On Wed, Jan 14 2015, Richard Biener <richard.guent...@gmail.com> wrote:
> On Tue, Jan 13, 2015 at 11:47 PM, Andrew Pinski <pins...@gmail.com> wrote: >> On Tue, Jan 13, 2015 at 2:41 PM, Rasmus Villemoes <r...@rasmusvillemoes.dk> >> wrote: >>> [My first attempt at submitting a patch for gcc, so please forgive me >>> if I'm not following the right protocol.] >> >> There are a few things missing. For one, a testcase or two for the >> added optimizations. I'll see what I can come up with. Thanks for the pointers. >>> Sometimes rounding a variable to the next even integer is written x += x >>> & 1. This usually means using an extra register (and hence at least an >>> extra mov instruction) compared to the equivalent x = (x + 1) & ~1. The >>> first pattern below tries to do this transformation. >>> >>> While playing with various ways of rounding down, I noticed that gcc >>> already optimizes all of x-(x&3), x^(x&3) and x&~(x&3) to simply >>> x&~3. > > Does it also handle x+(x&3)? I'm not sure what 'it' refers to, and I'm also not sure how you think x+(x&3) could be rewritten. > Where does it handle x - (x&3)? If by 'it' you mean gcc, I tried looking for a pattern matching this, but couldn't find it, so I don't know where it is handled. I can just see by running gcc-5.0 -fdump-tree-original -O2 -c opt.c that "x - (x & 3)" is rewritten as x & -4 (which is of course the same as x & ~3). Btw, I now see that neither x&~(x&3) or x&~(x&y) are rewritten that early, but objdump -d shows that the end result is the same. > That is, doesn't the pattern also work for constants other than 1? Here I assume that 'the pattern' refers to the first pattern, and the answer is 'not immediately'. To round up a number to the next multiple of 2^k we need to add the negative of that number modulo 2^k. It just so happens that for k=1 we have x==-x for both possible values of x. So with a little tweak, this does in fact lead to an optimization opportunity, namely x + ((-x) & m) -> (x + m) & ~m whenever m is one less than a power of 2. I don't know how to check for m satisfying this in the match.pd language. > Please put it before the abs simplifications after the last one handing > bit_and/bit_ior. OK, will do. Rasmus
