Why doesn't a compiler optimize floating-point *2 into an exponent increment?

Question

I\'ve often noticed gcc converting multiplications into shifts in the executable. Something similar might happen when multiplying an int and a float. F

Answer 1

On modern CPUs, multiplication typically has one-per-cycle throughput and low latency. If the value is already in a floating point register, there's no way you'll beat that by juggling it around to do integer arithmetic on the representation. If it's in memory to begin with, and if you're assuming neither the current value nor the correct result would be zero, denormal, nan, or infinity, then it might be faster to perform something like

addl $0x100000, 4(%eax)   # x86 asm example

to multiply by two; the only time I could see this being beneficial is if you're operating on a whole array of floating-point data that's bounded away from zero and infinity, and scaling by a power of two is the only operation you'll be performing (so you don't have any existing reason to be loading the data into floating point registers).