Gornak40's blog

By Gornak40, history, 4 months ago, translation, In English

Compiler GCC provides the ability to use assembler inserts. This can be useful, for example, for multiplying two 64-bit numbers by a 64-bit module.

The fact is that multiplying two 64-bit registers, the processor stores the result in a pair of registers rdx (upper part) and rax (lower part). Division works in a similar way: the divisible is taken from the registers rdx and rax, after which the quotient is stored in rax, and the remainder is stored in rdx.

Using this knowledge, you can implement an analog of the following function:

inline long long mul(long long a, long long b) {
	return (__int128)a * b % 1000000014018503;

In this way:

inline long long mul(long long a, long long b) {
	long long res;
		"mov %1, %%rax\n"
		"mov %2, %%rbx\n"
		"imul %%rbx\n"
		"mov $1000000014018503, %%rbx\n"
		"idiv %%rbx\n"
		"mov %%rdx, %0\n"
		:"a"(a), "b"(b)
	return res;

We indicate the use of variables res for writing, a and b for reading. They accordingly receive designations %0, %1, %2. Operations are written using the standard AT&T syntax.

Now you can write hashes using a 64-bit module, which is equivalent to using a pair using a 32-bit module, without using __int128.

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