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[Bug tree-optimization/45034] [4.3/4.4/4.5/4.6 Regression] "safe" conversion from unsigned to signed char gives broken code
- From: "rakdver at kam dot mff dot cuni dot cz" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 27 Jul 2010 23:09:13 -0000
- Subject: [Bug tree-optimization/45034] [4.3/4.4/4.5/4.6 Regression] "safe" conversion from unsigned to signed char gives broken code
- References: <bug-45034-7665@http.gcc.gnu.org/bugzilla/>
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
------- Comment #10 from rakdver at kam dot mff dot cuni dot cz 2010-07-27 23:09 -------
Subject: Re: [4.3/4.4/4.5/4.6 Regression]
"safe" conversion from unsigned to signed char gives broken code
> > ux = (unsigned char) x;
> > uy = (unsigned char) -(signed char) ux;
> > ...
> > }
> >
> > That is, the negation of unsigned char value is implemented by casting it to
> > signed char, which introduces signed overflow if the value of x is -128. As
> > far as I understand the C standard, this seems incorrect.
>
> It depends on how GCC interprets that cast and negation:
> - if the cast has C semantics, then (signed char)ux causes overflow
> - if the cast wraps, then it is fine and yields (signed char)-128
> - if the negation has C semantics, then (signed char)-128 is widened to int and
> then negated to 128
> - if the negation maps signed char to signed char, then it causes overflow
>
> IMO, a serious problem with the C standard is that
>
> signed char x = -1;
> signed char y = (signed char)(unsigned char)x;
>
> triggers signed overflow causing undefined behaviour.
no, it does not. The semantics of the cast in this case is not undefined, it
is implementation-defined. GCC defines it in the natural way (and induction
variable analysis takes that into account). The problem is with the negation,
which causes overflow.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45034