Bootstrapped and regtested on x86_64-pc-linux-gnu, is this something > > > > > > we > > > > > > want to consider for GCC 12? > > > > > > > > > > > > PR c++/96780 > > > > > > > > > > > > gcc/cp/ChangeLog: > > > > > > > > > > > > * cp-gimplify.cc (cp_fold) : When optimizing, > > > > > > fold calls to std::move/forward into simple casts. > > > > > > * cp-tree.h (is_std_move_p, is_std_forward_p): Declare. > > > > > > * typeck.cc (is_std_move_p, is_std_forward_p): Export. > > > > > > > > > > > > gcc/testsuite/ChangeLog: > > > > > > > > > > > > * g++.dg/opt/pr96780.C: New test. > > > > > > --- > > > > > > gcc/cp/cp-gimplify.cc | 18 ++++++++++++++++++ > > > > > > gcc/cp/cp-tree.h | 2 ++ > > > > > > gcc/cp/typeck.cc | 6 ++---- > > > > > > gcc/testsuite/g++.dg/opt/pr96780.C | 24 ++++++++++++++++++++++++ > > > > > > 4 files changed, 46 insertions(+), 4 deletions(-) > > > > > > create mode 100644 gcc/testsuite/g++.dg/opt/pr96780.C > > > > > > > > > > > > diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc > > > > > > index d7323fb5c09..0b009b631c7 100644 > > > > > > --- a/gcc/cp/cp-gimplify.cc > > > > > > +++ b/gcc/cp/cp-gimplify.cc > > > > > > @@ -2756,6 +2756,24 @@ cp_fold (tree x) > > > > > > case CALL_EXPR: > > > > > > { > > > > > > + if (optimize > > > > > > > > > > I think this should check flag_no_inline rather than optimize. > > > > > > > > Sounds good. > > > > > > > > Here's a patch that extends the folding to as_const and addressof (as > > > > well as __addressof, which I'm kind of unsure about since it's > > > > non-standard). I suppose it also doesn't hurt to verify that the return > > > > and argument type of the function are sane before we commit to folding. > > > > > > > > -- >8 -- > > > > > > > > Subject: [PATCH] c++: fold calls to std::move/forward [PR96780] > > > > > > > > A well-formed call to std::move/forward is equivalent to a cast, but the > > > > former being a function call means the compiler generates debug info for > > > > it, which persists even after the call has been inlined away, for an > > > > operation that's never interesting to debug. > > > > > > > > This patch addresses this problem in a relatively ad-hoc way by folding > > > > calls to std::move/forward and other cast-like functions into simple > > > > casts as part of the frontend's general expression folding routine. > > > > After this patch with -O2 and a non-checking compiler, debug info size > > > > for some testcases decreases by about ~10% and overall compile time and > > > > memory usage decreases by ~2%. > > > > > > > > PR c++/96780 > > > > > > > > gcc/cp/ChangeLog: > > > > > > > > * cp-gimplify.cc (cp_fold) : When optimizing, > > > > fold calls to std::move/forward and other cast-like functions > > > > into simple casts. > > > > > > > > gcc/testsuite/ChangeLog: > > > > > > > > * g++.dg/opt/pr96780.C: New test. > > > > --- > > > > gcc/cp/cp-gimplify.cc | 36 +++++++++++++++++++++++++++- > > > > gcc/testsuite/g++.dg/opt/pr96780.C | 38 > > > > ++++++++++++++++++++++++++++++ > > > > 2 files changed, 73 insertions(+), 1 deletion(-) > > > > create mode 100644 gcc/testsuite/g++.dg/opt/pr96780.C > > > > > > > > diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc > > > > index d7323fb5c09..efc4c8f0eb9 100644 > > > > --- a/gcc/cp/cp-gimplify.cc > > > > +++ b/gcc/cp/cp-gimplify.cc > > > > @@ -2756,9 +2756,43 @@ cp_fold (tree x) > > > > case CALL_EXPR: > > > > { > > > > - int sv = optimize, nw = sv; > > > > tree callee = get_callee_fndecl (x); > > > > + /* "Inline" calls to std::move/forward and other cast-like > > > > functions > > > > + by simply folding them into the corresponding cast determined by > > > > + their return type. This is cheaper than relying on the middle-end > > > > + to do so, and also means we avoid generating useless debug info for > > > > + them at all. > > > > + > > > > + At this point the argument has already been converted into a > > > > + reference, so it suffices to use a NOP_EXPR to express the > > > > + cast. */ > > > > + if (!flag_no_inline > > > > > > In our conversation yesterday it occurred to me that we might make this a > > > separate flag that defaults to the value of flag_no_inline; I was thinking > > > of > > > -ffold-simple-inlines. Then Vittorio et al can specify that explicitly at > > > -O0 > > > if they'd like. > > > > Makes sense, like so? Bootstrapped and regtested on x86_64-pc-linux-gnu. > > > > The patch defaults -ffold-simple-inlines according to the value of > > flag_no_inline at startup. IIUC this means that if the flag has been > > defaulted to set, then e.g. an optimize("O0") function attribute won't > > disable -ffold-simple-inlines for that function, since we only compute > > its default value once. > > > > I wonder if we therefore instead want to handle defaulting the flag > > when it's used, e.g. check > > > > (flag_fold_simple_inlines == -1 > > ? flag_no_inline > > : flag_fold_simple_inlines) > > > > instead of > > > > flag_fold_simple_inlines > > > > in cp_fold? > > I guess that makes sense, we can't add front-end options to the > default_options_table. But I think let's use OPTION_SET_P instead of checking > for -1. Done. > > > -- >8 -- > > > > Subject: [PATCH] c++: fold calls to std::move/forward [PR96780] > > > > A well-formed call to std::move/forward is equivalent to a cast, but the > > former being a function call means the compiler generates debug info for > > it, which persists even after the call has been inlined away, for an > > operation that's never interesting to debug. > > > > This patch addresses this problem by folding calls to std::move/forward > > and other cast-like functions into simple casts as part of the frontend's > > general expression folding routine. This behavior is controlled by a > > new flag -ffold-simple-inlines which defaults to the value of -fno-inline. > > > > After this patch with -O2 and a non-checking compiler, debug info size > > for some testcases (e.g. from range-v3 and cmcstl2) decreases by about > > ~10% and overall compile time and memory usage decreases by ~2%. > > Did you compare the reduction after handling more functions? The numbers are roughly the same, which I guess is not too surprising since calls to std::move/forward outnumber the other functions by about 10:1 in libstdc++, range-v3 and cmcstl2. The biggest reduction in debug object file size (measured by du) I've observed is 14% with range-v3's test/algorithm/stable_partition.cpp. The biggest reduction in peak memory usage is (measured by /usr/bin/time -v) is 5% with cmcstl's test/algorithm/set_symmetric_difference4.cpp. The biggest reduction in compile time (measured by perf stat) is about 3%, also from that testcase. -- >8 -- Subject: [PATCH] c++: fold calls to std::move/forward [PR96780] A well-formed call to std::move/forward is equivalent to a cast, but the former being a function call means the compiler generates debug info for it, which persists even after the call has been inlined away, for an operation that's never interesting to debug. This patch addresses this problem by folding calls to std::move/forward and other cast-like functions into simple casts as part of the frontend's general expression folding routine. This behavior is controlled by a new flag -ffold-simple-inlines, and otherwise by -fno-inline, so that users can enable such folding even with -O0 (which implies -fno-inline). After this patch with -O2 and a non-checking compiler, debug info size for some testcases (e.g. from range-v3 and cmcstl2) decreases by about ~10% and overall compile time and memory usage decreases by ~2%. PR c++/96780 gcc/c-family/ChangeLog: * c-opts.cc (c_common_post_options): Handle defaulting of flag_fold_simple_inlines. * c.opt: Add -ffold-simple-inlines. gcc/cp/ChangeLog: * cp-gimplify.cc (cp_fold) : Fold calls to std::move/forward and other cast-like functions into simple casts. gcc/testsuite/ChangeLog: * g++.dg/opt/pr96780.C: New test. --- gcc/c-family/c.opt | 4 ++++ gcc/cp/cp-gimplify.cc | 38 +++++++++++++++++++++++++++++- gcc/testsuite/g++.dg/opt/pr96780.C | 38 ++++++++++++++++++++++++++++++ 3 files changed, 79 insertions(+), 1 deletion(-) create mode 100644 gcc/testsuite/g++.dg/opt/pr96780.C diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt index 9cfd2a6bc4e..9a4828ebe37 100644 --- a/gcc/c-family/c.opt +++ b/gcc/c-family/c.opt @@ -1731,6 +1731,10 @@ Support dynamic initialization of thread-local variables in a different translat fexternal-templates C++ ObjC++ WarnRemoved +ffold-simple-inlines +C++ ObjC++ Optimization Var(flag_fold_simple_inlines) +Fold calls to simple inline functions. + ffor-scope C++ ObjC++ WarnRemoved diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc index d7323fb5c09..41daaf18725 100644 --- a/gcc/cp/cp-gimplify.cc +++ b/gcc/cp/cp-gimplify.cc @@ -41,6 +41,7 @@ along with GCC; see the file COPYING3. If not see #include "file-prefix-map.h" #include "cgraph.h" #include "omp-general.h" +#include "opts.h" /* Forward declarations. */ @@ -2756,9 +2757,44 @@ cp_fold (tree x) case CALL_EXPR: { - int sv = optimize, nw = sv; tree callee = get_callee_fndecl (x); + /* "Inline" calls to std::move/forward and other cast-like functions + by simply folding them into a corresponding cast to their return + type. This is cheaper than relying on the middle-end to do so, and + also means we avoid generating useless debug info for them at all. + + At this point the argument has already been converted into a + reference, so it suffices to use a NOP_EXPR to express the + cast. */ + if ((OPTION_SET_P (flag_fold_simple_inlines) + ? flag_fold_simple_inlines + : !flag_no_inline) + && call_expr_nargs (x) == 1 + && decl_in_std_namespace_p (callee) + && DECL_NAME (callee) != NULL_TREE + && (id_equal (DECL_NAME (callee), "move") + || id_equal (DECL_NAME (callee), "forward") + || id_equal (DECL_NAME (callee), "addressof") + /* This addressof equivalent is used heavily in libstdc++. */ + || id_equal (DECL_NAME (callee), "__addressof") + || id_equal (DECL_NAME (callee), "as_const"))) + { + r = CALL_EXPR_ARG (x, 0); + /* Check that the return and arguments types are sane before + folding. */ + if (INDIRECT_TYPE_P (TREE_TYPE (x)) + && INDIRECT_TYPE_P (TREE_TYPE (r))) + { + if (!same_type_p (TREE_TYPE (x), TREE_TYPE (r))) + r = build_nop (TREE_TYPE (x), r); + x = cp_fold (r); + break; + } + } + + int sv = optimize, nw = sv; + /* Some built-in function calls will be evaluated at compile-time in fold (). Set optimize to 1 when folding __builtin_constant_p inside a constexpr function so that fold_builtin_1 doesn't fold it to 0. */ diff --git a/gcc/testsuite/g++.dg/opt/pr96780.C b/gcc/testsuite/g++.dg/opt/pr96780.C new file mode 100644 index 00000000000..61e11855eeb --- /dev/null +++ b/gcc/testsuite/g++.dg/opt/pr96780.C @@ -0,0 +1,38 @@ +// PR c++/96780 +// Verify calls to std::move/forward are folded away by the frontend. +// { dg-do compile { target c++11 } } +// { dg-additional-options "-ffold-simple-inlines -fdump-tree-gimple" } + +#include + +struct A; + +extern A& a; +extern const A& ca; + +void f() { + auto&& x1 = std::move(a); + auto&& x2 = std::forward(a); + auto&& x3 = std::forward(a); + + auto&& x4 = std::move(ca); + auto&& x5 = std::forward(ca); + auto&& x6 = std::forward(ca); + + auto x7 = std::addressof(a); + auto x8 = std::addressof(ca); +#if __GLIBCXX__ + auto x9 = std::__addressof(a); + auto x10 = std::__addressof(ca); +#endif +#if __cpp_lib_as_const + auto&& x11 = std::as_const(a); + auto&& x12 = std::as_const(ca); +#endif +} + +// { dg-final { scan-tree-dump-not "= std::move" "gimple" } } +// { dg-final { scan-tree-dump-not "= std::forward" "gimple" } } +// { dg-final { scan-tree-dump-not "= std::addressof" "gimple" } } +// { dg-final { scan-tree-dump-not "= std::__addressof" "gimple" } } +// { dg-final { scan-tree-dump-not "= std::as_const" "gimple" } } -- 2.35.1.500.gb896f729e2