[Octave-bug-tracker] [bug #42583] log2() returns inaccurate result for many integer powers of 2, unlike Matlab

[Dmitri A. Sergatskov]

Follow-up Comment #21, bug #42583 (project octave):

I compiled with default optimization flags both 64-bit pointer and 32-bit
pointer. The result is the same. With 32-bit pointers I got the same answer as
Rik:

octave:1> __octave_config_info__ ("config_opts")
ans = 
octave:2> pow2 (-1026:-1020)
ans =

  1.4e-309  2.8e-309  5.6e-309  1.1e-308  2.2e-308  4.5e-308  8.9e-308


With 64-bit pointers:

ctave:1> __octave_config_info__ ("config_opts")
ans =  '--enable-64' '--with-blas=flexiblas64' '--with-qrupdate=qrupdate64'
'--with-suitesparseconfig=suitesparseconfig64' '--with-amd=amd64'
'--with-camd=camd64' '--with-colamd=colamd64' '--with-ccolamd=ccolamd64'
'--with-cholmod=cholmod64' '--with-spqr=spqr64' '--with-cxsparse=cxsparse64'
'--with-umfpack=umfpack64' '--with-klu=klu64' '--with-arpack=arpack64'
'--without-sundials_nvecserial' '--without-sundials_ida'
'--without-sundials_sunlinsolklu'
octave:2> pow2 (-1026:-1020)
ans =

   0   0   0   0  2.2e-308  4.5e-308  8.9e-308



I also tried clang (32-bit and 64-bit) -- the same discrepancy.

Trying smaller steps:


octave:37> pow2 (-(1022+257*eps))
ans = 0
octave:38> pow2 (-(1022+256*eps))
ans = 2.2251e-308
octave:39> format bit
octave:40> -(1022+257*eps)
ans = 1100000010001111111100000000000000000000000000000000000000000001
octave:41> -(1022+256*eps)
ans = 1100000010001111111100000000000000000000000000000000000000000000



Dmitri.
-- 



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