linux-kernel - [PATCH 3/8] perf clang: Do not use 'return std::move(something)'

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Message-Id: <20190205150708.9012-4-acme@kernel.org>
Date:   Tue,  5 Feb 2019 12:07:03 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Clark Williams <williams@...hat.com>, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Luis Cláudio Gonçalves 
        <lclaudio@...hat.com>, Namhyung Kim <namhyung@...nel.org>,
        Wang Nan <wangnan0@...wei.com>
Subject: [PATCH 3/8] perf clang: Do not use 'return std::move(something)'

From: Arnaldo Carvalho de Melo <acme@...hat.com>

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;
 }
 
 }
-- 
2.20.1