CODEWITHSUNDEEP
c++c++11gccgdb
oo_miguel
oo_miguel

Reputation: 2379

gcc instantiates template with long instead of __int128

I am dazed and confused. What is happening here in the second call to my big template?

template <class T> void big(T t) {  }

int main()
{
    big(9223372036854775808);                        // calls big<__int128>
    big(941832094813209483120);                      // calls big<long>
    big(239120938091238093219203810293801923832019); // calls big<__int128>
}

Why is a long template instantiated for 941832094813209483120, while the two other values get a __int128 template.

This value obviously does not fit inside the long and seems to result in an overflow (see full gdb session below):

big<long> (t=1048147054022350704) at blob.cpp:1

I observe this with gcc-5.2.0 as well as gcc-4.9.2, while I use gdb-7.7.1 for debugging.

This is my full gdb session:

Breakpoint 1, main () at blob.cpp:5
(gdb) s 
big<__int128> (t=0x00000000000000008000000000000000) at blob.cpp:1
(gdb)  
main () at blob.cpp:6
(gdb)  
big<long> (t=1048147054022350704) at blob.cpp:1
(gdb)  
main () at blob.cpp:7
(gdb)  
big<__int128> (t=0x0000000000000000d90567828f8ae8d3) at blob.cpp:1
(gdb)

Upvotes: 3

Views: 201

Answers (1)

Shafik Yaghmour
Shafik Yaghmour

Reputation: 158599

Since the OP has confirmed that long long is 64bit on their system we can see that the gcc docs on 128-bit Integers says:

There is no support in GCC for expressing an integer constant of type __int128 for targets with long long integer less than 128 bits wide.

So while I agree the behavior is odd, it is technically not a bug since gcc does not support this scenario and explicitly documents this.

Compilers may support extended signed integers, from the draft C++11 standard section 3.9.1:

There may also be implementation-defined extended signed integer types

but they are implementation-defined and wording for integer literals in section 2.14.2 says:

If an integer literal cannot be represented by any type in its list and an extended integer type (3.9.1) can represent its value, it may have that extended integer type [...]

with emphasis on may.

Upvotes: 1

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