linux-kernel - [tip:perf/core] perf clang: Do not use 'return std::move(something)'

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Date:   Sat, 9 Feb 2019 04:48:16 -0800
From:   tip-bot for Arnaldo Carvalho de Melo <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     adrian.hunter@...el.com, lclaudio@...hat.com,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, acme@...hat.com,
        namhyung@...nel.org, mingo@...nel.org, jolsa@...nel.org,
        wangnan0@...wei.com, hpa@...or.com
Subject: [tip:perf/core] perf clang: Do not use 'return
 std::move(something)'

Commit-ID:  5f40fa97669bb3d97a43cd5c8f69f520d8dcb106
Gitweb:     https://git.kernel.org/tip/5f40fa97669bb3d97a43cd5c8f69f520d8dcb106
Author:     Arnaldo Carvalho de Melo <acme@...hat.com>
AuthorDate: Mon, 4 Feb 2019 11:04:20 -0300
Committer:  Arnaldo Carvalho de Melo <acme@...hat.com>
CommitDate: Wed, 6 Feb 2019 10:00:39 -0300

perf clang: Do not use 'return std::move(something)'

It prevents copy elision, generating this warning when building with
fedora:rawhide's clang:

  clang version 7.0.1 (Fedora 7.0.1-2.fc30)
  Target: x86_64-unknown-linux-gnu
  Thread model: posix
  InstalledDir: /usr/bin
  Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-redhat-linux/9
  Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/9
  Selected GCC installation: /usr/bin/../lib/gcc/x86_64-redhat-linux/9
  Candidate multilib: .;@m64
  Candidate multilib: 32;@m32
  Selected multilib: .;@m64

  $ make -C tools/perf CC=clang LIBCLANGLLVM=1
  <SNIP>
  util/c++/clang.cpp: In function 'std::unique_ptr<llvm::SmallVectorImpl<char> > perf::getBPFObjectFromModule(llvm::Module*)':
  util/c++/clang.cpp:163:18: error: moving a local object in a return statement prevents copy elision [-Werror=pessimizing-move]
    163 |  return std::move(Buffer);
        |         ~~~~~~~~~^~~~~~~~
  util/c++/clang.cpp:163:18: note: remove 'std::move' call
  cc1plus: all warnings being treated as errors
  <SNIP>

References:

  http://www.cplusplus.com/forum/general/186411/#msg908572
  https://en.cppreference.com/w/cpp/language/return#Notes
  https://en.cppreference.com/w/cpp/language/copy_elision

Cc: Adrian Hunter <adrian.hunter@...el.com>
Cc: Jiri Olsa <jolsa@...nel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@...hat.com>
Cc: Namhyung Kim <namhyung@...nel.org>
Cc: Wang Nan <wangnan0@...wei.com>
Link: https://lkml.kernel.org/n/tip-lehqf5x5q96l0o8myhb6blz6@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>
---
 tools/perf/util/c++/clang.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/util/c++/clang.cpp b/tools/perf/util/c++/clang.cpp
index 89512504551b..39c0004f2886 100644
--- a/tools/perf/util/c++/clang.cpp
+++ b/tools/perf/util/c++/clang.cpp
@@ -160,7 +160,7 @@ getBPFObjectFromModule(llvm::Module *Module)
 	}
 	PM.run(*Module);
 
-	return std::move(Buffer);
+	return Buffer;
 }
 
 }