how can I get the ARM MULL instruction to produce its output in a uint64_t in gcc?

Question

Hello,

I would like to introduce some assembly code into a c99 codebase. I want to use the UMULL instruction from the ARM CPU to multiply 2 uint32_t and get the result immediately into a uint64_t.

Now a uint64_t needs 2 registers, so how do I specify the output and the constraints of the asm block?

Answer 1

The umull instruction produces its results into two 32-bit registers. I suggest explicitly reassembling the 64-bit value with something like that:

/* assuming the 64-bit result was stored in "hi" (upper
   half) and "lo" (lower half) */
uint64_t v = ((uint64_t)hi << 32) | (uint64_t)lo;

The compiler optimizer should notice that the left-shift is pure data routing, and the resulting code should be fine. To be sure, just use -S to check the compiler output.