[SOLVED] Is there a 128 bit integer in gcc?

For GCC before C23, a primitive 128-bit integer type is only ever available on 64-bit targets, so you need to check for availability even if you have already detected a recent GCC version. In theory gcc could support TImode integers on machines where it would take 4x 32-bit registers to hold one, but I don't think there are any cases where it does.

In C++, consider a library such as boost::multiprecision::int128_t, but it doesn't use compiler-specific wide types, always sign/magnitude (with a 128-bit magnitude plus a separate sign bit, so a wider value-range than 128-bit 2's complement). You might want an #ifdef to typedef GCC's __int128 or Clang's _BitInt(128) on systems that have that, only typedef to Boost as a fallback, if your use-case is ok with a wider value-range. See also @phuclv's answer on another question.

ISO C23 will let you typedef unsigned _BitInt(128) u128, modeled on clang's feature originally called _ExtInt() which works even on 32-bit machines; see a brief intro to it. GCC13 -std=gnu2x doesn't even support that syntax yet, but current nightly GCC14 trunk does support it with -std=gnu23 (Godbolt)

GCC 4.6 and later has a __int128 / unsigned __int128 defined as a built-in type. Use
#ifdef __SIZEOF_INT128__ to detect it.

GCC 4.1 and later define __int128_t and __uint128_t as built-in types. (You don't need #include <stdint.h> for these, either. Proof on Godbolt.)

I tested on the Godbolt compiler explorer for the first versions of compilers to support each of these 3 things (on x86-64). Godbolt only goes back to gcc4.1, ICC13, and clang3.0, so I've used <= 4.1 to indicate that the actual first support might have been even earlier.

         legacy               recommended(?)    |  One way of detecting support
        __uint128_t   |  [unsigned]  __int128   |  #ifdef __SIZEOF_INT128__
gcc        <=  4.1    |       4.6               |     4.6
clang      <=  3.0    |       3.1               |     3.3
ICC        <=  13     |     <= 13               |     16.  (Godbolt doesn't have 14 or 15)

If you compile for a 32-bit architecture like ARM, or x86 with -m32, no 128-bit integer type is supported with even the newest version of any of these compilers. So you need to detect support before using, if it's possible for your code to work at all without it.

The only direct CPP macro I'm aware of for detecting it is __SIZEOF_INT128__, but unfortunately some old compiler versions support it without defining it. (And there's no macro for __uint128_t, only the gcc4.6 style unsigned __int128). How to know if __uint128_t is defined

Some people still use ancient compiler versions like gcc4.4 on RHEL (RedHat Enterprise Linux), or similar crusty old systems. If you care about obsolete gcc versions like that, you probably want to stick to __uint128_t. And maybe detect 64-bitness in terms of sizeof(void*) == 8 as a fallback for __SIZEOF_INT128__ no being defined. (I think GNU systems always have CHAR_BIT==8, although I might be wrong about some DSPs). That will give a false negative on ILP32 ABIs on 64-bit ISAs (like x86-64 Linux x32, or AArch64 ILP32), but this is already just a fallback / bonus for people using old compilers that don't define __SIZEOF_INT128__.

There might be some 64-bit ISAs where gcc doesn't define __int128, or maybe even some 32-bit ISAs where gcc does define __int128, but I'm not aware of any.

The GCC internals are integer TI mode (GCC internals manual). (Tetra-integer = 4x width of a 32-bit int, vs. DImode = double width vs. SImode = plain int.) As the GCC manual points out, __int128 is supported on targets that support a 128-bit integer mode (TImode).

// __uint128_t is pre-defined equivalently to this
typedef unsigned uint128 __attribute__ ((mode (TI)));

There is an OImode in the manual, oct-int = 32 bytes, but current GCC for x86-64 complains "unable to emulate 'OI'" if you attempt to use it.

Random fact: ICC19 and g++/clang++ -E -dM define:

#define __GLIBCXX_TYPE_INT_N_0 __int128
#define __GLIBCXX_BITSIZE_INT_N_0 128

@MarcGlisse commented that's the way you tell libstdc++ to handle extra integer types (overload abs, specialize type traits, etc)

icpc defines that even with -xc (to compile as C, not C++), while g++ -xc and clang++ -xc don't. But compiling with actual icc (e.g. select C instead of C++ in the Godbolt language dropdown) doesn't define this macro.

The test function was:

#include <stdint.h>   // for uint64_t

#define uint128_t __uint128_t
//#define uint128_t unsigned __int128

uint128_t mul64(uint64_t a, uint64_t b) {
    return (uint128_t)a * b;
}

compilers that support it all compile it efficiently, to

    mov       rax, rdi
    mul       rsi
    ret                  # return in RDX:RAX which mul uses implicitly