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Subject[PATCH 3/8] perf clang: Do not use 'return std::move(something)'
Date
From: Arnaldo Carvalho de Melo <acme@redhat.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@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-lehqf5x5q96l0o8myhb6blz6@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.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
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