Re: Issue with __int128 in powerpc64le

[Richard Biener <richard dot guenther at gmail dot com>]

On Fri, Dec 19, 2014 at 10:13 AM, Roger Ferrer IbÃÃez
<rofirrim@gmail.com> wrote:
> Hi,
>
> I'm observing a weird behaviour in PowerPC64 Little Endian that does
> not seem to occur on other architectures supporting __int128. The
> following code, when compiled with -O1 generates wrong output.
>
> -- test.c
> #include <stdio.h>
>
> typedef unsigned __int128 uint128_t;
>
> #define PRINT(value) \
>     { union u { uint128_t i; unsigned long long l[2]; } _t = { .i = value }; \
>         fprintf(stderr, "%s => <%016llx, %016llx>\n", #value, _t.l[1],
> _t.l[0]); }
>
> __attribute__((noinline))
> uint128_t get_int(uint128_t value, unsigned int num_bytes)
> {
>     uint128_t mask = ~(uint128_t)0;
>     mask <<= (uint128_t)(8 * num_bytes); /* assuming 1 byte = 8 bits */
>     mask = ~mask;
>     value &= mask;
>
>     return value;
> }
>
> int main(int argc, char* argv[])
> {
>     uint128_t x = 0;
>     x = get_int(10, /* num_bytes */ 1);
>
>     PRINT(x);
>
>     return 0;
> }
> -- end of test.c
>
> $ gcc -v
> Using built-in specs.
> COLLECT_GCC=/home/Computational/rferrer/gcc/install/bin/gcc
> COLLECT_LTO_WRAPPER=/home/Computational/rferrer/gcc/install/libexec/gcc/powerpc64le-unknown-linux-gnu/5.0.0/lto-wrapper
> Target: powerpc64le-unknown-linux-gnu
> Configured with: ../gcc-src/configure
> --prefix=/home/Computational/rferrer/gcc/install
> --enable-languages=c,c++,fortran
> --with-gmp=/home/Computational/rferrer/gcc/install
> --with-mpfr=/home/Computational/rferrer/gcc/install
> --with-mpc=/home/Computational/rferrer/gcc/install --enable-multiarch
> --disable-multilib
> Thread model: posix
> gcc version 5.0.0 20141218 (experimental) (GCC)
>
> $ make
> gcc  -O0 -o test.O0 test.c
> ./test.O0
> x => <0000000000000000, 000000000000000a>
> gcc  -O1 -o test.O1 test.c
> ./test.O1
> x => <ffffffffffffffff, ffffffffffffff00>
>
> It looks like GCC somehow forgets to perform the logical not in the
> optimized version.
>
> I'd fill a PR but I'm not sure if I'm triggering some sort of
> undefined behaviour in the shift/not/and sequence in 'get_int'.
>
> Is this a bug in GCC or in the code above?

Please open a bug in either case.

Richard.

> Kind regards,
>
>
> --
> Roger Ferrer IbÃÃez