distributed/tria.cc: avoid FPE due to undefined behavior #16820
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The following construct ``` static_cast<types::subdomain_id>(this_sc_point[dim]) ``` triggers a floating point exception when the stored double is -1.0 (at least with gcc-11 and avx512 instructions enabled). Judging from cppreference [1] this is undefined behavior: """ A prvalue of floating-point type can be converted to a prvalue of any integer type. The fractional part is truncated, that is, the fractional part is discarded. If the truncated value cannot fit into the destination type, the behavior is undefined (even when the destination type is unsigned, modulo arithmetic does not apply). """ Thus, work around the issue by explicitly setting negative values to `numbers::invalid_subdomain_id` if we encounter -1. [1] https://en.cppreference.com/w/cpp/language/implicit_conversion
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/rebuild |
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All, this fixes the last pre-alignment avx test-failure on my side. I have marked it as Draft for a moment because I have to verify that I actually understood the code in question. (aka Matthias runs a full ctest while going out for dinner). |
kronbichler
approved these changes
Mar 30, 2024
bangerth
approved these changes
Mar 30, 2024
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The following construct
triggers a floating point exception when the stored double is -1.0 (at least with gcc-11 and avx512 instructions enabled).
Judging from cppreference [1] this is undefined behavior:
"""
A prvalue of floating-point type can be converted to a prvalue of any integer type. The fractional part is truncated, that is, the fractional part is discarded.
If the truncated value cannot fit into the destination type, the behavior is undefined (even when the destination type is unsigned, modulo arithmetic does not apply).
"""
Thus, work around the issue by explicitly setting negative values to
numbers::invalid_subdomain_id.Preparatory step to get the avx256 / avx512 regression tester variants online.
In reference to #16796
[1] https://en.cppreference.com/w/cpp/language/implicit_conversion