[Bug c/99268] New: shifted uint64_t becomes wrong value

[Bug c/99268] New: shifted uint64_t becomes wrong value

[spoettl at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99268

            Bug ID: 99268
           Summary: shifted uint64_t becomes wrong value
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: spoettl at hotmail dot com
  Target Milestone: ---

MY COMPILER VERSION
===================
stefan@Apollon:~$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/10/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
10.2.0-13ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-10/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-10
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new
--enable-gnu-unique-object --disable-vtable-verify --enable-plugin
--enable-default-pie --with-system-zlib --enable-libphobos-checking=release
--with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch
--disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-10-JvwpWM/gcc-10-10.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-10-JvwpWM/gcc-10-10.2.0/debian/tmp-gcn/usr,hsa
--without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.2.0 (Ubuntu 10.2.0-13ubuntu1) 

PROGRAMM PRODUCING (UNEXPECTED) WRONG VALUES BY SHIFTING UINT64_T
=================================================================
#include <stdint.h>
#include <stdio.h>


typedef uint8_t  N8;
typedef uint16_t N16;
typedef uint32_t N32;
typedef uint64_t N64;


// Counting the number of bits in n
// (leading zeros are not counted)
N8 numberBits(N64 n)
{
    N8 b = 1, lz = 0;
    // The compiler doesn't like 1 << 63!
    // (such that the most significant bit of n is set)
    N64 mk = 1 << 31;
    mk <<= 31;
    mk <<= 1;
    if (n > 0)
    {
       while ((n & mk) == 0)
       {
           lz++;
           mk >>= 1;
       }
       b = 64 - lz;
    }
    return b;
}


void numberBitsTest()
{
    N64 n;

    // Working Code:
    n = 1;
    printf("%lu has %2i bits\n", n, numberBits(n));
    n = 1 << 17;
    printf("%lu has %2i bits\n", n, numberBits(n));

    // Now the bug:
    n = 1 << 31;
    printf("n was set to (1 << 31) == 2**32\n");
    printf("%lu has %2i bits\n", n, numberBits(n));
    printf("Hexadecimal:\n");
    printf("%lX has %2i bits\n", n, numberBits(n));
    // Doen't disappear for bigger n:
    n <<= 30;
    printf("n was set to (1 << 61) == 2**62\n");
    printf("%lu has %2i bits\n", n, numberBits(n));
    printf("Hexadecimal:\n");
    printf("%lX has %2i bits\n", n, numberBits(n));    
}


int main()
{
    numberBitsTest();
    return 0;
}

[Bug c/99268] shifted uint64_t becomes wrong value

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99268

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
                 CC|                            |jakub at gcc dot gnu.org
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
User error, 1 << 31 is undefined behavior in C and even when it is not UB (e.g.
in C++20), the result when int is 32-bit is -2147483647 - 1 (i.e. INT_MIN,
negative value) which when cast to uint64_t is then 0xffffffff80000000ULL.
You could try using -fsanitize=undefined to get it diagnosed at runtime.