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Fix non-determinism in global placement caused by different FFT output values #4518

Merged
merged 2 commits into from
Jan 10, 2024
Merged

Fix non-determinism in global placement caused by different FFT output values #4518

merged 2 commits into from
Jan 10, 2024

Commits on Jan 10, 2024

  1. Adding a simple test for the FFT class. 

    The `fft_test` checks that the FFT output is deterministic despite using
floating point values.

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>
    @mithro
    mithro committed Jan 10, 2024
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  2. Add -ffp-contract=off compile flag. 

    GCC per default enables one optimization for x86_64 that can change the
result of floating-point operations: -ffp-contract=fast.

This allows the compiler to do floating-point expression contraction
such as combining multiplication and addition instructions with an FMA
instruction. However, the FMA instruction omits the rounding done in the
multiplication instruction, making the calculations produce different
results for some input values.

Disabling floating-point contraction with -ffp-contract=off makes GCC
generate code that produces a consistent result for x86_64.

See https://kristerw.github.io/2021/11/09/fp-contract/

The `fft_test` checks that this is working correctly.

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>
    @mithro
    mithro committed Jan 10, 2024
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