https://chromiumcodereview.appspot.com/9638018/diff/13001/src/utils.h File src/utils.h (right):
https://chromiumcodereview.appspot.com/9638018/diff/13001/src/utils.h#newcode112 src/utils.h:112: const DivMagicNumbers DivMagicNumberFor625 = {0x68db8bad, 8}; Fair question! I actually noticed a non-optimal pattern in the patch (1 instruction difference). Details below. Since there were a couple of other comments I'll fix that and upload a new patch. Optimized code for the pattern <k for which we have a magic number> * 2^n can require a bit more code (1 instr on ARM) than for <k for which we have a magic number> Using magic numbers for 6 spares one instruction compare to using those for 3 plus the code for the factor 2. For 10 it does not make a difference here. And actually, my implementation is non-optimal if divisor == 6... I first search for the greatest power of 2 dividing the divisor, and check if we have magic numbers for the odd part. That means the pattern '3 * 2' is used. One instruction less efficient :( On 2012/04/17 18:30:26, alexeif wrote:
On 2012/04/17 17:12:22, Alexandre wrote: > We can generate optimized code for numbers with an absolute value
equals that
is > representable by > <1, or k for which we have a magic number> * 2^n > So optimized code can be generated for these: > 60 = 15 * 2^2 > 100 = 25 * 2^2 > 1000 = 125 * 2^3 > > This is detailed in the arm files. > > On 2012/04/17 16:59:28, alexeif wrote: > > How about adding 60, 100 and 1000? They seem to have even higher
rate than
> 625. > Oh, I see, it has a shift anyway. Btw, why do you need constants for
evens 6 and
10?
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