[Bug c/60090] New: For expression without ~, gcc -O1 emits "comparison of promoted ~unsigned with unsigned"

[Bug c/60090] New: For expression without ~, gcc -O1 emits "comparison of promoted ~unsigned with unsigned"

[chengniansun at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60090

            Bug ID: 60090
           Summary: For expression without ~, gcc -O1 emits "comparison of
                    promoted ~unsigned with unsigned"
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: chengniansun at gmail dot com

For the expression "(l ^ a) != b", gcc -O0 emits no warning, but gcc -O1 emits
a warning "comparison of promoted ~unsigned with unsigned". After checking the
gimple code, I found the optimization transforms the expression to a bitwise
not expression. 

However, in a syntactic perspective, the warning message is confusing, given
the fact that there is no '~' in this expression. 

$: cat s.c
int fn1(unsigned char a, unsigned char b) {
  const unsigned l = 4294967295u;
  return (l ^ a) != b;
}
$: gcc-trunk -c -Wsign-compare s.c
$: gcc-trunk -c -Wsign-compare s.c -O1
s.c: In function ‘fn1’:
s.c:3:18: warning: comparison of promoted ~unsigned with unsigned
[-Wsign-compare]
   return (l ^ a) != b;
                  ^
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Subject: [Bug ipa/59469] [4.8/4.9 Regression] LLVM build failure with gcc LTO
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idY469

--- Comment #45 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
The bug here is that lto-cgraph.c still checks DECL_COMDAT as a condition if
symbol is duplicated or partitioned.  We really need to get the logic into
lto-cgraph.c and use same test at both places... Will do that tomorrow.

[Bug c/60090] For expression without ~, gcc -O1 emits "comparison of promoted ~unsigned with unsigned"

[mpolacek at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60090

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-02-06
                 CC|                            |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
This (and a bunch of other related PRs) is about early folding in the FE.  
c_fully_fold when optimize calls fold_build2 -> fold_binary_loc that transforms
the expression and then we issue those seemingly unrelated warnings.  I'd say
that while we want the exprs to be folded, we should issue diagnostics for the
unfolded exprs.  (I think it's been discussed numerous times in the past.)

Nothing for stage4, but we/I should address this in gcc 5.0.

[Bug c/60090] For expression without ~, gcc -O1 emits "comparison of promoted ~unsigned with unsigned"

[mpolacek at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60090

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
C++ folds while parsing and here for both -O0 -O we get

y.c: In function ‘int fn1(unsigned char, unsigned char)’:
y.c:3:18: warning: comparison of promoted ~unsigned with unsigned
[-Wsign-compare]
   return (l ^ a) != b;
                  ^
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Subject: [Bug c/60090] For expression without ~, gcc -O1 emits "comparison of promoted ~unsigned with unsigned"
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id`090

--- Comment #3 from Chengnian Sun <chengniansun at gmail dot com> ---
Thanks, Marek.

May I ask another question on the Gcc optimizations and warnings? Is there a
policy that the warnings should be independent of the optimization levels? That
is, for all optimization levels, Gcc should always emit consistent warnings.

I know that Clang has such a policy. But based on what I have observed on Gcc,
it seems that Gcc does not have this policy.

[Bug c/60090] For expression without ~, gcc -O1 emits "comparison of promoted ~unsigned with unsigned"

[mpolacek at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60090

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I believe we strive for the warnings be independent of the optimization level,
but it's not always possible, we have tons of bugs where -Wuninitialized
depends on the optimization level, sometimes -Warray-bounds warns only on -O3,
some warnings depend on VRP or SRA, on -O some uses are constant-propagated
etc.  In the middle-end we often warn only about variables in SSA form which
are made, as a part of some optimization, non-addressable.

[Bug c/60090] For expression without ~, gcc -O1 emits "comparison of promoted ~unsigned with unsigned"

[manu at gcc dot gnu.org]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60090

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu.org

--- Comment #5 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Marek Polacek from comment #1)
> This (and a bunch of other related PRs) is about early folding in the FE.  
> c_fully_fold when optimize calls fold_build2 -> fold_binary_loc that
> transforms the expression and then we issue those seemingly unrelated
> warnings.  I'd say that while we want the exprs to be folded, we should
> issue diagnostics for the unfolded exprs.  (I think it's been discussed
> numerous times in the past.)
> 
> Nothing for stage4, but we/I should address this in gcc 5.0.

I think this should be the plan:

http://gcc.gnu.org/ml/gcc/2013-11/msg00253.html

but someone has to implement it ;-)

In those cases where folding helps to avoid false positives, it would be nice
to be able to still fold on-demand or to delay the warnings until folding can
happen.
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id`078

--- Comment #9 from Bernd Edlinger <bernd.edlinger at hotmail dot de> ---
Created attachment 32065
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id2065&actioníit
possible fix

well, I don't know if the Finalize method are supposed
to be called in a sequential manner, which GNAT does obviously not
guarantee.
But how about this, for a fix?

[Bug c/60090] For expression without ~, gcc -O1 emits "comparison of promoted ~unsigned with unsigned"

[mpolacek at gcc dot gnu.org]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60090

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to Manuel López-Ibáñez from comment #5)
> http://gcc.gnu.org/ml/gcc/2013-11/msg00253.html

Exactly.  I hope I can tackle at least a part of it in next stage 1.

> In those cases where folding helps to avoid false positives, it would be nice
> to be able to still fold on-demand or to delay the warnings until folding can 
> happen.

Sure.
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id`092

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Created attachment 32066
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id2066&actioníit
part #2, C11 aligned_alloc

It was noted that aligned_alloc is standard enough to be supported (and with
nicer interface albeit possibly clobbering errno ...).

[Bug c/60090] For expression without ~, gcc -O1 emits "comparison of promoted ~unsigned with unsigned"

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60090

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This one still happens on the trunk even with PR 107465 fixed. The reason is
because even though a warning here is correct, it is not wanted due to
requiring constant folding. Note you can get also the incorrect warning wording
at -O0 with constexpr in GCC 13+ (and -std=c2x).