[llvm-dev] Strange local variable cv::VideoCapture allocated

[Adrian Tong via llvm-dev]

Hi

I am using clang-6 to compile this C++ code and I see a strange temporary
variable allocated at expression address 0x7ff1131536e8.

If I change the ternary operator at line 483 to an if-else, the temporary
is not allocated.

Thanks

Variables:

=========

FFMPEGVideoCapture ffmpeg_video_capture_;

cv::VideoCapture opencv_video_capture_;



Function:

========

bool StreamData::readFileFrame(Mat& curr_frame, int& frame_id) {

    // read frame from file

    ++frame_id;

483: enable_ffmpeg_decoder_ ? (ffmpeg_video_capture_.read(curr_frame))

           : opencv_video_capture_ >> curr_frame;

    return !curr_frame.empty();

}



Generated AST from ast-dump:

===========================

-CXXMethodDecl 0x7ff113148810 parent 0x7ff114400cb0 prev 0x7ff114401e30
<line:480:1, line:486:1> line:480:18 used readFileFrame 'bool (cv::Mat &,
int &)'


| |-ParmVarDecl 0x7ff113148708 <col:32, col:37> col:37 used curr_frame
'cv::Mat &'


| |-ParmVarDecl 0x7ff113148780 <col:49, col:54> col:54 used frame_id 'int &'


| `-CompoundStmt 0x7ff1131537d8 <col:64, line:486:1>


|   |-UnaryOperator 0x7ff113148930 <line:482:5, col:7> 'int' lvalue prefix
'++'


|   | `-DeclRefExpr 0x7ff113148908 <col:7> 'int' lvalue ParmVar
0x7ff113148780 'frame_id' 'int &'


|   |-ExprWithCleanups 0x7ff1131536e8 <line:483:5, line:484:32>
'cv::VideoCapture'


|   | `-ConditionalOperator 0x7ff1131536b8 <line:483:5, line:484:32>
'cv::VideoCapture'


|   |   |-ImplicitCastExpr 0x7ff113153178 <line:483:5> 'bool'
<LValueToRValue>


|   |   | `-MemberExpr 0x7ff113148968 <col:5> 'bool' lvalue
->enable_ffmpeg_decoder_ 0x7ff114221ef0


|   |   |   `-CXXThisExpr 0x7ff113148950 <col:5> 'xyz::StreamData *' this


|   |   |-CXXBindTemporaryExpr 0x7ff113153620 <col:30, col:69>
'cv::VideoCapture' (CXXTemporary 0x7ff113153618)


|   |   | `-CXXConstructExpr 0x7ff1131535e0 <col:30, col:69>
'cv::VideoCapture' 'void (const cv::VideoCapture &) noexcept(false)'
elidable


|   |   |   `-MaterializeTemporaryExpr 0x7ff1131535c8 <col:30, col:69>
'const cv::VideoCapture' lvalue


|   |   |     `-ImplicitCastExpr 0x7ff1131535b0 <col:30, col:69> 'const
cv::VideoCapture' <NoOp>


|   |   |       `-CXXBindTemporaryExpr 0x7ff113153590 <col:30, col:69>
'cv::VideoCapture' (CXXTemporary 0x7ff113153588)


|   |   |         `-CXXConstructExpr 0x7ff113153550 <col:30, col:69>
'cv::VideoCapture' 'void (const cv::VideoCapture &) noexcept(false)'
elidable


|   |   |           `-MaterializeTemporaryExpr 0x7ff113153238 <col:30,
col:69> 'const cv::VideoCapture' lvalue


|   |   |             `-ImplicitCastExpr 0x7ff113153220 <col:30, col:69>
'const cv::VideoCapture' <NoOp>


|   |   |               `-CXXBindTemporaryExpr 0x7ff113153200 <col:30,
col:69> 'cv::VideoCapture' (CXXTemporary 0x7ff1131531f8)


|   |   |                 `-ImplicitCastExpr 0x7ff1131531e0 <col:30,
col:69> 'cv::VideoCapture' <ConstructorConversion>


|   |   |                   `-CXXConstructExpr 0x7ff1131531a8 <col:30,
col:69> 'cv::VideoCapture' 'void (int)'


|   |   |                     `-ImplicitCastExpr 0x7ff113153190 <col:30,
col:69> 'int' <IntegralCast>


|   |   |                       `-ParenExpr 0x7ff113148a80 <col:30, col:69>
'bool'


|   |   |                         `-CXXMemberCallExpr 0x7ff113148a50
<col:31, col:68> 'bool'


|   |   |                           |-MemberExpr 0x7ff1131489f0 <col:31,
col:53> '<bound member function type>' .read 0x7ff1143a6f38


|   |   |                           | `-MemberExpr 0x7ff1131489b8 <col:31>
'FFMPEGVideoCapture' lvalue ->ffmpeg_video_capture_ 0x7ff114402a38


|   |   |                           |   `-CXXThisExpr 0x7ff1131489a0
<col:31> 'xyz::StreamData *' this


|   |   |                           `-DeclRefExpr 0x7ff113148a28 <col:58>
'cv::Mat' lvalue ParmVar 0x7ff113148708 'curr_frame' 'cv::Mat &'


|   |   `-CXXBindTemporaryExpr 0x7ff113153698 <line:484:7, col:32>
'cv::VideoCapture' (CXXTemporary 0x7ff113153690)


|   |     `-CXXConstructExpr 0x7ff113153658 <col:7, col:32>
'cv::VideoCapture' 'void (const cv::VideoCapture &) noexcept(false)'


|   |       `-ImplicitCastExpr 0x7ff113153640 <col:7, col:32> 'const
cv::VideoCapture' lvalue <NoOp>


|   |         `-CXXOperatorCallExpr 0x7ff113153130 <col:7, col:32>
'cv::VideoCapture' lvalue


|   |           |-ImplicitCastExpr 0x7ff113153118 <col:29>
'cv::VideoCapture &(*)(cv::Mat &)' <FunctionToPointerDecay>


|   |           | `-DeclRefExpr 0x7ff1131530c0 <col:29> 'cv::VideoCapture
&(cv::Mat &)' lvalue CXXMethod 0x55be741b2b60 'operator>>'
'cv::VideoCapture &(cv::Mat &)'


|   |           |-MemberExpr 0x7ff113148ab8 <col:7>
'cv::VideoCapture':'cv::VideoCapture' lvalue ->opencv_video_capture_
0x7ff114402b00


|   |           | `-CXXThisExpr 0x7ff113148aa0 <col:7> 'xyz::StreamData *'
this


|   |           `-DeclRefExpr 0x7ff113148af0 <col:32> 'cv::Mat' lvalue
ParmVar 0x7ff113148708 'curr_frame' 'cv::Mat &'
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