3 ms·
Last time I checked LLVM had surprisingly bad codegen for this using int128. On x86 you only need two instructions: __asm ( "mulq %[multiplier]\n"
by cmovq 8mo ago
Last time I checked LLVM had surprisingly bad codegen for this using int128. On x86 you only need two instructions:
__asm (
"mulq %[multiplier]\n"
"divq %[divisor]\n"
: "=a"(result)
: "a"(num), [multiplier]"r"(multiplier), [divisor]"r"(divisor)
: "rdx"
);
The intermediate 128bit number is in rdx:rax.
- bonzini 8mo agoThat only works if you are sure to have a 64-bit result. If you can have divisor < multiplier and need to detect overflow, it's more complicated.