whoopdedo on Why doesn't GCC optimize a*a*a*a*a*a to (a*a*a)*(a*a*a)? (2013)

Comment on Why doesn't GCC optimize a*a*a*a*a*a to (a*a*a)*(a*a*a)? (2013)parent

If you want an exact representation, C99 has the %a format specifier for base-16 floating point. It seems that Python 3 doesn't support it but Lua 5.2 does.

    Lua 5.2.2  Copyright (C) 1994-2013 Lua.org, PUC-Rio
    > print(string.format("  0.1 (%a)\n+ 0.2 (%a)\n= %a\n+ 0.3 (%a)\n= %a", 0.1,0.2,0.1+0.2, 0.3, 0.1+0.2+0.3))
      0.1 (0x1.999999999999ap-4)
    + 0.2 (0x1.999999999999ap-3)
    = 0x1.3333333333334p-2
    + 0.3 (0x1.3333333333333p-2)
    = 0x1.3333333333334p-1
    > print(string.format("  0.2 (%a)\n+ 0.3 (%a)\n= %a\n+ 0.1 (%a)\n= %a", 0.2,0.3,0.2+0.3, 0.1, 0.2+0.3+0.1))
      0.2 (0x1.999999999999ap-3)
    + 0.3 (0x1.3333333333333p-2)
    = 0x1p-1
    + 0.1 (0x1.999999999999ap-4)
    = 0x1.3333333333333p-1

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