[PATCH 1/1] gdb, testsuite: Adapt gdb.base/callfuncs.exp for new clang warnings.

[Willgerodt, Felix]

> -----Original Message-----
> From: Andrew Burgess <aburgess@redhat.com>
> Sent: Dienstag, 19. Juli 2022 11:14
> To: Willgerodt, Felix <felix.willgerodt@intel.com>; Bruno Larsen
> <blarsen@redhat.com>; gdb-patches@sourceware.org
> Subject: RE: [PATCH 1/1] gdb, testsuite: Adapt gdb.base/callfuncs.exp for
> new clang warnings.
> 
> "Willgerodt, Felix via Gdb-patches" <gdb-patches@sourceware.org> writes:
> 
> >> -----Original Message-----
> >> From: Andrew Burgess <aburgess@redhat.com>
> >> Sent: Montag, 18. Juli 2022 15:37
> >> To: Willgerodt, Felix <felix.willgerodt@intel.com>; Bruno Larsen
> >> <blarsen@redhat.com>; gdb-patches@sourceware.org
> >> Subject: RE: [PATCH 1/1] gdb, testsuite: Adapt gdb.base/callfuncs.exp for
> >> new clang warnings.
> >>
> >> "Willgerodt, Felix via Gdb-patches" <gdb-patches@sourceware.org>
> writes:
> >>
> >> >> -----Original Message-----
> >> >> From: Bruno Larsen <blarsen@redhat.com>
> >> >> Sent: Mittwoch, 8. Juni 2022 14:59
> >> >> To: Willgerodt, Felix <felix.willgerodt@intel.com>; gdb-
> >> >> patches@sourceware.org
> >> >> Subject: Re: [PATCH 1/1] gdb, testsuite: Adapt gdb.base/callfuncs.exp
> for
> >> >> new clang warnings.
> >> >>
> >> >>
> >> >> On 6/8/22 04:17, Felix Willgerodt via Gdb-patches wrote:
> >> >> > This patch fixes two issues with callfuncs.exp, which are both related
> >> >> > to new Clang warnings:
> >> >>
> >> >> Hi Felix!
> >> >>
> >> >> Thanks for this!
> >> >>
> >> >> >
> >> >> > 1) Clang 15.0.0 added a new warning for deprecated non-prototype
> >> >> functions:
> >> >> > https://reviews.llvm.org/D122895
> >> >> > Callfuncs.exp is impacted and won't run due to new warnings:
> >> >> >
> >> >> > callfuncs.c:339:5: warning: a function declaration without a prototype
> is
> >> >> > deprecated in all versions of C and is not supported in C2x
> >> >> > [-Wdeprecated-non-prototype]
> >> >> > int t_float_values (float_arg1, float_arg2)
> >> >> >
> >> >> > This patch disables those warnings with -Wno-deprecated-non-
> >> prototype.
> >> >> > Removing the test for deprecated syntax would also be an option.
> But I
> >> will
> >> >> > leave that for others to decide.
> >> >>
> >> >> I like your solution with -Wno-deprecated-non-prototype.
> >> >>
> >> >> I think it is important (at least for now) to keep this test, since we have
> to
> >> >> support very old setups.
> >> >>
> >> >> >
> >> >> > 2) The other new warnings are about comparing a define with floats
> and
> >> >> doubles:
> >> >> >
> >> >> > callfuncs.c:518:1: warning: floating-point comparison is always true;
> >> >> constant
> >> >> > cannot be represented exactly in type 'float' [-Wliteral-range]
> >> >> > DEF_FUNC_VALUES_3(fc, float, crealf, cimagf)
> >> >> >
> >> >> > This can be fixed by making the define a float.
> >> >>
> >> >> Genuine question, would this not cause a problem for the times where
> >> the
> >> >> parameters are doubles and DELTA is a float? If it isn't a problem, I'm
> fine
> >> >> with this.
> >> >>
> >> >
> >> > I was wondering about that as well, but it still passes with GCC, clang and
> >> Intel
> >> > compilers and there are no compiler warnings about it.
> >> > I couldn't really figure it out if it would actually a problem somewhere. I
> >> could only
> >> > test on linux x86 though. My best guess was, as it is the "smaller
> precision"
> >> the
> >> > compiler will do the right thing.
> >>
> >> Indeed, my understanding of type promotion is that the compiler will
> >> promote the float argument to double or long double as needed.
> >>
> >> But then, prior to this patch, when DELTA was just (0.001), and would be
> >> considered a double, I would have expected, in any comparison between
> a
> >> float and DELTA, for the float to be converted to double, so I don't
> >> really understand that part of the original patch.
> >>
> >> Thanks,
> >> Andrew
> >
> > Thanks for your input.
> > What I didn't quite realize when writing the patch, is that float literals
> without
> > suffix are of type double. I double checked the ISO C11 draft and it is
> mentioned
> > there as well.
> >
> > So to me this is likely a compiler bug. Or did I miss anything?
> 
> I did wonder the same, but I don't claim to be an expert on language
> details.  I don't have immediate access to clang 15, but it might be
> interesting to try and reduce the test program down to just that
> error/warning case, and see if the code makes sense.
> 
> Thanks,
> Andrew

I did this experiment:


$ cat float.c
#define DELTA 0.001

int main() {
float a = 0.3;

if (a == DELTA)
  return 1;
else
  return 0;
}
$ clang float.c 
float.c:7:7: warning: floating-point comparison is always false; constant
cannot be represented exactly in type 'float' [-Wliteral-range]
if (a == DELTA)
    ~ ^  ~~~~~
1 warning generated.
$


When I make the define a variable, there is no warning:


$ cat double.c 
double DELTA = 0.001;

int main() {
float a = 0.3;

if (a == DELTA)
  return 1;
else
  return 0;
}
$ clang double.c 
$

Thanks,
Felix

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