On Monday, May 18, 2015 11:26:05 AM Matt Turner wrote: > On Fri, May 8, 2015 at 3:36 AM, Kenneth Graunke <kenn...@whitecape.org> wrote: > > According to Glenn, shifts on R600 have 5x the throughput as multiplies. > > > > Intel GPUs have strange integer multiplication restrictions - on most > > hardware, MUL actually only does a 32-bit x 16-bit multiply. This > > means the arguments aren't commutative, which can limit our constant > > propagation options. SHL has no such restrictions. > > > > Shifting is probably reasonable on most people's hardware, so let's just > > do that. > > > > i965 shader-db results (using NIR for VS): > > total instructions in shared programs: 7432587 -> 7388982 (-0.59%) > > instructions in affected programs: 1360411 -> 1316806 (-3.21%) > > helped: 5772 > > HURT: 0 > > Just to close the loop, I ran shader-db with this patch on top of my > integer multiplication series, and it doesn't change any instruction > counts on i965. (I also tried with all other power-of-two > multiplications for shift values < 31.) > > We may want to do it for other reasons though.
If we're going to do it because shifts are faster/nicer than multiplies, then we should probably just do it for powers-of-two in general. Unfortunately, opt_algebraic doesn't really lend itself to that without adding some sort of "power of two" infrastructure. I guess we could optimize things like: a * 2^n => a << n a % 2^n => a & (n-1) a / 2^n => a >> n (possibly only for unsigned? (*)) ...others? The first is a clear win on r600, and the latter are clear wins on i965, though they may be rather rare... We could add a custom NIR pass. Or, we could just or just have backends check for an immediate second operand and do this sort of stuff. Or optimize it themselves. *shrug* (*) http://lists.freedesktop.org/archives/mesa-dev/2014-April/057364.html
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