libm: Add frexpf16

[tgross35]

No description provided.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[tgross35]

As expected, the algorithm change gets back the perf lost in #862 with some tiny improvements on top of it. Aarch64:

icount::icount_bench_frexp_group::icount_bench_frexp logspace:setup_frexp()
  Baselines:                      softfloat|softfloat (old)
  Instructions:                        7719|8219                 (-6.08347%) [-1.06478x]
  L1 Hits:                             8334|8833                 (-5.64927%) [-1.05988x]
  LL Hits:                                2|2                    (No change)
  RAM Hits:                               5|6                    (-16.6667%) [-1.20000x]
  Total read+write:                    8341|8841                 (-5.65547%) [-1.05994x]
  Estimated Cycles:                    8519|9053                 (-5.89860%) [-1.06268x]
icount::icount_bench_frexpf128_group::icount_bench_frexpf128 logspace:setup_frexpf128()
  Baselines:                      softfloat|softfloat (old)
  Instructions:                       15219|15219                (No change)
  L1 Hits:                            23825|21829                (+9.14380%) [+1.09144x]
  LL Hits:                                5|1                    (+400.000%) [+5.00000x]
  RAM Hits:                               8|8                    (No change)
  Total read+write:                   23838|21838                (+9.15835%) [+1.09158x]
  Estimated Cycles:                   24130|22114                (+9.11640%) [+1.09116x]
icount::icount_bench_frexpf16_group::icount_bench_frexpf16 logspace:setup_frexpf16()
  Baselines:                      softfloat|softfloat (old)
  Instructions:                        7848|N/A                  (*********)
  L1 Hits:                             8415|N/A                  (*********)
  LL Hits:                                1|N/A                  (*********)
  RAM Hits:                               6|N/A                  (*********)
  Total read+write:                    8422|N/A                  (*********)
  Estimated Cycles:                    8630|N/A                  (*********)
icount::icount_bench_frexpf_group::icount_bench_frexpf logspace:setup_frexpf()
  Baselines:                      softfloat|softfloat (old)
  Instructions:                        7739|8251                 (-6.20531%) [-1.06616x]
  L1 Hits:                             8354|8873                 (-5.84921%) [-1.06213x]
  LL Hits:                                2|1                    (+100.000%) [+2.00000x]
  RAM Hits:                               5|7                    (-28.5714%) [-1.40000x]
  Total read+write:                    8361|8881                 (-5.85520%) [-1.06219x]
  Estimated Cycles:                    8539|9123                 (-6.40140%) [-1.06839x]

x86:

icount::icount_bench_frexp_group::icount_bench_frexp logspace:setup_frexp()
  Baselines:                      softfloat|softfloat (old)
  Instructions:                       11189|11689                (-4.27753%) [-1.04469x]
  L1 Hits:                            12776|13273                (-3.74444%) [-1.03890x]
  LL Hits:                                1|3                    (-66.6667%) [-3.00000x]
  RAM Hits:                               6|7                    (-14.2857%) [-1.16667x]
  Total read+write:                   12783|13283                (-3.76421%) [-1.03911x]
  Estimated Cycles:                   12991|13533                (-4.00502%) [-1.04172x]
icount::icount_bench_frexpf128_group::icount_bench_frexpf128 logspace:setup_frexpf128()
  Baselines:                      softfloat|softfloat (old)
  Instructions:                       17194|17694                (-2.82582%) [-1.02908x]
  L1 Hits:                            24780|25279                (-1.97397%) [-1.02014x]
  LL Hits:                                0|2                    (-100.000%) [---inf---]
  RAM Hits:                               7|6                    (+16.6667%) [+1.16667x]
  Total read+write:                   24787|25287                (-1.97730%) [-1.02017x]
  Estimated Cycles:                   25025|25499                (-1.85890%) [-1.01894x]
icount::icount_bench_frexpf16_group::icount_bench_frexpf16 logspace:setup_frexpf16()
  Baselines:                      softfloat|softfloat (old)
  Instructions:                       12996|N/A                  (*********)
  L1 Hits:                            15649|N/A                  (*********)
  LL Hits:                                1|N/A                  (*********)
  RAM Hits:                               6|N/A                  (*********)
  Total read+write:                   15656|N/A                  (*********)
  Estimated Cycles:                   15864|N/A                  (*********)
icount::icount_bench_frexpf_group::icount_bench_frexpf logspace:setup_frexpf()
  Baselines:                      softfloat|softfloat (old)
  Instructions:                       10218|10726                (-4.73616%) [-1.04972x]
  L1 Hits:                            11806|12320                (-4.17208%) [-1.04354x]
  LL Hits:                                1|2                    (-50.0000%) [-2.00000x]
  RAM Hits:                               5|6                    (-16.6667%) [-1.20000x]
  Total read+write:                   11812|12328                (-4.18559%) [-1.04368x]
  Estimated Cycles:                   11986|12540                (-4.41786%) [-1.04622x]