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*From*: Canqun Yang <canqun at nudt dot edu dot cn>*To*: Geert Bosch <bosch at adacore dot com>*Cc*: fortran at gcc dot gnu dot org, gcc at gcc dot gnu dot org, gcc-patches at gcc dot gnu dot org,wilson at specifixinc dot com*Date*: Fri, 8 Apr 2005 17:54:54 +0800 (HKT)*Subject*: Re: Inline round for IA64*Reply-to*: Canqun Yang <canqun at nudt dot edu dot cn>

Geert Bosch <bosch@adacore.com>: > As far as I can seem from this patch, it rounds incorrectly. > This is a problem with the library version as well, I believe. > > The issue is that one cannot round a positive float to int > by adding 0.5 and truncating. (Same issues with negative values > and subtracting 0.5, of course). This gives an error for the > predecessor of 0.5. The between Pred (0.5) and 0.5 is half that of > pred (1.0) and 1.0. So the value of Pred (0.5) + 0.5 lays exactly > halfway Pred (1.0) and 1.0. The CPU rounds this halfway value to > even, or 1.0 in this case. > > So try rounding .499999999999999944488848768742172978818416595 4589843750 > using IEEE double on non-x86 platform, and you'll see it gets rounded > to 1.0. Do you mean the correct value should be 0.0 ? > A similar problem exists with large odd integers between 2^52+1 and > 2^53-1, > where adding 0.5 results in a value exactly halfway two integers, > rounding up to the nearest even integer. So, for IEEE double, > 4503599627370497 would round to 4503599627370498. Do you mean 4503599627370498 is a wrong result? > > These issues can be fixed by not adding/subtracting 0.5, but Pred (0.5). > As shown above, this rounds to 1.0 correctly for 0.5. For larger values > halfway two integers, the gap with the next higher representable number > will > only decrease so the result will always be rounded up to the next higher > integer. For this technique to work, however, it is necessary that the > addition will be rounded to the target precision according to IEEE > round-to-even semantics. On platforms such as x86, where GCC implicitly > widens intermediate results for IEEE double, the rounding to integer > should be performed entirely in long double mode, using the long double > predecessor of 0.5. > > See ada/trans.c around line 5340 for an example of how Ada does this. > > -Geert > > On Apr 7, 2005, at 05:38, Canqun Yang wrote: > > Gfortran translates the Fortran 95 intrinsic DNINT to > > round operation with double precision type argument > > and return value. Inline round operation will speed up > > the SPEC CFP2000 benchmark 189.lucas which contains > > function calls of intrinsic DNINT from 706 (SPEC > > ratio) to 783 on IA64 1GHz system. > > > > I have implemented the double precison version of > > inline round. If it is worth doing, I can go on to > > finish the other precision mode versions. > > I attached an example for intrinsic DNINT with its output. Would you please check it, and tell me whether the result is correct. Canqun Yang Creative Compiler Research Group. National University of Defense Technology, China.

! Test case for inline round subroutine dnint_ex (a, b, n) real*8 a(n), b(n) integer n do i = 1, n b(i) = dnint (a(i)) enddo end program round_test real*8 a(2), b(2) a(:) = (/.4999999999999999444888487687421729788184165954589843750_8,& 4503599627370497.0_8/) call dnint_ex (a, b, 2) write (*,*) b end The output is: 0.00000000000000 4.503599627370497E+015

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