Commits on Sep 25, 2021

1. BUG: Resolve Divide by Zero on Apple silicon + test failures (numpy#1… …

   …9926)
   
   * Resolve divide by zero in reciprocal numpy#18555
   
   clang has an optimization bug where a vector that is only partially loaded / stored will get narrowed to only the lanes used, which can be fine in some cases. However, in numpy's `reciprocal` function a partial load is explicitly extended to the full width of the register (filled with '1's) to avoid divide-by-zero. clang's optimization ignores the explicit filling with '1's.
   
   The changes here insert a dummy `volatile` variable. This convinces clang not to narrow the load and ignore the explicit filling of '1's.
   
   `volatile` can be expensive since it forces loads / stores to refresh contents whenever the variable is used. numpy has its own template / macro system that'll expand the loop function below for sqrt, absolute, square, and reciprocal. Additionally, the loop can be called on a full array if there's overlap between src and dst. Consequently, we try to limit the scope that we need to apply `volatile`. Intention is it should only be needed when compiling with clang, against Apple arm64, and only for the `reciprocal` function. Moreover, `volatile` is only needed when a vector is partially loaded.
   
   Testing:
   Beyond fixing the cases mentioned in the GitHub issue, the changes here also resolve several failures in numpy's test suite.
   
   Before:
   ```
   FAILED numpy/core/tests/test_scalarmath.py::TestBaseMath::test_blocked - RuntimeWarning: divide by zero encountered in reciprocal
   FAILED numpy/core/tests/test_ufunc.py::TestUfuncGenericLoops::test_unary_PyUFunc_O_O_method_full[reciprocal] - AssertionError: FloatingPointError not raised
   FAILED numpy/core/tests/test_umath.py::TestPower::test_power_float - RuntimeWarning: divide by zero encountered in reciprocal
   FAILED numpy/core/tests/test_umath.py::TestSpecialFloats::test_tan - AssertionError: FloatingPointError not raised by tan
   FAILED numpy/core/tests/test_umath.py::TestAVXUfuncs::test_avx_based_ufunc - RuntimeWarning: divide by zero encountered in reciprocal
   FAILED numpy/linalg/tests/test_linalg.py::TestNormDouble::test_axis - RuntimeWarning: divide by zero encountered in reciprocal
   FAILED numpy/linalg/tests/test_linalg.py::TestNormSingle::test_axis - RuntimeWarning: divide by zero encountered in reciprocal
   FAILED numpy/linalg/tests/test_linalg.py::TestNormInt64::test_axis - RuntimeWarning: divide by zero encountered in reciprocal
   8 failed, 14759 passed, 204 skipped, 1268 deselected, 34 xfailed in 69.90s (0:01:09)
   ```
   
   After:
   ```
   FAILED numpy/core/tests/test_umath.py::TestSpecialFloats::test_tan - AssertionError: FloatingPointError not raised by tan
   1 failed, 14766 passed, 204 skipped, 1268 deselected, 34 xfailed in 70.37s (0:01:10)
   ```
   
   * Enhancement on top of workaround for clang bug in reciprocal
   
   Enhancement on top of workaround for clang bug in reciprocal (numpy#18555)
   Numpy's FP unary loops use a partial load / store on every iteration. The partial load / store helpers each insert a switch statement to know how many elements to handle. This causes a lot of unnecessary branches to be inserted in the loops. The partial load / store is only needed on the final iteration of the loop if it isn't a full vector.
   
   The changes here breakout the final iteration to use the partial load / stores. The loop has been changed to use full load / stores. Additionally, this means we don't need to conditionalize the volatile workaround in the loop.
   
   * Address Azure CI failures with older versions of clang
   
   - -ftrapping-math is default enabled for Numpy, but support in clang is mainly for x86_64
   - Apple Clang and Clang have different, but overlapping versions
   - Non-Apple Clang versions come from looking at when they started supporting -ftrapping-math for x86_64
   
   Testing was done against Apple Clang versions
   - v11 / x86_64 - failed previously, now passes (azure failure)
   - v12+ / x86_64 - passes before and after
   - v13 / arm64 - failed before initial patch, passes after

   

   Developer-Ecosystem-Engineering authored and charris committed Sep 25, 2021
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