Alan Cox <[EMAIL PROTECTED]> writes: > > Good compilers even in the 1990's would defer the divide and try and > propogate it out as a multiply the other side for constants, and they'll > also use shifts when possible.
gcc has an algorithm that tends to generate a near perfect shift/add etc. code sequence and also knows the obvious x / y ==> x*1/y However it doesn't do that with -Os, prefering smaller code on x86 (idiv is fairly small compared to the expanded sequences for non power of two dividends) and kernels are usually compiled with -Os these days. We've had a few cases in the past where this showed up as regression against older kernels that still used -O2. > Thus they'll turn > > (ptr.element - base.element) < NELEM > > into > (ptr.char - base.char) < (constant) [NELEM *sizeof(element) ] > > > at least for constant operations. Dunno if gcc is that clever It is. However a few more complex transformations I would have liked in the past are missing -- in particular x / (cond ? const1 : const2) ==> cond ? (x / const1) : (x / const2) -Andi - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
