Issue with __int128 in powerpc64le

[Roger Ferrer Ibáñez]

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?

Kind regards,


-- 
Roger Ferrer Ibáñez