gcc 7.3 128-bit unsigned integer operation

问题

I'm confused with the usage of 128-bit integer. Please look at the test code:

uint128_t test_data = 0x00000000FFFFFFFF0101010101010101;
uint64_t test_data_h = test_data  >> 64;
uint64_t test_data_l = test_data ;
printf("test_data 0x %016llx %016llx\n", test_data_h, test_data_l);

I expect the test_data_h to be 0x00000000FFFFFFFF, but the result is :

test data 0x 0000000000000000 0101010101010101

Why is this?

回答1:

Many compilers do not support 128 bit constants.

Instead

// uint128_t test_data = 0x00000000FFFFFFFF0101010101010101;
uint128_t test_data = (((uint128_t) 0x00000000FFFFFFFF) << 64) | 0x0101010101010101;

Tip: enable all compiler warnings.

回答2:

GCC doesn't support 128-bit integer literal

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.

128-bit Integers

So you'll have to construct it from smaller parts (high << 64) | low

来源：https://stackoverflow.com/questions/60860827/gcc-7-3-128-bit-unsigned-integer-operation