Looks ok to me. I think it might actually be ok to omit the version check, we use __builtin_popcount without it too (and that seems to be gcc 3.4 too, so presumably gcc 3.4 is a requirement). So what about __builtin_clz optimized logbase2 :-) I think that should just be 31 - __builtin_clz(n | 1)
I wonder though if these microptimizations are really worth it... Roland Am 08.06.2011 13:38, schrieb Benjamin Bellec: > Le 06/06/2011 23:54, Roland Scheidegger a écrit : >> Am 06.06.2011 23:18, schrieb Tormod Volden: >>> On Sun, Jun 5, 2011 at 1:14 AM, Benjamin Bellec wrote: >>>> So here is a v2 patch with a builtin GCC optimization which is the >>>> fastest (thx Matt to point me to this solution). >>>> >>> >>> From patch: >>> + return (1 << (32 - __builtin_clz(x - 1))); >>> >>> I don't know if the use of gcc guarantees that int will always be 32 >>> bit, otherwise maybe use sizeof(int)*8 instead of 32? Or even >>> sizeof(int)*CHAR_BIT for good measures. Although probably the robots >>> have taken over before this becomes necessary :) >>> >> >> Hmm I think a lot more things will break if that's not 32bit. >> There's another problem though, gcc docs say this: >> >> — Built-in Function: int __builtin_clz (unsigned int x) >> >> Returns the number of leading 0-bits in x, starting at the most >> significant bit position. If x is 0, the result is undefined. >> >> Which means it's now undefined for x == 1 too - not handling x == 0 >> correctly might not be much of a problem in practice, but the same >> certainly cannot be said for x == 1. So that should probably be >> >> +#if defined(PIPE_CC_GCC) >> + if (x <= 1) >> + return 1; >> + else >> + return (1 << (32 - __builtin_clz(x - 1))); >> >> >> Also I believe this builtin requires gcc 3.4 - not sure though if the >> rest of the code compiles on older gcc. >> >> Roland >> _______________________________________________ >> mesa-dev mailing list >> mesa-dev@lists.freedesktop.org >> http://lists.freedesktop.org/mailman/listinfo/mesa-dev > > Here is the v4 patch. > > Benjamin _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
