Re: Issue with __int128 in powerpc64le

[Roger Ferrer IbÃÃez <rofirrim at gmail dot com>]

Done, it is PR64358.

Kind regards,

2014-12-19 12:21 GMT+01:00 Richard Biener <richard.guenther@gmail.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



-- 
Roger Ferrer IbÃÃez