[Bug math/13932] dbl-64 pow unexpectedly slow for some inputs

["vincent-srcware at vinc17 dot net" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=13932

--- Comment #13 from Vincent Lefèvre <vincent-srcware at vinc17 dot net> ---
From the bug report, it is __ieee754_pow_sse2 that was used, and if I
understand correctly, the code is in "sysdeps/x86_64/fpu/multiarch/e_pow.c",
which uses "sysdeps/ieee754/dbl-64/e_pow.c". The slow path would imply that
__exp1 returns a non-positive value. __exp1 is defined in
"sysdeps/ieee754/dbl-64/e_exp.c". There, the rounding test is:

      if (res == (res + cor * (1.0 + error + err_1)))

For the example of slow case pow(0.562500, 1.5), the exact result is 0.421875 =
27/64, so that cor should be much smaller than ulp(res) and a positive value
should be returned, which doesn't explain the behavior.

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