Right, signed integer overflow is undefined in C. Try using -fwrapv to
make the compiler treat it more sanely, or do your arithmetic using
unsigned types and just cast to a signed type at the end.
--David Grayson
On Mon, Dec 9, 2024, 11:46 AM Hannes Domani via Mingw-w64-public <
mingw-w64-public@lists.sourceforge.net> wrote:
> Am Montag, 9. Dezember 2024 um 19:50:19 MEZ hat Johannes Khoshnazar-Thoma
> <johan...@johannesthoma.com> Folgendes geschrieben:
>
> > Hi:
> >
> > When compiling following C program:
> >
> > #include <stdio.h>
> >
> > int main(int argc, char ** argv)
> > {
> > volatile unsigned long long a = 0;
> > volatile unsigned long long b = a + (1ULL << 63);
> >
> > if (((long long) a - (long long) b) >= 0)
> > printf("wrong\n");
> > else
> > printf("right\n");
> >
> > return 0;
> > }
> >
> > with mingw gcc compiler:
> >
> > /home/johannes/.zeranoe/mingw-w64/x86_64/bin/x86_64-w64-mingw32-gcc
> --version
> > x86_64-w64-mingw32-gcc (GCC) 12.3.1 20230814
> >
> > and -O2 the output of the program is:
> >
> > wrong
> >
> > When compiling without -O2 or compiling on another gcc (the one that
> comes
> > with Linux Ubuntu for example) the result is:
> >
> > right
> >
> > Is the above comparision in the C program something that is undefined?
> >
> > Looking at the assembly code the difference seems to be the
> > conditional jump expression: it is
> >
> > js .L4
> >
> > in the right case and
> >
> > jl .L4
> >
> > in the wrong case.
> >
> > My 8086 assembly is almost about 35-40 years old, however I recall
> > that jl jumps if operand 2 is less than operand 1 (in System-V
> > syntax ...).
> >
> > Is this maybe a mingw gcc bug?
>
> UBSAN [1] says it's undefined behavior, and I would tend to agree:
>
> example.c:8:24: runtime error: signed integer overflow: 0 -
> -9223372036854775808 cannot be represented in type 'long long int'
>
> [1] https://godbolt.org/z/TcvYsec8j
>
>
> Hannes
>
>
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