With gfortran and g++ the computation of cos(x) and sin(x) is "optimized" by taking the real and imaginary parts of cexpi(x) (at least it is what I understand). This is working if and only if the computation of cexpi(x) is faster than the sum of the separate computations of cos(x) and sin(x).
Now consider the following code: integer, parameter :: n=1000000 integer :: i real(8) :: pi, ss, sc, t, dt pi = acos(-1.0d0) dt=pi/n sc=0 ss=0 t=0 do i= 1, 100*n sc = sc + cos(t-dt) ss = ss + sin(t) t = t+dt end do print *, sc, ss end the result is (G5 1.8Ghz, OSX 10.3.9): [karma] bug/timing% gfc -O3 sincos.f90 [karma] bug/timing% time a.out -6.324121638644320E-002 -2.934958087315009E-003 13.020u 0.050s 0:13.59 96.1% 0+0k 0+2io 0pf+0w It is easy to see that I have fooled the optimizer with the line sc = sc + cos(t-dt) If I replace it by: sc = sc + cos(t) the result is now (over a 50% increase of the CPU time): [karma] bug/timing% gfc -O3 sincos_o.f90 [karma] bug/timing% time a.out -6.324121573032526E-002 -2.934958087315009E-003 21.740u 0.080s 0:22.18 98.3% 0+0k 0+2io 0pf+0w to be compared with the result of the code: integer, parameter :: n=1000000 integer :: i real(8) :: pi, ss, sc, t, dt complex(8) :: z, dz pi = acos(-1.0d0) dt=pi/n dz=cmplx(0.0d0,dt,8) sc=0 ss=0 z=0 do i= 1, 100*n sc = sc + real(exp(z)) ss = ss + aimag(exp(z)) z = z+dz end do print *, sc, ss end is [karma] bug/timing% gfc -O3 cexp.f90 [karma] bug/timing% time a.out -6.324121573032526E-002 -2.934958087315009E-003 20.850u 0.110s 0:21.45 97.7% 0+0k 0+2io 0pf+0w Following the comments in PR #30969, 30980, and 31161, I have understood that on OSX cexpi "fallback" to cexp in perfect agreement with the above timings. So it would probably nice to disable the sincos "optimisation" on platforms that do not support fast cexpi such as OSX (as presently configured). Note that on Sat, 30 Sep 2006 in http://gcc.gnu.org/ml/fortran/2006-09/msg00454.html I have reported (in vain) a timing regression for the fatigue.f90 polyhedron test case. Is this related to this pseudo-optimization or to another change? -- Summary: pseudo-optimzation with sincos/cexpi Product: gcc Version: 4.3.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: middle-end AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: dominiq at lps dot ens dot fr GCC target triplet: powerpc-apple-darwin7 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31249
