[libcamera-devel] [PATCH v2] libcamera: pipeline: uvcvideo: Avoid reference to temporary object

[Laurent Pinchart]

Hi Khem,

On Sat, Mar 27, 2021 at 10:42:07PM -0700, Khem Raj wrote:
> Thanks Laurent, for this, I know it was on my plate to address your
> feedback but I could not get to it thus far

No worries. I was investigating this issue, to have a better
understanding of why the lifetime extension was a problem, and have
reworded the commit message as a result, so I thought it was worth
posting a v2. I'd appreciate if you could test with gcc-11 when you'll
have a chance, but there's no urgency.

> On Sat, Mar 27, 2021 at 7:58 PM Laurent Pinchart wrote:
> >
> > From: Khem Raj <raj.khem at gmail.com>
> >
> > A range-based for loop whose range expression is an array of char
> > pointers and range variable declaration is a const reference to a
> > std::string creates a temporary string from the char pointer and binds
> > the range variable reference to it. This creates a const reference to a
> > temporary, which is valid in C++, and extends the lifetime of the
> > temporary to the lifetime of the reference.
> >
> > However, lifetime extension in range-based for loops is considered as a
> > sign of a potential issue, as a temporary is created for every
> > iteration, which can be costly, and the usage of a reference in the
> > range declaration doesn't make it obvious that the code isn't simply
> > binding a reference to an existing object. gcc 11, with the
> > -Wrange-loop-construct option, flags this:
> >
> > uvcvideo.cpp:432:33: error: loop variable 'name' of type 'const string&' {aka 'const std::__cxx11::basic_string<cha
> > r>&'} binds to a temporary constructed from type 'const char* const' [-Werror=range-loop-construct]
> > |   432 |         for (const std::string &name : { "idVendor", "idProduct" }) {
> > |       |                                 ^~~~
> >
> > To please the compiler, make the range variable a const char *. This may
> > bring a tiny performance improvement, as the name is only used once, in
> > a location where the compiler can use
> >
> >         operator+(const std::string &, const char *)
> >
> > instead of
> >
> >         operator+(const std::string &, const std::string &)
> >
> > Signed-off-by: Khem Raj <raj.khem at gmail.com>
> > Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> >
> > Use a const char * type instead of auto, and update the commit message
> > accordingly.
> >
> > Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > ---
> >  src/libcamera/pipeline/uvcvideo/uvcvideo.cpp | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/src/libcamera/pipeline/uvcvideo/uvcvideo.cpp b/src/libcamera/pipeline/uvcvideo/uvcvideo.cpp
> > index 031f96e28449..b6c6ade5ebaf 100644
> > --- a/src/libcamera/pipeline/uvcvideo/uvcvideo.cpp
> > +++ b/src/libcamera/pipeline/uvcvideo/uvcvideo.cpp
> > @@ -429,7 +429,7 @@ std::string PipelineHandlerUVC::generateId(const UVCCameraData *data)
> >
> >         /* Creata a device ID from the USB devices vendor and product ID. */
> >         std::string deviceId;
> > -       for (const std::string &name : { "idVendor", "idProduct" }) {
> > +       for (const char *name : { "idVendor", "idProduct" }) {
> >                 std::ifstream file(path + "/../" + name);
> >
> >                 if (!file.is_open())

-- 
Regards,

Laurent Pinchart