Re: Warn about conversion from floating point to integers

[Andrea Bocci <andrea.bocci@cern.ch>]

On Mon, 22 Aug 2022 at 19:33, Segher Boessenkool <segher@kernel.crashing.org>
wrote:

> Hi!
>
> On Sat, Aug 20, 2022 at 11:21:50AM +0200, Andrea Bocci wrote:
> > is there a way to ask GCC to produce a warning when there is a conversion
> > from a floating point type (float or double) to an integer type (char,
> > short, int, long, etc.), but not when there is a conversion from double
> to
> > float ?
> >
> > The reason is that we often do computations in double precision and store
> > the result in single precision, so I'd rather not warn about that.
> >
> > -Wconversion warns about both kinds and more.
> > -Wfloat-conversion warns about both kinds.
>
> There is no separate option.  But, you can just grep the error messages?
>
> fc.c: In function 'f':
> fc.c:1:28: warning: conversion from 'double' to 'float' may change value
> [-Wfloat-conversion]
>     1 | float f(double x) { return x; }
>       |                            ^
> fc.c: In function 'g':
> fc.c:2:26: warning: conversion from 'double' to 'int' may change value
> [-Wfloat-conversion]
>     2 | int g(double x) { return x; }
>       |                          ^
>
> Not ideal of course, but this should be workable?  (You may want to set
> LANG=C to get parsable warning messages).
>
> If you want a separate -W option for this, please file a PR?
> See <https://gcc.gnu.org/bugs.html>.  Thanks!
>


Hi Segher,
thank you for your message !

From a test build of our code base with -Wfloat-conversion we got over 700k
warnings from 65k unique locations, the vast majority being about "warning:
conversion from 'double' to 'float' may change value".
So, while we can run an ad-hoc build with -Wfloat-conversion and
post-process the compiler messages, it's not something we can really keep
enabled all the time.

Making a PR for this might be a tad complicated, but I agree it would be
the best way forward... so thanks for the suggestion :-)

Ciao,
.Andrea


-- 

Dr. Andrea Bocci
Applied Physicist, CMS Experiment
<https://cms.cern/>EP-CMD, CERN
<https://home.cern/>168990 (+41 75 411 8990)
CERN 40/1-B01 <https://maps.cern.ch/mapsearch/mapsearch.htm?n=['40/1-B01']>
E27810

Esplanade des particules, 1, CH-1211 Geneve 23, Switzerland
<https://goo.gl/maps/AMjHdhXzYk2itSu7A>